Publications by authors named "Hugo Palmans"

67 Publications

A Geant4 Fano test for novel very high energy electron beams.

Phys Med Biol 2021 Nov 29. Epub 2021 Nov 29.

Radiation Science, National Physical Laboratory, Teddington, UNITED KINGDOM OF GREAT BRITAIN AND NORTHERN IRELAND.

The boundary crossing algorithm available in Geant4 10.07-p01 general purpose Monte Carlo code has been investigated for a 12 MeV and 200 MeV electron source by the application of a Fano cavity test.Fano conditions were enforced through all simulations whilst varying individual charged particle transport parameters which control particle step size, ionisation and single scattering.At 12 MeV, Geant4 was found to return excellent dose consistency within 0.1% even with the default parameter configurations. The 200 MeV case, however, showed significant consistency issues when default physics parameters were employed with deviations from unity of more than 6%. The effect of the inclusion of nuclear interactions was also investigated for the 200 MeV beam and was found to return good consistency for a number of parameter configurations.The Fano test is a necessary investigation to ensure the consistency of charged particle transport available in Geant4 before detailed detector simulations can be conducted.
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http://dx.doi.org/10.1088/1361-6560/ac3e0fDOI Listing
November 2021

Experimental determination of the effective point of measurement of the PTW-31010 ionization chamber in proton and carbon ion beams.

Med Phys 2021 Nov 24. Epub 2021 Nov 24.

MedAustron Ion Therapy Center, Marie Curie-Straße 5, Wiener Neustadt, Austria.

Purpose: The accurate knowledge of the effective point of measurement (P ) is particularly important for measurements in proximity to high dose gradients such as in the distal fall-off of particle beams. For plane-parallel ionization chambers (IC), P is well known and located at the center of the inner surface of the entrance window. For cylindrical ICs, P is shifted from the chamber's center towards the beam source. According to IAEA TRS-398, this shift can be calculated as 0.75·r for light ions with r being the radius of the cavity. For proton beams and in absence of a dose gradient, no shift is recommended. We have experimentally determined P for the 0.125 cc Semiflex IC in both proton and carbon ion beams.

Methods: The first method consisted of simultaneous irradiation of a plane-parallel IC and the Semiflex in a 4 cm wide spread-out Bragg peak. In the second method, a single-energy beam was used and both ICs were positioned successively at the same measurement depths. For both approaches, the shift of the distal edge of the depth ionization distributions recorded by the two chambers at different reference points was used to calculate P of the Semiflex. Both methods were applied in carbon ion beams and only the latter was applied in proton beams.

Results: Both methods yielded a similar P for carbon ions, 0.88·r and 0.84·r , which results in a difference of only 0.1 mm. The difference to the recommended value of 0.75·r is 0.4 and 0.3 mm respectively, which is larger than the positioning uncertainty. In the proton beam, a P of 0.92·r was obtained.

Conclusions: The P for the 0.125 cc Semiflex IC is shifted further from the cavity center as recommended by IAEA TRS-398 for light ions, with the shift for proton beams being even larger than for carbon ion beams. This article is protected by copyright. All rights reserved.
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http://dx.doi.org/10.1002/mp.15377DOI Listing
November 2021

Dose calculation accuracy in particle therapy: Comparing carbon ions with protons.

Med Phys 2021 Nov 23;48(11):7333-7345. Epub 2021 Sep 23.

Department of Radiation Oncology, Medical University of Vienna, Vienna, Austria.

Purpose: This work presents the validation of an analytical pencil beam dose calculation algorithm in a commercial treatment planning system (TPS) for carbon ions by measurements of dose distributions in heterogeneous phantom geometries. Additionally, a comparison study of carbon ions versus protons is performed considering current best solutions in commercial TPS.

Methods: All treatment plans were optimized and calculated using the RayStation TPS (RaySearch, Sweden). The dose distributions calculated with the TPS were compared with measurements using a 24-pinpoint ionization chamber array (T31015, PTW, Germany). Tissue-like inhomogeneities (bone, lung, and soft tissue) were embedded in water, while a target volume of 4 x 4 x 4 cm was defined at two different depths behind the heterogeneities. In total, 10 different test cases, with and without range shifter as well as different air gaps, were investigated. Dose distributions inside as well as behind the target volume were evaluated.

Results: Inside the target volume, the mean dose difference between calculations and measurements, averaged over all test cases, was 1.6% for carbon ions. This compares well to the final agreement of 1.5% obtained in water at the commissioning stage of the TPS for carbon ions and is also within the clinically acceptable interval of 3%. The mean dose difference and maximal dose difference obtained outside the target area were 1.8% and 13.4%, respectively. The agreement of dose distributions for carbon ions in the target volumes was comparable or better to that between Monte Carlo (MC) dose calculations and measurements for protons. Percentage dose differences of more than 10% were present outside the target area behind bone-lung structures, where the carbon ion calculations systematically over predicted the dose. MC dose calculations for protons were superior to carbon ion beams outside the target volumes.

Conclusion: The pencil beam dose calculations for carbon ions in RayStation were found to be in good agreement with dosimetric measurements in heterogeneous geometries for points of interest located within the target. Large local discrepancies behind the target may contribute to incorrect dose predictions for organs at risk.
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http://dx.doi.org/10.1002/mp.15209DOI Listing
November 2021

Reply to comment on 'Lateral response heterogeneity of Bragg peak ionization chambers for narrow-beam photon and proton dosimetry'.

Phys Med Biol 2021 Aug 2;66(16). Epub 2021 Aug 2.

MedAustron Ion Therapy Center, Wiener Neustadt, Austria.

Gomà (2020) commented on our paper '' (Kuess20179189-206) which describes a method to determine the response pattern of large-area ionization chambers using a collimated x-ray beam. Gomà performed a simple Monte Carlo simulation to investigate the energy transferred by secondary electrons within the detector, deducing that our conclusion, that the chamber has a non-uniform response, is not supported by our results. We appreciate the work performed by Gomà very much and believe that the transport of secondary electrons in the chamber is an important contribution to understand the non-uniformity response of large-area chambers in narrow beams. However, we disagree with the conclusions drawn by Gomà that the radial response is homogenous. His simulation actually demonstrates that the response is non-uniform in an x-ray beam.
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http://dx.doi.org/10.1088/1361-6560/ac16bfDOI Listing
August 2021

Correction of the measured current of a small-gap plane-parallel ionization chamber in proton beams in the presence of charge multiplication.

Z Med Phys 2021 May 13;31(2):192-202. Epub 2021 Mar 13.

National Physical Laboratory, Medical Radiation Science, Teddington, United Kingdom; MedAustron Ion Therapy Center, Medical Physics, Wiener Neustadt, Austria.

Purpose: The aims of this work are to study the response of a small-gap plane-parallel ionization chamber in the presence of charge multiplication and suggest an experimental method to determine the product of the recombination correction factor (k) and the charge multiplication correction factor (k) in order to investigate the latter.

Methods: Experimental data were acquired in scanned proton beams and in a Cobalt-60 beam. Measurements were carried out using an IBA PPC05 chambers of which the electrode gap is 0.6mm. The study is based on the determination of Jaffé plots by operating the chambers at different voltages. Experimental results are compared to theoretical equations describing initial and volume recombination as well as charge multiplication for continuous and pulsed beams.

Results: Results obtained in protons and Cobalt-60 with the same PPC05 chamber indicate that the charge multiplication effect is independent of the beam quality, while results obtained in different proton beams with two different PPC05 chambers show that the charge multiplication effect is chamber dependent.

Conclusions: The approach to be taken when using a small-gap plane-parallel ionization chamber with a high voltage (e.g. 300V or 500V) for reference dosimetry in scanned proton beams depends on which correction factors were applied to the chamber response during its calibration in terms of absorbed dose to water: In both cases, it is recommended to use the ionization chamber at the same operating voltage used during its N-calibration. Another solution consists of operating the PPC05 chamber at a lower voltage (e.g. 50V) with larger k and smaller k and determining the product of both factors with higher accuracy using a linear extrapolation method.
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http://dx.doi.org/10.1016/j.zemedi.2021.01.008DOI Listing
May 2021

Results of an independent dosimetry audit for scanned proton beam therapy facilities.

Z Med Phys 2021 May 9;31(2):145-153. Epub 2021 Mar 9.

MedAustron Ion Therapy Center, Medical Physics, Wiener Neustadt, Austria.

Purpose: An independent dosimetry audit based on end-to-end testing of the entire chain of radiation therapy delivery is highly recommended to ensure consistent treatments among proton therapy centers. This study presents an auditing methodology developed by the MedAustron Ion Beam Therapy Center (Austria) in collaboration with the National Physical Laboratory (UK) and audit results for five scanned proton beam therapy facilities in Europe.

Methods: The audit procedure used a homogeneous and an anthropomorphic head phantom. The phantoms were loaded either with an ionization chamber or with alanine pellets and radiochromic films. Homogeneously planned doses of 10Gy were delivered to a box-like target volume in the homogeneous phantom and to two clinical scenarios with increasing complexity in the head phantom.

Results: For all tests the mean of the local differences of the absolute dose to water determined with the alanine pellets compared to the predicted dose from the treatment planning system installed at the audited institution was determined. The mean value taken over all tests performed was -0.1±1.0%. The measurements carried out with the ionization chamber were consistent with the dose determined by the alanine pellets with a mean deviation of -0.5±0.6%.

Conclusion: The developed dosimetry audit method was successfully applied at five proton centers including various "turn-key" Cyclotron solutions by IBA, Varian and Mevion. This independent audit with extension to other tumour sites and use of the correspondent anthropomorphic phantoms may be proposed as part of a credentialing procedure for future clinical trials in proton beam therapy.
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http://dx.doi.org/10.1016/j.zemedi.2021.01.003DOI Listing
May 2021

Monte Carlo computation of 3D distributions of stopping power ratios in light ion beam therapy using GATE-RTion.

Med Phys 2021 May 27;48(5):2580-2591. Epub 2021 Mar 27.

MedAustron Ion Therapy Center, Marie Curie-Straße 5, Wiener Neustadt, A-2700, Austria.

Purpose: This paper presents a novel method for the calculation of three-dimensional (3D) Bragg-Gray water-to-detector stopping power ratio (s ) distributions for proton and carbon ion beams.

Methods: Contrary to previously published fluence-based calculations of the stopping power ratio, the s calculation method used in this work is based on the specific way GATE/Geant4 scores the energy deposition. It only requires the use of the so-called DoseActor, as available in GATE, for the calculation of the s at any point of a 3D dose distribution. The simulations are performed using GATE-RTion v1.0, a dedicated GATE release that was validated for the clinical use in light ion beam therapy.

Results: The Bragg-Gray water-to-air stopping power ratio (s ) was calculated for monoenergetic proton and carbon ion beams with the default stopping power data in GATE-RTion v1.0 and the new ICRU90 recommendation. The s differences between the use of the default and the ICRU90 configuration were 0.6% and 5.4% at the physical range (R - 80% dose level in the distal dose fall-off) for a 70 MeV proton beam and a 120 MeV/u carbon ion beam, respectively. For protons, the s results for lithium fluoride, silicon, gadolinium oxysulfide, and the active layer material of EBT2 (radiochromic film) were compared with the literature and a reasonable agreement was found. For a real patient treatment plan, the 3D distributions of s in proton beams were calculated.

Conclusions: Our method was validated by comparison with available literature data. Its equivalence with Bragg-Gray cavity theory was demonstrated mathematically. The capability of GATE-RTion v1.0 for the s calculation at any point of a 3D dose distribution for simple and complex proton and carbon ion plans was presented.
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http://dx.doi.org/10.1002/mp.14726DOI Listing
May 2021

MR-guided proton therapy: Impact of magnetic fields on the detector response.

Med Phys 2021 May 3;48(5):2572-2579. Epub 2021 Apr 3.

Division of Medical Physics, Department of Radiation Oncology, Medical University of Vienna, 1090, Vienna, Austria.

Purpose: To investigate the response of detectors for proton dosimetry in the presence of magnetic fields.

Material And Methods: Four ionization chambers (ICs), two thimble-type and two plane-parallel-type, and a diamond detector were investigated. All detectors were irradiated with homogeneous single-energy-layer fields, using 252.7 MeV proton beams. A Farmer IC was additionally irradiated in the same geometrical configuration, but with a lower nominal energy of 97.4 MeV. The beams were subjected to magnetic field strengths of 0, 0.25, 0.5, 0.75, and 1 T produced by a research dipole magnet placed at the room's isocenter. Detectors were positioned at 2 cm water equivalent depth, with their stem perpendicular to both the magnetic field lines and the proton beam's central axis, in the direction of the Lorentz force. Normality and two sample statistical Student's t tests were performed to assess the influence of the magnetic field on the detectors' responses.

Results: For all detectors, a small but significant magnetic field-dependent change of their response was found. Observed differences compared to the no magnetic field case ranged from +0.5% to -0.7%. The magnetic field dependence was found to be nonlinear and highest between 0.25 and 0.5 T for 252.7 MeV proton beams. A different variation of the Farmer chamber response with magnetic field strength was observed for irradiations using lower energy (97.4 MeV) protons. The largest magnetic field effects were observed for plane-parallel ionization chambers.

Conclusion: Small magnetic field-dependent changes in the detector response were identified, which should be corrected for dosimetric applications.
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http://dx.doi.org/10.1002/mp.14660DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8251909PMC
May 2021

Gradient corrections for reference dosimetry using Farmer-type ionization chambers in single-layer scanned proton fields.

Med Phys 2020 Dec 14;47(12):6531-6539. Epub 2020 Nov 14.

MedAustron Ion Therapy Center, Wiener Neustadt, Austria.

Purpose: The local depth dose gradient and the displacement correction factor for Farmer-type ionization chambers are quantified for reference dosimetry at shallow depth in single-layer scanned proton fields.

Method: Integrated radial profiles as a function of depth (IRPDs) measured at three proton therapy centers were smoothed by polynomial fits. The local relative depth dose gradient at measurement depths from 1 to 5 cm were derived from the derivatives of those fits. To calculate displacement correction factors, the best estimate of the effective point of measurement was derived from reviewing experimental and theoretical determinations reported in the literature. Displacement correction factors for the use of Farmer-type ionization chambers with their reference point (at the center of the cavity volume) positioned at the measurement depth were derived as a ratio of IRPD values at the measurement depth and at the effective point of measurement.

Results: Depth dose gradients are as low as 0.1-0.4% per mm at measurement depths from 1 to 5 cm in the highest clinical proton energies (with residual ranges higher than 15 cm) and increase to 1% per mm at a residual range of 4 cm and become larger than 3% per mm for residual ranges lower than 2 cm. The literature review shows that the effective point of measurement of Farmer-type ionization chambers is, similarly as for carbon ion beams, located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. If a maximum displacement correction of 2% is deemed acceptable to be included in calculated beam quality correction factors, Farmer-type ICs can be used at measurements depths from 1 to 5 cm for which the residual range is 4 cm or larger. If one wants to use the same beam quality correction factors as applicable to the conventional measurement point for scattered beams, located at the center of the SOBP, the relative standard uncertainty on the assumption that the displacement correction factor is unity can be kept below 0.5% for measurement depths of at least 2 cm and for residual ranges of 15 cm or higher.

Conclusion: The literature review confirmed that for proton beams the effective point of measurement of Farmer-type ionization chambers is located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. Based on the findings in this work, three options can be recommended for reference dosimetry of scanned proton beams using Farmer-type ionization chambers: (a) positioning the effective point of measurement at the measurement depth, (b) positioning the reference point at the measurement depth and applying a displacement correction factor, and (c) positioning the reference point at the measurement depth without applying a displacement correction factor. Based on limiting the acceptable uncertainty on the gradient correction factor to 0.5% and the maximum deviation of the displacement perturbation correction factor from unity to 2%, the first two options can be allowed for residual ranges of at least 4 cm while the third option only for residual ranges of at least 15 cm.
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http://dx.doi.org/10.1002/mp.14554DOI Listing
December 2020

Beam monitor calibration of a synchrotron-based scanned light-ion beam delivery system.

Z Med Phys 2021 May 31;31(2):154-165. Epub 2020 Jul 31.

MedAustron Ion Therapy Center, Marie Curie-Straße 5, 2700 Wiener Neustadt, Austria; National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom of Great Britain and Northern Ireland, United Kingdom.

Purpose: This paper presents the implementation and comparison of two independent methods of beam monitor calibration in terms of number of particles for scanned proton and carbon ion beams.

Methods: In the first method, called the single-layer method, dose-area-product to water (DAP) is derived from the absorbed dose to water determined using a Roos-type plane-parallel ionization chamber in single-energy scanned beams. This is considered the reference method for the beam monitor calibration in the clinically relevant proton and carbon energy ranges. In the second method, called the single-spot method, DAP of a single central spot is determined using a Bragg-peak (BP) type large-area plane-parallel ionization chamber. Emphasis is given to the detailed characterization of the ionization chambers used for the beam monitor calibration. For both methods a detailed uncertainty budget on the DAP determination is provided as well as on the derivation of the number of particles.

Results: Both calibration methods agreed on average within 1.1% for protons and within 2.6% for carbon ions. The uncertainty on DAP using single-layer beams is 2.1% for protons and 3.1% for carbon ions with major contributions from the available values of k and the average spot spacing in both lateral directions. The uncertainty using the single-spot method is 2.2% for protons and 3.2% for carbon ions with major contributions from the available values of k and the non-uniformity of the BP chamber response, which can lead to a correction of up-to 3.2%. For the number of particles, an additional dominant uncertainty component for the mean stopping power per incident proton (or the CEMA) needs to be added.

Conclusion: The agreement between both methods enhances confidence in the beam monitor calibration and the estimated uncertainty. The single-layer method can be used as a reference and the single-spot method is an alternative that, when more accumulated knowledge and data on the method becomes available, can be used as a redundant dose monitor calibration method. This work, together with the overview of information from the literature provided here, is a first step towards comprehensive information on the single-spot method.
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http://dx.doi.org/10.1016/j.zemedi.2020.06.005DOI Listing
May 2021

The practical radius of a pencil beam in proton therapy.

Z Med Phys 2021 May 7;31(2):166-174. Epub 2020 Jul 7.

Division Medical Radiation Physics, Department of Radiation Oncology, Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna/AKH Wien, Währinger Gürtel 18-20, 1090 Vienna, Austria.

The central Gaussian shaped high dose region of a pencil beam (PB) in light ion beam therapy (LIBT) is enveloped by a low dose region causing non-negligible field size effects and impairs the dose calculation accuracy considerably if the low dose envelope is not well modeled. The purpose of this study was to calculate the practical radius, R, at which a PB does not influence a field more than a certain accuracy level. Lateral dose profiles of proton beams in water were simulated using GATE/Geant4. Those lateral dose profiles were integrated numerically and used to calculate field size factors (FSFs). The R was then determined such, that the lateral dose at radii exceeding R can be neglected without compromising the FSF of a 20cm×20cm field more than a desired accuracy level c. The practical radius R yielding c=0.5% was compared to the frequently applied concept of full width at a ratio x of the maximum (FWxM). The sensitivity to variations of the beam width was tested by increasing the initial beam width σ of the clinical beam model by 0.5 and 1mm, respectively. Neglecting the dose at radii exceeding R resulted in the desired FSF accuracy, whereas using the FW0.01%M cut resulted in varying accuracy. In order to yield a constant FSF accuracy, the ratio x in FWxM ranged from 0.003% to 0.065% of the maximum. In contrast to R, FWxM was sensitive to variations of the initial beam width. The maximum R over all depths was less than 7cm for the low(62.4MeV) and medium(148.2MeV) proton energy beam, which suggests that a plane parallel ionization chamber exceeding that radius is sufficient to acquire laterally integrated depth dose distributions for those energies. However, this holds not true for the highest energy (252.7MeV) or when including a range shifter (RaShi). The values of R are specific to our beam line configuration as the maximum R was depending on both, the scattering material in the Nozzle as well as the distance of the air-gap between Nozzle and phantom.
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http://dx.doi.org/10.1016/j.zemedi.2020.06.003DOI Listing
May 2021

Characterization of the PTW-34089 type 147 mm diameter large-area ionization chamber for use in light-ion beams.

Phys Med Biol 2020 09 4;65(17):17NT02. Epub 2020 Sep 4.

Division of Medical Radiation Physics, Department of Radiation Oncology, Medical University of Vienna, Austria.

A newly-designed large-area plane-parallel ionization chamber (of type PTW 34089), denoted BPC150, with a nominal active volume diameter of 147 mm is characterized in this study. Such chambers exhibit benefits compared to smaller chambers in the field of scanned light-ion beam dosimetry because they capture a larger fraction of the laterally-spread beam fragments and ease positioning with respect to small fields. The chamber was characterized in Co, 200 kV x-ray, proton and carbon ion beams. The chamber-specific beam-quality correction factor k was determined. To investigate the homogeneity of the chamber's response, a radial response map was acquired. An edge correction was applied when the proton beam only partly impinged on the chamber's active surface. The measured response map showed that the response in the chamber's center is 3% lower than the average response over the total active area. Furthermore, percentage depth dose (PDD) curves in carbon ions were acquired and compared to those obtained with smaller-diameter chambers (i.e. 81.6 mm and 39.6 mm) as well as with results from Monte Carlo simulations. The measured absorbed dose to water cross calibration coefficients resulted in a k of 0.981 ± 0.020. Regarding carbon ion PDD curves, relative differences between the BPC150 and smaller chambers were observed, especially for higher energies and in the fragmentation tail. These differences reached 10%-22% in the fragmentation tail (compared to the 81.6 mm diameter chamber). Differences increased when comparing to a chamber with 39.6 mm diameter. The provided results characterize the BPC150 thoroughly for usage in scanned light-ion beam dosimetry and demonstrate its advantage of capturing a larger fraction of the laterally-integrated dose in the fragmentation tail.
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http://dx.doi.org/10.1088/1361-6560/ab9852DOI Listing
September 2020

Dose- rather than fluence-averaged LET should be used as a single-parameter descriptor of proton beam quality for radiochromic film dosimetry.

Med Phys 2020 Jun 13;47(5):2289-2299. Epub 2020 Mar 13.

MedAustron Ion Therapy Centre/EBG MedAustron, Marie-Curie-Straße 5, 2700, Wiener Neustadt, Austria.

Purpose: The dose response of Gafchromic EBT3 films exposed to proton beams depends on the dose, and additionally on the beam quality, which is often quantified with the linear energy transfer (LET) and, hence, also referred to as LET quenching. Fundamentally different methods to determine correction factors for this LET quenching effect have been reported in literature and a new method using the local proton fluence distribution differential in LET is presented. This method was exploited to investigate whether a more practical correction based on the dose- or fluence-averaged LET is feasible in a variety of clinically possible beam arrangements.

Methods: The relative effectiveness (RE) was characterized within a high LET spread-out Bragg peak (SOBP) in water made up by the six lowest available energies (62.4-67.5 MeV, configuration " ") resulting in one of the highest clinically feasible dose-averaged LET distributions. Additionally, two beams were measured where a low LET proton beam (252.7 MeV) was superimposed on " ", which contributed either 50% of the initial particle fluence or 50% of the dose in the SOBP, referred to as configuration " " and " ," respectively. The proton LET spectrum was simulated with GATE/Geant4 at all measurement positions. The net optical density change differential in LET was integrated over the local proton spectrum to calculate the net optical density and therefrom the beam quality correction factor. The LET dependence of the film response was accounted for by an LET dependence of one of the three parameters in the calibration function and was determined from inverse optimization using measurement " ." This method was then validated on the measurements of " " and " " and subsequently used to calculate the RE at 900 positions in nine clinically relevant beams. The extrapolated RE set was used to derive a simple linear correction function based on dose-averaged LET ( ) and verify the validity in all points of the comprehensive RE set.

Results: The uncorrected film dose deviated up to 26% from the reference dose, whereas the corrected film dose agreed within 3% in all three beams in water (" ", " " and " "). The LET dependence of the calibration function started to strongly increase around 5 keV/μm and flatten out around 30 keV/μm. All REs calculated from the proton fluence in the nine simulated beams could be approximated with a linear function of dose-averaged LET (RE = 1.0258-0.0211 μm/keV ). However, no functional relationship of RE- and fluence-averaged LET could be found encompassing all beam energies and modulations.

Conclusions: The film quenching was found to be nonlinear as a function of proton LET as well as of the dose-averaged LET. However, the linear relation of RE on dose-averaged LET was a good approximation in all cases. In contrast to dose-averaged LET, fluence-averaged LET could not describe the RE when multiple beams were applied.
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http://dx.doi.org/10.1002/mp.14097DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7318138PMC
June 2020

A GATE/Geant4 beam model for the MedAustron non-isocentric proton treatment plans quality assurance.

Phys Med 2020 Mar 29;71:115-123. Epub 2020 Feb 29.

EBG MedAustron GmbH, Marie-Curie Straße 5, 2700 Wiener Neustadt, Austria.

Purpose: To present a reference Monte Carlo (MC) beam model developed in GATE/Geant4 for the MedAustron fixed beam line. The proposed model includes an absolute dose calibration in Dose-Area-Product (DAP) and it has been validated within clinical tolerances for non-isocentric treatments as routinely performed at MedAustron.

Material And Methods: The proton beam model was parametrized at the nozzle entrance considering optic and energy properties of the pencil beam. The calibration in terms of absorbed dose to water was performed exploiting the relationship between number of particles and DAP by mean of a recent formalism. Typical longitudinal dose distribution parameters (range, distal penumbra and modulation) and transverse dose distribution parameters (spot sizes, field sizes and lateral penumbra) were evaluated. The model was validated in water, considering regular-shaped dose distribution as well as clinical plans delivered in non-isocentric conditions.

Results: Simulated parameters agree with measurements within the clinical requirements at different air gaps. The agreement of distal and longitudinal dose distribution parameters is mostly better than 1 mm. The dose difference in reference conditions and for 3D dose delivery in water is within 0.5% and 1.2%, respectively. Clinical plans were reproduced within 3%.

Conclusion: A full nozzle beam model for active scanning proton pencil beam is described using GATE/Geant4. Absolute dose calibration based on DAP formalism was implemented. The beam model is fully validated in water over a wide range of clinical scenarios and will be inserted as a reference tool for research and for independent dose calculation in the clinical routine.
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http://dx.doi.org/10.1016/j.ejmp.2020.02.006DOI Listing
March 2020

The influence of lack of reference conditions on dosimetry in pre-clinical radiotherapy with medium energy x-ray beams.

Phys Med Biol 2020 04 23;65(8):085016. Epub 2020 Apr 23.

National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, United Kingdom. Author to whom any correspondence should be addressed.

Despite well-established dosimetry in clinical radiotherapy, dose measurements in pre-clinical and radiobiology studies are frequently inadequate, thus undermining the reliability and reproducibility of published findings. The lack of suitable dosimetry protocols, coupled with the increasing complexity of pre-clinical irradiation platforms, undermines confidence in preclinical studies and represents a serious obstacle in the translation to clinical practice. To accurately measure output of a pre-clinical radiotherapy unit, appropriate Codes of Practice (CoP) for medium energy x-rays needs to be employed. However, determination of absorbed dose to water (D) relies on application of backscatter factor (B) employing in-air method or carrying out in-phantom measurement at the reference depth of 2 cm in a full backscatter (i.e. 30 × 30 × 30 cm) condition. Both of these methods require thickness of at least 30 cm of underlying material, which are never fulfilled in typical pre-clinical irradiations. This work is focused on evaluation the effects of the lack of recommended reference conditions in dosimetry measurements for pre-clinical settings and is aimed at extending the recommendations of the current CoP to practical experimental conditions and highlighting the potential impact of the lack of correct backscatter considerations on radiobiological studies.
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http://dx.doi.org/10.1088/1361-6560/ab7b30DOI Listing
April 2020

An analytical formalism for the assessment of dose uncertainties due to positioning uncertainties.

Med Phys 2020 Mar 26;47(3):1357-1363. Epub 2020 Jan 26.

EBG MedAustron GmbH, Marie-Curie Straße 5, 2700, Wiener Neustadt, Austria.

Purpose: To present an analytical formalism for the in depth assessment of uncertainties of field output factors in small fields related to detector positioning based on dose profile measurements. Additionally, a procedure for the propagation of these uncertainties was developed.

Methods: Based on the assumption that one dimensional and two dimensional second-order polynomial functions can be fitted to dose profiles of small photon beams, equations for the calculation of the expectation value, the variance, and the standard deviation were developed. The following fitting procedures of the dose profiles were considered: A one-dimensional case (1D), a quasi two-dimensional case (2Dq) based on independently measured line profiles and a full 2D case (2Df) which also considers cross-correlations in a two-dimensional dose distribution. A rectangular and a Gaussian probability density function (PDF) characterizing the probability of possible positions of the detector relative to the maximum dose were used. Uncertainty components such as the finite resolution of the scanning water phantom, the reproducibility of the determination of the position of the maximum dose, and the reproducibility of the collimator system were investigated. This formalism was tested in a 0.5 x 0.5 cm photon field where dose profiles were measured using a radiochromic film, a synthetic diamond detector, and an unshielded diode detector. Additionally, the dose distribution measured with the radiochromic film was convoluted with a convolution kernel mimicking the active volume of the unshielded diode.

Results: Analytic expressions for the calculation of uncertainties on field output factors were found for the 1D, the 2Dq, and the 2Df case. The uncertainty of the field output factor related to the relative position of the detector to the maximum dose increased quadratically with increasing limits of possible detector positions. Analysis of the radiochromic film showed that the 2Dq case gave a more conservative assessment of the uncertainty compared to the 2Df case with a difference of < 0.1%. The 2Dq case applied to the film measurements agreed well with the same approach as was applied to the unshielded diode. The investigated uncertainty components propagated to an uncertainty of the field output factors of 0.5% and 0.4% for the synthetic diamond and the unshielded diode, respectively. Additionally, the expectation value was lower than the maximum dose. The difference was 0.4% and 0.3% for the synthetic diamond and the unshielded diode, respectively.

Conclusions: The assessment of uncertainties of field output factors related to detector positioning is feasible using the proposed formalism. The 2Dq case is applicable when using online detectors. Accurate positioning in small fields is essential for accurate dosimetry as its related uncertainty increases quadratically. The observed drop of the expectation value needs to be considered in small field dosimetry.
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http://dx.doi.org/10.1002/mp.13991DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7078844PMC
March 2020

Clinical implementation and commissioning of the MedAustron Particle Therapy Accelerator for non-isocentric scanned proton beam treatments.

Med Phys 2020 Feb 29;47(2):380-392. Epub 2019 Dec 29.

EBG MedAustron GmbH, Marie Curie-Straße 5, A-2700, Wiener Neustadt, Austria.

Purpose: This paper describes the clinical implementation and medical commissioning of the MedAustron Particle Therapy Accelerator (MAPTA) for non-isocentric scanned proton beam treatments.

Methods: Medical physics involvement during technical commissioning work is presented. Acceptance testing procedures, including advanced measurement methods of intra-spill beam variations, are defined. Beam monitor calibration using two independent methods based on a dose-area product formalism is described. Emphasis is given to the medical commissioning work and the specificities related to non-isocentric irradiation, since a key feature of MedAustron is the routine delivery of non-isocentric scanned proton beam treatments.

Results: Key commissioning results and beam stability trend lines for more than 2 yr of clinical operation have been provided. Intra-spill beam range, size, and position variations were within specifications of 0.3 mm, 15%, and 0.5 mm, respectively. The agreement between two independent beam monitor calibration methods was better than 1.0%. Non-isocentric treatment delivery allowed lateral penumbra reduction of up to about 30%. Daily QA measurements of the beam range, size, position, and dose were always within 1 mm, 10%, 1 mm, and 2% from the baseline data, respectively.

Conclusions: Non-isocentric treatments have been successfully implemented at MedAustron for routine scanned proton beam therapy using horizontal and vertical fixed beamlines. Up to now every patient was treated in non-isocentric conditions. The presented methodology to implement a new Scanned Ion Beam Delivery (SIBD) system into clinical routine for proton therapy may serve as a guidance for other centers.
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http://dx.doi.org/10.1002/mp.13928DOI Listing
February 2020

Phantom design and dosimetric characterization for multiple simultaneous cell irradiations with active pencil beam scanning.

Radiat Environ Biophys 2019 11 20;58(4):563-573. Epub 2019 Sep 20.

Department of Radiation Oncology, Medical University of Vienna, Vienna, Austria.

A new phantom was designed for in vitro studies on cell lines in horizontal particle beams. The phantom enables simultaneous irradiation at multiple positions along the beam path. The main purpose of this study was the detailed dosimetric characterization of the phantom which consists of various heterogeneous structures. The dosimetric measurements described here were performed under non-reference conditions. The experiment involved a CT scan of the phantom, dose calculations performed with the treatment planning system (TPS) RayStation employing both the Pencil Beam (PB) and Monte Carlo (MC) algorithms, and proton beam delivery. Two treatment plans reflecting the typical target location for head and neck cancer and prostate cancer treatment were created. Absorbed dose to water and dose homogeneity were experimentally assessed within the phantom along the Bragg curve with ionization chambers (ICs) and EBT3 films. LET distributions were obtained from the TPS. Measured depth dose distributions were in good agreement with the Monte Carlo-based TPS data. Absorbed dose calculated with the PB algorithm was 4% higher than the absorbed dose measured with ICs at the deepest measurement point along the spread-out Bragg peak. Results of experiments using melanoma (SKMel) cell line are also presented. The study suggested a pronounced correlation between the relative biological effectiveness (RBE) and LET, where higher LET leads to elevated cell death and cell inactivation. Obtained RBE values ranged from 1.4 to 1.8 at the survival level of 10% (RBE). It is concluded that dosimetric characterization of a phantom before its use for RBE experiments is essential, since a high dosimetric accuracy contributes to reliable RBE data and allows for a clearer differentiation between physical and biological uncertainties.
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http://dx.doi.org/10.1007/s00411-019-00813-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6768893PMC
November 2019

Reply to Comment on 'Lateral response heterogeneity of Bragg peak ionization chambers for narrow-beam photon and proton dosimetry'.

Phys Med Biol 2019 09 26;64(19):198002. Epub 2019 Sep 26.

Division Medical Physics, Department of Radiation Oncology, Medical University Vienna, Vienna, Austria. Author to whom any correspondence should be addressed.

Shen (2019) commented on our paper 'Lateral response heterogeneity of Bragg peak ionization chambers for narrow-beam photon and proton dosimetry' regarding the impact of the low dose tail of the collimated x-ray beam we used to acquire individual response maps of large area ionization chambers. The behavior of this low dose tail was measured and compared to the simulations performed by Shen (2019). It was shown that the model of the tail by Shen (2019) is too simplistic and overestimates its effect.
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http://dx.doi.org/10.1088/1361-6560/ab3ba0DOI Listing
September 2019

Characterization of EBT3 radiochromic films for dosimetry of proton beams in the presence of magnetic fields.

Med Phys 2019 Jul 31;46(7):3278-3284. Epub 2019 May 31.

Department of Radiotherapy, Medical University of Vienna/AKH, Vienna, Austria.

Purpose: Radiochromic film dosimetry is extensively used for quality assurance in photon and proton beam therapy. So far, GafchromicTM EBT3 film appears as a strong candidate to be used in future magnetic resonance (MR) based therapy systems. The response of Gafchromic EBT3 films in the presence of magnetic fields has already been addressed for different MR-linacs systems. However, a detailed evaluation of the influence of external magnetic fields on the film response and calibration curves for proton therapy has not yet been reported. This study aims to determine the dose responses of EBT3 films for clinical proton beams exposed to magnetic field strengths up to 1 T in order to investigate the feasibility of EBT3 film as an accurate dosimetric tool for a future MR particle therapy system (MRPT).

Methods: The dosimetric characteristics of EBT3 films were studied for a proton beam passing through magnetic field strengths of B = 0, 0.5, and 1 T. Absorbed dose calibration and measurements were performed using clinical proton beams in the nominal energy range of 62.4-252.6 MeV. Irradiations were done using an in-house developed PMMA slab phantom placed in the center of a dipole research magnet. Monte Carlo (MC) simulations using the GATE/Geant4 toolkit were performed to predict the effect of magnetic fields on the energy deposited by proton beams in the phantom. Planned and measured doses from 3D box cube irradiations were compared to assess the accuracy of the dosimetric method using EBT3 films with/without the external magnetic field.

Results: Neither for the mean pixel value nor for the net optical density, any significant deviations were observed due to the presence of an external magnetic field (B ≤ 1T) for doses up to 10 Gy. Dose-response curves for the red channel were fitted by a three-parameter function for the field-free case and for B = 1T, showing for both cases an R-square coefficient of unity and almost identical fitting parameters. Independently of the magnetic field, EBT3 films showed an under-response as high as 8% in the Bragg peak region, similarly to previously reported effects for particle therapy. No noticeable influence of the magnetic field strength was observed on the quenching effect of the EBT3 films.

Conclusions: For the first time detailed absorbed dose calibrations of EBT3 films for proton beams in magnetic field regions were performed. Results showed that EBT3 films represent an attractive solution for the dosimetry of a future MRPT system. As film response functions for protons are not affected by the magnetic field strenght, they can be used for further investigations to evaluate the dosimetric effects induced due to particle beams bending in magnetic fields regions.
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http://dx.doi.org/10.1002/mp.13567DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6852248PMC
July 2019

Evaluation of electromagnetic and nuclear scattering models in GATE/Geant4 for proton therapy.

Med Phys 2019 May 15;46(5):2444-2456. Epub 2019 Apr 15.

MedAustron Ion Therapy Centre/EBG MedAustron, Marie-Curie-Straße 5, 2700, Wiener Neustadt, Austria.

Purpose: The dose core of a proton pencil beam (PB) is enveloped by a low dose area reaching several centimeters off the central axis and containing a considerable amount of the dose. Adequate modeling of the different components of the PB profile is, therefore, required for accurate dose calculation. In this study, we experimentally validated one electromagnetic and two nuclear scattering models in GATE/Geant4 for dose calculation of proton beams in the therapeutic energy window (62-252 MeV) with and without range shifter (RaShi).

Methods: The multiple Coulomb scattering (MCS) model was validated by lateral dose core profiles measured for five energies at up to four depths from beam plateau to Bragg peak region. Nuclear halo profiles of single PBs were evaluated for three (62.4, 148.2, and 252.7 MeV) and two (97.4 and 124.7 MeV) energies, without and with RaShi, respectively. The influence of the dose core and nuclear halo on field sizes varying from 2-20 cm was evaluated by means of output factors (OFs), namely frame factors (FFs) and field size factors (FSFs), to quantify the relative increase of dose when increasing the field size.

Results: The relative increase in the dose core width in the simulations deviated negligibly from measurements for depths until 80% of the beam range, but was overestimated by up to 0.2 mm in σ toward the end of range for all energies. The dose halo region of the lateral dose profile agreed well with measurements in the open beam configuration, but was notably overestimated in the deepest measurement plane of the highest energy or when the beam passed through the RaShi. The root-mean-square deviations (RMSDs) between the simulated and the measured FSFs were less than 1% at all depths, but were higher in the second half of the beam range as compared to the first half or when traversing the RaShi. The deviations in one of the two tested hadron physics lists originated mostly in elastic scattering. The RMSDs could be reduced by approximately a factor of two by exchanging the default elastic scattering cross sections for protons.

Conclusions: GATE/Geant4 agreed satisfyingly with most measured quantities. MCS was systematically overestimated toward the end of the beam range. Contributions from nuclear scattering were overestimated when the beam traversed the RaShi or at the depths close to the end of the beam range without RaShi. Both, field size effects and calculation uncertainties, increased when the beam traversed the RaShi. Measured field size effects were almost negligible for beams up to medium energy and were highest for the highest energy beam without RaShi, but vice versa when traversing the RaShi.
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http://dx.doi.org/10.1002/mp.13472DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6850424PMC
May 2019

Reply to "Comments on the TRS-483 Protocol on Small field Dosimetry" [Med. Phys. 45(12), 5666-5668 (2018)].

Med Phys 2018 12;45(12):5669-5671

Dosimetry and Medical Radiation Physics Section, International Atomic Energy Agency, A-1400, Vienna, Austria.

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http://dx.doi.org/10.1002/mp.13235DOI Listing
December 2018

Dose detectors, sensors, and their applications.

Med Phys 2018 Nov;45(11):e1051-e1072

National Physical Laboratory, Medical Radiation Science, Hampton Road, Teddington, Middlesex, TW11 0LW, UK.

This article is intended to present the different types of particle and radiation detectors available for applications in particle therapy. Several types of detectors and sensors exist for measurements of absorbed dose in reference and nonreference conditions and the ones in use for beam monitoring. Therefore, this manuscript focuses on the following applications: (a) primary methods for dosimetry in ion beams, (b) measurements of absorbed dose in realistic patient-specific scenarios with different dose delivery configurations, and (c) beam monitoring. Water and graphite calorimeters, Faraday cups, ionization based gas detectors are described first, followed by the description of the protocols for proton and ion dosimetry. Then films, scintillator and solid-state detectors are reviewed. Finally, several types of ionization chambers (large, pinpoint, arrays of chambers arranged in regular 1D or 2D pattern, parallel-plate configuration with large integral electrodes or with anode segmented in strips or pixels, multi-wire, and the multi-gap prototype) have been considered. New beam monitors to deal with a wide range of intensity and pulsed beams expected from new accelerators, different dose fractionation and advanced delivery techniques are presented. The existing detectors available for particle therapy have been described taking into account the different requirements for devices used in reference and nonreference conditions and the ones designed for beam monitoring.
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http://dx.doi.org/10.1002/mp.13089DOI Listing
November 2018

The influence of nuclear interactions on ionization chamber perturbation factors in proton beams: FLUKA simulations supported by a Fano test.

Med Phys 2019 Feb 7;46(2):885-891. Epub 2018 Dec 7.

Medical Radiation Science, National Physical Laboratory, Teddington, TW11 0LW, UK.

Purpose: In all recent protocols for the reference dosimetry of clinical proton beams, ionization chamber perturbation factors are assumed to be unity. In this work, such factors were computed using the FLUKA Monte Carlo code for three ionization chamber types, with particular attention to the influence of nuclear interactions.

Methods: The accuracy of the transport algorithms implemented in FLUKA was first evaluated by performing a Fano cavity test. Ionization chamber perturbation factors were computed for the PTW-34001 Roos and the PTW-34070 and PTW-34073 Bragg peak chambers for proton beams of 60-250 MeV using the same transport parameters that were needed to pass the Fano test.

Results: FLUKA was found to pass the Fano test within 0.15%. Ionization chamber simulation results show that the presence of the air cavity and the wall results in dose perturbations of the order of 0.6% and 0.8%, respectively. The perturbation factors are shown to be energy dependent and nuclear interactions must be taken into account for accurate calculation of the ionization chamber's response.

Conclusion: Ionization chamber perturbations can amount to 1% in high-energy proton beams and therefore need to be considered in dosimetry procedures.
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http://dx.doi.org/10.1002/mp.13281DOI Listing
February 2019

Dosimetry of small static fields used in external photon beam radiotherapy: Summary of TRS-483, the IAEA-AAPM international Code of Practice for reference and relative dose determination.

Med Phys 2018 Nov 17;45(11):e1123-e1145. Epub 2018 Oct 17.

Dosimetry and Medical Radiation Physics Section, International Atomic Energy Agency, A-1400, Vienna, Austria.

Purpose: A joint IAEA/AAPM international working group has developed a Code of Practice (CoP) for the dosimetry of small static fields used in external megavoltage photon beam radiotherapy, published by the IAEA as TRS-483. This summary paper introduces and outlines the main aspects of the CoP.

Methods: IAEA TRS-483 is a condensation of the wide range of different approaches that have been described in the literature for the reference dosimetry of radiotherapy machines with nominal accelerating potential up to 10 MV that cannot establish the conventional 10 cm × 10 cm reference field, and for the determination of field output factors for relative dosimetry in small static photon fields. The formalism used is based on that developed by Alfonso et al. [Med Phys. 2008;35:5179-5186] for this modality.

Results: Three introductory sections describe the rationale and context of the CoP, the clinical use of small photon fields, and the physics of small-field dosimetry. In the fourth section, definitions of terms that are specific to the CoP (as compared to previous CoPs for broad-beam reference dosimetry, such as IAEA TRS-398 and AAPM TG-51) are given; this section includes a list of the symbols and equivalences between IAEA and AAPM nomenclature to facilitate the practical implementation of the CoP by end users of IAEA TRS-398 and AAPM TG-51. The fifth section summarizes the equations and procedures that are recommended in the CoP and the sixth section provides an overview of the methods used to derive the data provided in IAEA TRS-483.

Conclusions: This is the first time an international Code of Practice for the dosimetry of small photon fields based on comprehensive data and correction factors has been published. This joint IAEA/AAPM CoP will ensure consistent reference dosimetry traceable to the international System of Units and enable common and internationally harmonized procedures to be followed by radiotherapy centers worldwide for the dosimetry of small static megavoltage photon fields.
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http://dx.doi.org/10.1002/mp.13208DOI Listing
November 2018

Characteristic of EBT-XD and EBT3 radiochromic film dosimetry for photon and proton beams.

Phys Med Biol 2018 03 15;63(6):065007. Epub 2018 Mar 15.

Division of Medical Radiation Physics, Department of Radiotherapy, Medical University of Vienna, Vienna, Austria. Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria. Division of Radiation Therapy, Faculty of Medicine Ramathibodi Hospital, Department of Diagnostic and Therapeutic Radiology, Mahidol University, Bangkok, Thailand.

Recently, a new type of radiochromic film, the EBT-XD film, has been introduced for high dose radiotherapy. The EBT-XD film contains the same structure as the EBT3 film but has a slightly different composition and a thinner active layer. This study benchmarks the EBT-XD against EBT3 film for 6 MV and 10 MV photon beams, as well as for 97.4 MeV and 148.2 MeV proton beams and 15-100 kV x-rays. Dosimetric and film reading characteristics, such as post irradiation darkening, film orientation effect, lateral response artifact (LRA), film sensitivity, energy and beam quality dependency were investigated. Furthermore, quenching effects in the Bragg peak were investigated for a single proton beam energy for both film types, in addition measurements were performed in a spread-out Bragg peak. EBT-XD films showed the same characteristic on film darkening as EBT3. The effects between portrait and landscape orientation were reduced by 3.1% (in pixel value) for EBT-XD compared to EBT3 at a dose of 2000 cGy. The LRA is reduced for EBT-XD films for all investigated dose ranges. The sensitivity of EBT-XD films is superior to EBT3 for doses higher than 500 cGy. In addition, EBT-XD showed a similar dosimetric response for photon and proton irradiation with low energy and beam quality dependency. A quenching effect of 10% was found for both film types. The slight decrease in the thickness of the active layer and different composition configuration of EBT-XD resulted in a reduced film orientation effect and LRA, as well as a sensitivity increase in high-dose regions for both photon and proton beams. Overall, the EBT-XD film improved regarding film reading characteristics and showed advantages in the high-dose region for photon and proton beams.
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http://dx.doi.org/10.1088/1361-6560/aab1eeDOI Listing
March 2018

On the conversion of dose to bone to dose to water in radiotherapy treatment planning systems.

Phys Imaging Radiat Oncol 2018 Jan 9;5:26-30. Epub 2018 Feb 9.

National Physics Laboratory, Acoustics and Ionising Radiation Division, Teddington, TW 11 0LW, UK.

Background And Purpose: Conversion factors between dose to medium (D) and dose to water (D) provided by treatment planning systems that model the patient as water with variable electron density are currently based on stopping power ratios. In the current paper it will be illustrated that this conversion method is not correct.

Materials And Methods: Monte Carlo calculations were performed in a phantom consisting of a 2 cm bone layer surrounded by water. D was obtained by modelling the bone layer as water with the electron density of bone. Conversion factors between D and D were obtained and compared to stopping power ratios and ratios of mass-energy absorption coefficients in regions of electronic equilibrium and interfaces. Calculations were performed for 6 MV and 20 MV photon beams.

Results: In the region of electronic equilibrium the stopping power ratio of water to bone (1.11) largely overestimates the conversion obtained using the Monte Carlo calculations (1.06). In that region the MC dose conversion corresponds to the ratio of mass energy absorption coefficients. Near the water to bone interface, the MC ratio cannot be determined from stopping powers or mass energy absorption coefficients.

Conclusion: Stopping power ratios cannot be used for conversion from D to D provided by treatment planning systems that model the patient as water with variable electron density, either in regions of electronic equilibrium or near interfaces. In regions of electronic equilibrium mass energy absorption coefficient ratios should be used. Conversions at interfaces require detailed MC calculations.
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http://dx.doi.org/10.1016/j.phro.2018.01.004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7807555PMC
January 2018

Implementation of dosimetry equipment and phantoms at the MedAustron light ion beam therapy facility.

Med Phys 2018 Jan 23;45(1):352-369. Epub 2017 Nov 23.

EBG MedAustron GmbH, Marie Curie-Straße 5, A-2700, Wiener Neustadt, Austria.

Purpose: To describe the implementation of dosimetry equipment and phantoms into clinical practice of light ion beam therapy facilities. This work covers not only standard dosimetry equipment such as computerized water scanners, films, 2D-array, thimble, and plane parallel ionization chambers, but also dosimetry equipment specifically devoted to the pencil beam scanning delivery technique such as water columns, scintillating screens or multilayer ionization chambers.

Method: Advanced acceptance testing procedures developed at MedAustron and complementary to the standard acceptance procedures proposed by the manufacturer are presented. Detailed commissioning plans have been implemented for each piece of dosimetry equipment and include an estimate of the overall uncertainty budget for the range of clinical use of each device. Some standard dosimetry equipment used in many facilities was evaluated in detail: for instance, the recombination of a 2D-array or the potential use of a microdiamond detector to measure reference transverse dose profiles in water in the core of the primary pencil beams and in the low-dose nuclear halo (over four orders of magnitude in dose).

Results: The implementation of dosimetry equipment as described in this work allowed determining absolute spot sizes and spot positions with an uncertainty better than 0.3 mm. Absolute ranges are determined with an uncertainty comprised of 0.2-0.6 mm, depending on the measured range and were reproduced with a maximum difference of 0.3 mm over a period of 12 months using three different devices.

Conclusion: The detailed evaluation procedures of dosimetry equipment and phantoms proposed in this work could serve as a guidance for other medical physicists in ion beam therapy facilities and also in conventional radiation therapy.
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http://dx.doi.org/10.1002/mp.12653DOI Listing
January 2018

Lateral response heterogeneity of Bragg peak ionization chambers for narrow-beam photon and proton dosimetry.

Phys Med Biol 2017 Nov 21;62(24):9189-9206. Epub 2017 Nov 21.

Department of Radiotherapy, Division Medical Physics, Medical University of Vienna, Waehringer Guertel 18-20, 1090 Vienna, Austria. Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Waehringer Guertel 18-20, 1090 Vienna, Austria.

Large area ionization chambers (LAICs) can be used to measure output factors of narrow beams. Dose area product measurements are proposed as an alternative to central-axis point dose measurements. Using such detectors requires detailed information on the uniformity of the response along the sensitive area. Eight LAICs were investigated in this study: four of type PTW-34070 (LAIC) and four of type PTW-34080 (LAIC). Measurements were performed in an x-ray unit using peak voltages of 100-200 kVp and a collimated beam of 3.1 mm (FWHM). The LAICs were moved with a step size of 5 mm to measure the chamber response at lateral positions. To account for beam positions where only a fraction of the beam impinged within the sensitive area of the LAICs, a corrected response was calculated which was the basis to calculate the relative response. The impact of a heterogeneous LAIC response, based on the obtained response maps was henceforth investigated for proton pencil beams and small field photon beams. A pronounced heterogeneity of the responses was observed in the investigated LAICs. The response of LAIC generally decreased with increasing radius, resulting in a response correction of up to 5%. This correction was more pronounced and more diverse (up to 10%) for LAIC. Considering a proton pencil beam the systematic offset for reference dosimetry was 2.4-4.1% for LAIC and  -9.5 to 9.4% for LAIC. For relative dosimetry (e.g. integral depth-dose curves) systematic response variation by 0.8-1.9% were found. For a decreasing photon field size the systematic offset for absolute dose measurements showed a 2.5-4.5% overestimation of the response for 6  ×  6 mm field sizes for LAIC. For LAIC the response varied even over a range of 20%. This study highlights the need for chamber-dependent response maps when using LAICs for absolute and relative dosimetry with proton pencil beams or small photon beams.
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http://dx.doi.org/10.1088/1361-6560/aa955eDOI Listing
November 2017

Equivalent (uniform) square field sizes of flattening filter free photon beams.

Phys Med Biol 2017 Sep 15;62(19):7694-7713. Epub 2017 Sep 15.

Department of Radiation Oncology, Division Medical Physics, Medical University of Vienna, Spitalgasse 23, 1090 Vienna, Austria. Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Vienna, Austria.

Various types of treatment units, such as CyberKnife, TomoTherapy and C-arm linear accelerators (LINACs) are operated using flattening filter free (FFF) photon beams. Their reference dosimetry, however, is currently based on codes of practice that provide data which were primarily developed and tested for high-energy photon beams with flattening filter (WFF). The aim of this work was to introduce equivalent uniform square field sizes of FFF beams to serve as a basis of a unified reference dosimetry procedure applicable to all aforementioned FFF machines. For this purpose, in-house determined experimental data together with published data of the ratio of doses at depths of 20 cm and 10 cm in water (D ) were used to characterize the depth dose distribution of 6 and 10 MV WFF and FFF beams. These data were analyzed for field sizes ranging from 2  ×  2 cm to 40  ×  40 cm. A scatter function that takes the lateral profiles of the individual beams into account was fitted to the experimental data. The lateral profiles of the WFF beams were assumed to be uniform, while those of the FFF beams were approximated using fourth or sixth order polynomials. The scatter functions of the FFF beams were recalculated using a uniform lateral profile (the same as the physical profile of the WFF beams), and are henceforth denoted as virtual uniform FFF beams (VUFFF). The field sizes of the VUFFF beams having the same scatter contribution as the corresponding FFF beams at a given field size were defined as the equivalent uniform square field (EQUSF) size. Data from four different LINACs with 18 different beams in total, as well as a CyberKnife beam, were analyzed. The average values of EQUSFs over all investigated LINACs of the conventional 10  ×  10 cm reference fields of 6 MV and 10 MV FFF beams for C-arm LINACs and machine-specific reference fields for CyberKnife and TomoTherapy were 9.5 cm, 9 cm, 5.0 cm and 6.5 cm respectively. The standard deviation of the mean of these EQUSFs was below 0.1 cm. It has been shown that with the introduction of a VUFFF beam, EQUSFs can be consistently defined for a variety of energies and collimations. These EQUSFs can form the basis for a unified reference dosimetry protocol for all different types of FFF machines.
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http://dx.doi.org/10.1088/1361-6560/aa83f5DOI Listing
September 2017
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