Publications by authors named "F Fracchiolla"

22 Publications

Experimental assessment of inter-centre variation in stopping-power and range prediction in particle therapy.

Radiother Oncol 2021 Jul 27;163:7-13. Epub 2021 Jul 27.

OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany; Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology - OncoRay, Dresden, Germany; Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Germany; German Cancer Consortium (DKTK), partner site Dresden, and German Cancer Research Center (DKFZ), Heidelberg, Germany.

Purpose: Experimental assessment of inter-centre variation and absolute accuracy of stopping-power-ratio (SPR) prediction within 17 particle therapy centres of the European Particle Therapy Network.

Material And Methods: A head and body phantom with seventeen tissue-equivalent materials were scanned consecutively at the participating centres using their individual clinical CT scan protocol and translated into SPR with their in-house CT-number-to-SPR conversion. Inter-centre variation and absolute accuracy in SPR prediction were quantified for three tissue groups: lung, soft tissues and bones. The integral effect on range prediction for typical clinical beams traversing different tissues was determined for representative beam paths for the treatment of primary brain tumours as well as lung and prostate cancer.

Results: An inter-centre variation in SPR prediction (2σ) of 8.7%, 6.3% and 1.5% relative to water was determined for bone, lung and soft-tissue surrogates in the head setup, respectively. Slightly smaller variations were observed in the body phantom (6.2%, 3.1%, 1.3%). This translated into inter-centre variation of integral range prediction (2σ) of 2.9%, 2.6% and 1.3% for typical beam paths of prostate-, lung- and primary brain-tumour treatments, respectively. The absolute error in range exceeded 2% in every fourth participating centre. The consideration of beam hardening and the execution of an independent HLUT validation had a positive effect, on average.

Conclusion: The large inter-centre variations in SPR and range prediction justify the currently clinically used margins accounting for range uncertainty, which are of the same magnitude as the inter-centre variation. This study underlines the necessity of higher standardisation in CT-number-to-SPR conversion.
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July 2021

Clinical validation of a GPU-based Monte Carlo dose engine of a commercial treatment planning system for pencil beam scanning proton therapy.

Phys Med 2021 Aug 23;88:226-234. Epub 2021 Jul 23.

Protontherapy Department, Trento Hospital, Trento, Italy; TIFPA - Trento Institute for Fundamental Physics and Applications, 14, Via Sommarive, 38123 Povo TN, Italy.

Purpose: To perform the validation of the GPU-based (Graphical Processing Unit based) proton Monte Carlo (MC) dose engine implemented in a commercial TPS (RayStation 10B) and to report final dose calculation times for clinical cases.

Materials And Methods: 440 patients treated at the Proton Therapy Center of Trento, Italy, between 2018 and 2019 were selected for this study. 636 approved plans with 3361 beams computed with the clinically implemented CPU-MC dose engine (version 4.2 and 4.5), were used for the validation of the new algorithm. For each beam, the dose was recalculated using the new GPU-MC dose engine with the initial CPU computation settings and compared to the original CPU-MC dose. Beam dose difference distributions were studied to ensure that the two dose distributions were equal within the expected fluctuations of the MC statistical uncertainty (s) of each computation. Plan dose distributions were compared with respect to the dosimetric indices D, D and D of all ROIs defined as targets. A complete assessment of the computation time as a function of s and dose grid voxel size was done.

Results: The median over all mean beam dose differences between CPU- and GPU-MC was -0.01% and the median of the corresponding standard deviations was close to (√2s) both for simulations with an s of 0.5% and 1.0% per beam. This shows that the two dose distributions can be considered equal. All the DVH indices showed an average difference below 0.04%. About half of the plans were computed with 1.0% statistical uncertainty on a 2 mm dose calculation grid, for which the median computation time was 5.2 s. The median computational speed for all plans in the study was 8.4 million protons/second.

Conclusion: A validation of a clinical MC algorithm running on GPU was performed on a large pool of patients treated with pencil beam scanning proton therapy. We demonstrated that the differences with the previous CPU-based MC were only due to the intrinsic statistical fluctuations of the MC method, which translated to insignificant differences on plan dose level. The significant increase in dose calculation speed is expected to facilitate new clinical workflows.
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August 2021

Technical Note: CT calibration for proton treatment planning by cross-calibration with proton CT data.

Med Phys 2021 Mar 6;48(3):1349-1355. Epub 2021 Feb 6.

Istituto Nazionale di Fisica Nucleare sezione di Firenze, Via G. Sansone 1, Sesto Fiorentino, Italy.

Purpose: This study explores the possibility of a new method for x-ray computed tomography (CT) calibration by means of cross-calibration with proton CT (pCT) data. The proposed method aims at a more accurate conversion of CT Hounsfield Units (HU) into proton stopping power ratio (SPR) relative to water to be used in proton-therapy treatment planning.

Methods: X-ray CT scan was acquired on a synthetic anthropomorphic phantom, composed of different tissue equivalent materials (TEMs). A pCT apparatus was instead adopted to obtain a reference three-dimensional distribution of the phantom's SPR values. After rigid registration, the x-ray CT was artificially blurred to the same resolution of pCT. Then a scatter plot showing voxel-by-voxel SPR values as a function of HU was employed to link the two measurements and thus obtaining a cross-calibrated x-ray CT calibration curve. The cross-calibration was tested at treatment planning system and then compared with a conventional calibration based on exactly the same TEMs constituting the anthropomorphic phantom.

Results: Cross-calibration provided an accurate SPR mapping, better than by conventional TEMs calibration. The dose distribution of single beams optimized on the reference SPR map was recomputed on cross-calibrated CT, showing, with respect to conventional calibration, minor deviation at the dose fall-off (lower than 1%).

Conclusions: The presented data demonstrated that, by means of reference pCT data, a heterogeneous phantom can be used for CT calibration, paving the way to the use of biological samples, with their accurate description of patients' tissues. This overcomes the limitations of conventional CT calibration requiring homogenous samples, only available by synthetic TEMs, which fail in accurately mimicking the properties of biological tissues. Once a heterogeneous biological sample is provided with its corresponding reference SPR maps, a cross-calibration procedure could be adopted by other PT centers, even when not equipped with a pCT system.
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March 2021

Proton pencil beam scanning reduces secondary cancer risk in breast cancer patients with internal mammary chain involvement compared to photon radiotherapy.

Radiat Oncol 2020 Oct 2;15(1):228. Epub 2020 Oct 2.

Department of Physics, University of Trento, Via Sommarive, 14, 38123, Povo, TN, Italy.

Purpose: Proton pencil beam scanning (PBS) represents an interesting option for the treatment of breast cancer (BC) patients with nodal involvement. Here we compare tangential 3D-CRT and VMAT to PBS proton therapy (PT) in terms of secondary cancer risk (SCR) for the lungs and for contralateral breast.

Methods: Five BC patients including supraclavicular (SVC) nodes in the target (Group 1) and five including SVC plus internal-mammary-nodes (IMNs, Group 2) were considered. The Group 1 patients were planned by PT versus tangential 3D-CRT in free-breathing (FB). The Group 2 patients were planned by PT versus VMAT considering both FB and deep-inspiration breath hold (DIBH) irradiation. The prescription dose to the target volume was 50 Gy (2 Gy/fraction). A constant RBE = 1.1 was assumed for PT. The SCR was evaluated with the excess absolute risk (EAR) formalism, considering also the age dependence. A cumulative EAR was finally computed.

Results: According to the linear, linear-exponential and linear-plateau dose response model, the cumulative EAR for Group 1 patients after PT was equal to 45 ± 10, 17 ± 3 and 15 ± 3, respectively. The corresponding relative increase for tangential 3D-CRT was equal to a factor 2.1 ± 0.5, 2.1 ± 0.4 and 2.3 ± 0.4. Group 2 patients showed a cumulative EAR after PT in FB equal to 65 ± 3, 21 ± 1 and 20 ± 1, according to the different models; the relative risk obtained with VMAT increased by a factor 3.5 ± 0.2, 5.2 ± 0.3 and 5.1 ± 0.3. Similar values emerge from DIBH plans. Contrary to photon radiotherapy, PT appears to be not sensitive to the age dependence due to the very low delivered dose.

Conclusions: PBS PT is associated to significant SCR reduction in BC patients compared to photon radiotherapy. The benefits are maximized for young patients with both SVC and IMNs involvement. When combined with the improved sparing of the heart, this might contribute to the establishment of effective patient-selection criteria for proton BC treatments.
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October 2020

Clinical implementation of pencil beam scanning proton therapy for liver cancer with forced deep expiration breath hold.

Radiother Oncol 2021 01 22;154:137-144. Epub 2020 Sep 22.

Azienda Provinciale per i Servizi Sanitari (APSS) Protontherapy Department, Trento, Italy; TIFPA Trento Institute for Fundamental Physics and Applications, Italy.

Purpose: To present our technique for liver cancer treatments with proton therapy in pencil beam scanning mode and to evaluate the impact of uncertainties on plan quality.

Materials And Methods: Seventeen patients affected by liver cancer were included in this study. Patients were imaged and treated in forced breath-hold using the Active Breathing Coordinator system and monitored with an optical tracking system. Three simulation CTs were acquired to estimate the anatomical variability between breath-holds and generate an internal target volume (ITV). The treatment plans were optimized with a Single Field Optimization technique aimed at minimizing the use of range shifter. Plan robustness was tested simulating systematic range and setup uncertainties, as well as the interplay effect between breath-holds. The appropriateness of margin was further verified based on the actual positioning data acquired during treatment.

Results: The dose distributions of the nominal plans achieved a satisfactory target coverage in 11 out of 17 patients, while in the remaining 6 D to the PTV was affected by the constraint on mean liver dose. The constraints for all other organs at risk were always within tolerances. The interplay effect had a limited impact on the dose distributions: the worst case scenario showed a D reduction in the ITV < 3.9 GyRBE and no OAR with D > 105% of the prescription dose. The robustness analysis showed that for 13 out of 17 patients the ITV coverage in terms of D was better than D of the PTV in the nominal plan. For the remaining 4 patients, the maximum difference between ITV D and PTV D was ≤0.7% even for the largest simulated setup error and it was deemed clinically acceptable. Hot spots in the OARs were always lower than 105% of the prescription dose. Positioning images confirmed that the breath hold technique and the PTV margin were adequate to compensate for inter- and intra-breath-hold variations in liver position.

Conclusion: We designed and clinically applied a technique for the treatment of liver cancer with proton pencil beam scanning in forced deep expiration breath-hold. The initial data on plan robustness and patient positioning suggest that the choices in terms of planning technique and treatment margins are able to reach the desired balance between target coverage and organ at risk sparing.
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January 2021