Publications by authors named "Haluk Utku"

3 Publications

  • Page 1 of 1

Determination of inflection points of CyberKnife dose profiles within acceptability criteria of deviations in measurements.

Rep Pract Oncol Radiother 2020 Jan-Feb;25(1):6-12. Epub 2019 Nov 12.

Institute of Nuclear Sciences, Hacettepe University, Ankara, Turkey.

Aim: The aim of this study was to determine the Inflection Points (IPs) of flattening filter free (FFF) CyberKnife dose profiles for cone-based streotactic radiotherapy. In addition, dosimetric field sizes were determined.

Background: The increased need for treatment in the early stages of cancer necessitated the treatment of smaller tumors. However, efforts in that direction required the modeling accuracy of the beam. Removal of the flattening filter (FF) from the path of x-ray beam has provided the solution to those efforts, but required a different normalization approach for the beam to ensure the delivery of the dose accurately. As a solution, researchers proposed a normalization factor based on IPs.

Materials And Methods: Measurements using microDiamond (PTW 60019), Diode SRS (PTW 60018) and Monte Carlo (MC) calculations of dose profiles were completed at SAD 80 cm and 5 cm depth for 15-60 mm cones. Performance analysis of detectors with respect to MC calculation was carried out. Gamma evaluation method was used to determine achievable acceptability criteria for FFF CyberKnife beams.

Results: Acceptability within (3%-0.5 mm) was found to be anachievable criterion for all dose profile measurements of the cone beams used in this study. To determine the IP, the first and second derivatives of the dose profile were determined via the cubic spline interpolation technique.

Conclusion: Derivatives of the interpolated profiles showed that locations of IPs and 50% isodose points coincide.
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November 2019

Evaluation of NanoDot Optically Stimulated Luminescence Dosimeter for Cone-shaped Small-field Dosimetry of Cyberknife Stereotactic Radiosurgery Unit: A Monte Carlo Simulation and Dosimetric Verification Study.

J Med Phys 2019 Jan-Mar;44(1):27-34

Institute of Nuclear Science, Hacettepe University, Ankara, Turkey.

Aim: The aim of this study was to investigate the adequacy of nanoDot optically stimulated luminescence (OSL) dosimeter for small field dosimetry before its applications in CyberKnife SRS unit.

Materials And Methods: A PTW 60018 SRS Diode, 60019 microDiamond, and Gafchromic EBT3 films were used along with a nanoDot carbon-doped aluminum oxide OSL dosimeter to collect and compare beam data. In addition, the EGSnrc/BEAMnrc code was employed to simulate 6-MV photon beams of CyberKnife SRS system.

Results: All detectors showed good consistency with each other in output factor measurements for cone sizes of 15 mm or more. The differences were maintained within 3% for these cones. However, OSL output factors showed higher discrepancies compared to those of other detectors for smaller cones wherein the difference reached nearly 40% for cone size of 5 mm. Depending on the performance of OSL dosimeter in terms of output factors, percentage depth doses (PDDs) were only measured for cones equal to or larger than 15 mm. The differences in PDD measurements were within 5% for depths in the range of 5-200 mm.

Conclusion: Its low reliable readings for cones smaller than 15 mm should be considered before its applications of Cyberknife system.
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April 2019

Discrimination of brain's neuroelectric responses by a decision-making function.

J Neurosci Methods 2002 Feb;114(1):25-31

Institute of Nuclear Sciences, Hacettepe University, Beytepe 06532, Ankara, Turkey.

The aim of this work was to develop a mathematical decision-making procedure that might become a basis for real-time pattern recognition studies of the brain's neuroelectric responses. Data were collected from 77 volunteers under the auditory oddball paradigm with standard (1000 Hz) and deviant (2000 Hz) stimuli. The participants counted the deviants and reported them at the end of the experimental session. Event-related potentials (ERPs) were recorded and filtered with a bandpass between 0.16 and 70 Hz (3 dB down, 12 dB/octave) at Fz- and Pz-recording sites. The most significant potential values that discriminated the responses to the deviant stimuli group were at 136, 224, 328, 348, and 350 ms for Fz, and at 166, 220, and 350 ms for the Pz. The 328, 348, and 350 ms potential values define the curvature of the P300 peak; the 224 ms potential at Fz, and the 220 ms at Pz define that of the N2b peak. The differentiation between the deviant and standard group was checked through a discriminant function that allowed prediction of group membership; 98% of the ERP responses were correctly identified. The results showed that statistically derived time-points were congruent with the P300 and N2b ERP curvature.
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February 2002