Gavin Cranmer-Sargison1, Paul H Charles, Jamie V Trapp, David I Thwaites. 1. Department of Medical Physics, Saskatchewan Cancer Agency, Saskatoon, Canada; Academic Unit of Medical Physics, Faculty of Medicine and Health, University of Leeds, UK. Electronic address: gavin.cranmer-sargison@saskcancer.ca.
Abstract
PURPOSE: The goal of this work was to set out a methodology for measuring and reporting small field relative output and to assess the application of published correction factors across a population of linear accelerators. METHODS AND MATERIALS: Measurements were made at 6 MV on five Varian iX accelerators using two PTW T60017 unshielded diodes. Relative output readings and profile measurements were made for nominal square field sizes of side 0.5 to 1.0 cm. The actual in-plane (A) and cross-plane (B) field widths were taken to be the FWHM at the 50% isodose level. An effective field size, defined as √FS eff=A · B, was calculated and is presented as a field size metric. FSeff was used to linearly interpolate between published Monte Carlo (MC) calculated [Formula in text] values to correct for the diode over-response in small fields. RESULTS: The relative output data reported as a function of the nominal field size were different across the accelerator population by up to nearly 10%. However, using the effective field size for reporting showed that the actual output ratios were consistent across the accelerator population to within the experimental uncertainty of ± 1.0%. Correcting the measured relative output using [Formula in text] at both the nominal and effective field sizes produce output factors that were not identical but differ by much less than the reported experimental and/or MC statistical uncertainties. CONCLUSIONS: In general, the proposed methodology removes much of the ambiguity in reporting and interpreting small field dosimetric quantities and facilitates a clear dosimetric comparison across a population of linacs.
PURPOSE: The goal of this work was to set out a methodology for measuring and reporting small field relative output and to assess the application of published correction factors across a population of linear accelerators. METHODS AND MATERIALS: Measurements were made at 6 MV on five Varian iX accelerators using two PTW T60017 unshielded diodes. Relative output readings and profile measurements were made for nominal square field sizes of side 0.5 to 1.0 cm. The actual in-plane (A) and cross-plane (B) field widths were taken to be the FWHM at the 50% isodose level. An effective field size, defined as √FS eff=A · B, was calculated and is presented as a field size metric. FSeff was used to linearly interpolate between published Monte Carlo (MC) calculated [Formula in text] values to correct for the diode over-response in small fields. RESULTS: The relative output data reported as a function of the nominal field size were different across the accelerator population by up to nearly 10%. However, using the effective field size for reporting showed that the actual output ratios were consistent across the accelerator population to within the experimental uncertainty of ± 1.0%. Correcting the measured relative output using [Formula in text] at both the nominal and effective field sizes produce output factors that were not identical but differ by much less than the reported experimental and/or MC statistical uncertainties. CONCLUSIONS: In general, the proposed methodology removes much of the ambiguity in reporting and interpreting small field dosimetric quantities and facilitates a clear dosimetric comparison across a population of linacs.
Authors: Wolfgang Lechner; Paulina Wesolowska; Godfrey Azangwe; Mehenna Arib; Victor Gabriel Leandro Alves; Luo Suming; Daniela Ekendahl; Wojciech Bulski; José Luis Alonso Samper; Sumanth Panyam Vinatha; Srimanoroth Siri; Milan Tomsej; Mikko Tenhunen; Julie Povall; Stephen F Kry; David S Followill; David I Thwaites; Dietmar Georg; Joanna Izewska Journal: Phys Imaging Radiat Oncol Date: 2018-03-06
Authors: Luis Muñoz; Tomas Kron; Marco Petasecca; Joseph Bucci; Michael Jackson; Peter Metcalfe; Anatoly B Rosenfeld; Giordano Biasi Journal: J Appl Clin Med Phys Date: 2021-01-13 Impact factor: 2.102