Validation of the geometric equivalent field concept in total scatter factor calculations, for half‐, quarter‐ and off‐isocenter asymmetric square fields
<h2>Objective</h2><p dir="ltr">Monitor unit (MU) verification for any symmetric or asymmetric field is performed using a total scatter factor (S<sub>cp</sub>), that is calculated based on the geometric equivalent square field (GESF) concept. In this study, we...
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2023
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| Summary: | <h2>Objective</h2><p dir="ltr">Monitor unit (MU) verification for any symmetric or asymmetric field is performed using a total scatter factor (S<sub>cp</sub>), that is calculated based on the geometric equivalent square field (GESF) concept. In this study, we measured the S<sub>cp</sub> of various asymmetric square fields (ASF<sub>s</sub>) and their respective GESFs.</p><h2>Methods</h2><p dir="ltr">Square half‐fields (SHF<sub>s</sub>), square quarter‐fields (SQF<sub>s</sub>) and square off‐isocenter fields (SOF<sub>s</sub>), with sizes ranging from 3×3 cm<sup>2</sup> to 20×20 cm<sup>2</sup> were created, by varying the collimator jaws of two Varian iX Linacs (6/18 and 6/23 MV). A semi‐flex ion chamber was used to measure S<sub>cp</sub> at a depth of 10 cm within a water phantom, at the effective field center (EFC) of all ASF<sub>s</sub>, and at the isocenter (IC) of their respective GESFs. The later S<sub>cp</sub> values were corrected by the off‐axis ratio [OAR(r)] of the 40×40 cm<sup>2</sup> field size, where r is the distance between EFC and IC.</p><h2>Results</h2><p dir="ltr">The results show that the S<sub>cp</sub> (EFC) is independent of the type of the ASF (SHF, SQF, or SOF) and no significant difference exists between the 18 and 23 MV beams. Compared with the S<sub>cp</sub> (IC), the S<sub>cp</sub> (EFC) increased with increasing r, by up to 2% and 4% for 18/23 and 6 MV, respectively.</p><h2>Conclusions</h2><p dir="ltr">The GESF concept provides acceptable accuracy (< 2%) for the calculation of S<sub>cp</sub> of the ASFs used in most clinical situations (except from SOF with EFC at large r), and thus can be used in MU verification calculations.</p><h2>Other Information</h2><p dir="ltr">Published in: Journal of Applied Clinical Medical Physics<br>License: <a href="http://creativecommons.org/licenses/by/4.0/" target="_blank">http://creativecommons.org/licenses/by/4.0/</a><br>See article on publisher's website: <a href="https://dx.doi.org/10.1002/acm2.14103" target="_blank">https://dx.doi.org/10.1002/acm2.14103</a></p> |
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