Objective: Monitor unit (MU) verification for any symmetric or asymmetric field is performed using a total scatter factor (S ), that is calculated based on the geometric equivalent square field (GESF) concept. In this study, we measured the S of various asymmetric square fields (ASF ) and their respective GESFs.
Methods: Square half-fields (SHF ), square quarter-fields (SQF ) and square off-isocenter fields (SOF ), with sizes ranging from 3×3 cm to 20×20 cm 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 at a depth of 10 cm within a water phantom, at the effective field center (EFC) of all ASF , and at the isocenter (IC) of their respective GESFs. The later S values were corrected by the off-axis ratio [OAR(r)] of the 40×40 cm field size, where r is the distance between EFC and IC.
Results: The results show that the S (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 (IC), the S (EFC) increased with increasing r, by up to 2% and 4% for 18/23 and 6 MV, respectively.
Conclusions: The GESF concept provides acceptable accuracy (< 2%) for the calculation of S 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.
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| http://dx.doi.org/10.1002/acm2.14103 | DOI Listing |
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