Validation of the geometric equivalent field concept in total scatter factor calculations, for half‐, quarter‐ and off‐isocenter asymmetric square fields

OBJECTIVE: Monitor unit (MU) verification for any symmetric or asymmetric field is performed using a total scatter factor (S(cp)), that is calculated based on the geometric equivalent square field (GESF) concept. In this study, we measured the S(cp) of various asymmetric square fields (ASF(s)) and their respective GESFs. METHODS: Square half‐fields (SHF(s)), square quarter‐fields (SQF(s)) and square off‐isocenter fields (SOF(s)), with sizes ranging from 3×3 cm(2) to 20×20 cm(2) 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(cp) at a depth of 10 cm within a water phantom, at the effective field center (EFC) of all ASF(s), and at the isocenter (IC) of their respective GESFs. The later S(cp) values were corrected by the off‐axis ratio [OAR(r)] of the 40×40 cm(2) field size, where r is the distance between EFC and IC. RESULTS: The results show that the S(cp) (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(cp) (IC), the S(cp) (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(cp) 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.