Validation of the geometric equivalent field concept in total scatter factor calculations, for half-, quarter- and...

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.

Square half-fields (SHF_s ), square quarter-fields (SQF_s ) and square off-isocenter fields (SOF_s ), with sizes ranging from 3×3 cm² to 20×20 cm² 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 cm² field size, where r is the distance between EFC and IC.

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.

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.