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

Hmodi, Mohammad Samir; Nahili, Majeda; Tsalafoutas, Ioannis A.; Saad, Bassam; Hasan, Ali; Anjak, Ousamah; Shamout, Karlos

doi:10.1002/acm2.14103

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

journal contribution

revised on 2024-01-09, 06:50 and posted on 2024-01-09, 06:52 authored by Mohammad Samir Hmodi, Majeda Nahili, Ioannis A. Tsalafoutas, Bassam Saad, Ali Hasan, Ousamah Anjak, Karlos Shamout

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 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.

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.

Other Information

Published in: Journal of Applied Clinical Medical Physics
License: http://creativecommons.org/licenses/by/4.0/
See article on publisher's website: https://dx.doi.org/10.1002/acm2.14103

Funding

Open Access funding provided by the Qatar National Library.

History

Language

English

Publisher

Wiley

Publication Year

2023

License statement

This Item is licensed under the Creative Commons Attribution 4.0 International License.

Institution affiliated with

Hamad Medical Corporation

Methodology

Square half‐fields (SHFs), square quarter‐fields (SQFs) and square off‐isocenter fields (SOFs), 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 Scp at a depth of 10 cm within a water phantom, at the effective field center (EFC) of all ASFs, and at the isocenter (IC) of their respective GESFs. The later Scp 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.