Measuring the medical linear accelerator head scatter for asymmetric fields radiotherapy

Abstract

In this paper we were measured head scatter (H_S) of Varian medical accelerator when radiotherapy with an asymmetric square half-field (ASH-F) and quarter-field (ASQ-F) and compared with the equivalent symmetric square field (SSF) where treatment planning systems depend on it for asymmetric fields.

H_S was measured in air using 0.6cc ion-chamber and two brass build-up caps with thicknesses of 1.8mm and 3.5mm for 6MV and 18MV, respectively. The measurement was performed in the iso-center (IC) for SSF and at effective fields center (EFC) for ASH-F and ASQ-F.

The results confirmed that the H_S value differs when using ASH-F and ASQ-F compared to SSF for the fields exceeding 10×10 cm² and this difference reaches 2.3% and 3.8% for 6MV, while it is 1.5% and 1.7% for 18MV. H_S is not related to the photon beam energy as the difference was less than 1% except ASQ-F where the difference was 2% for fields larger than the reference field.

This study indicated that improving the quality of radiotherapy requires additional measurements of asymmetric fields whose dimensions exceed the area of ​​the reference field, and their inclusion in the treatment planning system. Thus, the equivalent

field cannot be relied for this cases.