Monte Carlo Evaluation for the Effect of Positional Inaccuracy of Source on Patient's Dose in High-Dose-Rate Brachytherapy

Although the potential inaccuracies of source position in high-dose-rate (HDR) brachytherapy could affect the dose to the target and normal tissues, the dose variation to the target and critical organs due to the source position error have not been evaluated. Therefore, the effect of the positional inaccuracy on the dose is evaluated for uterine cervical cancer patients. Ten patients who were eligible for intracavitary brachytherapy were involved in this study. Conventional treatment plans (PLATO planning system) for a Fletcher-type metal applicator (one tandem and two ovoids) were performed to deliver 100 % of the prescribed dose to point A. The planned source dwell positions were shifted by 2.5 mm, and the doses to point A, point B, the external cervical os, the bladder, and the rectum were calculated with both MCNPX and PLATO planning system. As a result, point A has a dose difference within 5 % in all patients. The bladder and the rectum had dose differences within 10 %. PLATO and MCNPX had dose differences within 25 % at all reference points. The results show that the positional inaccuracy of the source more affect organ more than points A and B.