ESTRO 2024 - Abstract Book

S353

Beachytherapy - Physics

ESTRO 2024

1 Hospital de la Santa Creu i Sant Pau, Medical Physics, Barcelona, Spain. 2 Hospital de la Santa Creu i Sant Pau, Radiation Oncology, Barcelona, Spain

Purpose/Objective:

Currently, vaginal cylinder high-dose-rate (HDR) brachytherapy, either as a standalone treatment or as an adjuvant to external beam radiotherapy (EBRT), is one of the most common procedures for the management of postoperative endometrial carcinoma. Traditionally, in our center, a customized HDR brachytherapy treatment plan is calculated for each fraction using one-channel cylindrical applicator, which is fixed with external gauzes. The American Brachytherapy Society (ABS) guidelines 1 suggest performing a customized treatment plan only for the first fraction for fixed applicators. For this reason, the aim of this study is to evaluate the dosimetric impact of employing an individual fraction optimization (IFO) or first fraction optimization (FFO) approach.

Material/Methods:

A total of 31 patients were included in the study, of whom 17 received exclusive brachytherapy treatment (7 Gy in 3 fractions), while 14 patients underwent brachytherapy (5.5 Gy in 2 fractions) as an adjuvant treatment to EBRT. A single-channel cylindrical applicator was used, with different diameters and lengths tailored to the patients' anatomy, fixed with external gauzes. The patients were simulated using a Philips Big Bore CT scanner and treated with a Varian Bravos system. Individualized optimization was performed prior to each fraction (IFO method) using TG-43 dose calculation formalism in a homogeneous water medium in Eclipse BrachyVision software version 15.6.8 (Varian). Retrospectively, the dwell times of the treatment plan for the first fraction were applied to the simulation images of the subsequent fractions (FFO method). To assess the dosimetric impact between both methods throughout the treatment plan, relevant DVH dosimetric parameters were analyzed to evaluate the HR-CTV coverage (D 98% and D 90% ) and the maximum dose received (D 2cc and D 0.01cc ) by critical organs at risk (OARs) such as the bladder, rectum, sigmoid and bowel. Dosimetric differences between both methods were also assessed using a two-sided Wilcoxon signed-rank non-parametric test, after checking the non-normality of the data samples through Shapiro-Wilk test. Spearman's test was used to determine whether there was a correlation between inter-fractional dosimetric differences in FFO method with respect to the difference in the angle at which the applicator was inserted in the subsequent fractions compared to the first fraction. Significance level was set to 0.005 due to multiple test comparisons using Bonferroni method.

Results:

Dosimetric differences for HR-CTV and OARs are displayed in Figure 1. The maximum differences in EQD 2 were less than 1 Gy for D 90% and D 98% in HR-CTV and D 2cc in OARs. Larger dosimetric differences, up to 3 Gy, were observed for D 0.01cc in OARs, as this parameter is more sensitive to inter-fractional anatomical changes.