ESTRO 2024 - Abstract Book

S343

Beachytherapy - Physics

ESTRO 2024

At our institute, MR-guided high dose rate brachytherapy is used as a boost in combination with external beam radiation for prostate cancer. Clinically, the brachytherapy treatment plans are generated in Oncentra Prostate based on MR-images taken just after catheter insertion. Dwell positions are established and dwell times are intended to be set optimally. The treatment plans are optimized automatically using Oncentra Prostate’s optimization algorithm followed by manual adjustments with the focus of adhering to several dose aims listed in the clinical protocol (Table 1). BRIGHT is an automated treatment planning method for brachytherapy driven by the evolutionary algorithm GOMEA 3, which directly optimizes dosimetric indices within a bi-objective optimization model. The objectives of this model are the Least Coverage Index (LCI) and Least Sparing Index (LSI), denoting the dosimetric indices having the largest deviation from the clinical protocol in terms of coverage and sparing, respectively. The result of optimization with BRIGHT is a patient-specific manifold of high-quality plans, each according to a trade-off between target coverage and organ sparing. An illustration of the plan selection from the manifold can be seen in Figure 1. This study is the first to perform a retrospective clinical validation of BRIGHT on different patients, and a different protocol, than at the Amsterdam UMC where BRIGHT was first clinically introduced. Clinical aims regarding the maximum dose to an organ at risk (OAR), i.e., bladder V200% and rectum V100%, are not included in the optimization objectives, but rather set as hard constraints. For this study data from 17 fractions given to 10 patients is used. For each case, optimization with BRIGHT is performed with its default clinical settings (100,000 dose calculation points and a time limit of 180 seconds). Then, three plans are selected by a clinical physicist from the manifold each fulfilling the clinical aims but with a different balance between target coverage and organ sparing. Next, from these three plans, one is selected by a medical doctor. This selected plan is referred to as the BRIGHT plan. The BRIGHT plan is imported to Oncentra Prostate, where it is compared to the clinical plan in terms of target coverage and organ sparing.

Results:

We find that all plans achieve the target coverage aims of the protocol. Meaning that the prostate and the CTV (prostate plus 5 mm isotropic margin) are sufficiently covered with the 8.5 Gy isodose curve for both Oncentra