ESTRO 2024 - Abstract Book

S358

Beachytherapy - Physics

ESTRO 2024

constraint, we allow for a single such volume. This offers a more general alternative to a classically hardcoded “pear” region of interest (ROI) (fixed to 5mm around the applicator), reasoning that dose distribution contiguity around the target volume is more important than its location around the applicator. Furthermore, an aim can be added for the CTV HR core (CTV HR excluding its outer 6 mm margin) to localize the contiguity at the target center.

BRIGHT is run with four different customized sets of aims:

1. Tri-objective method with the set of aims in the Table below [3] as third objective, 2. Additionally to 1: pear-shaped ROI aim: V200% > 90-99.5%,

3. Additionally to 1: one 250% contiguous volume, 4. Additionally to 3: core-CTV HR aim: V200% > 99.5%.

DVH parameter

Aim (% of planning aim dose)

ROI

Purpose

50% isodose around CTV IR

CTV IR

V50%

> 99.9

Targets

Mid-CTV IR

V100%

< 25

100% isodose around CTV HR

CTV HR

V100%

> 99.9

Bottom-normal-tissue

D90%

< 25

limit normal tissue dose

OARs

Mid-normal-tissue

V100%

< 0.1

Top-normal-tissue

V100%

< 0.2

Planning is done retrospectively on 259 cases (123 patients), treated with MRI-guided BT in four fractions of 7 Gy HDR each. For evaluation, the associated EMBRACE-II DVH values are inspected.

For six representative patients, the resulting set of plans for each customization was presented to two radiation oncologists, a medical physicist, and a radiation technologist. For each patient, they assessed whether there is at least one acceptable plan.

Results:

A boxplot over all cases of obtained EMBRACE-II DVH values, pertaining to the equally balanced trade-off plan, is shown in Figure 1; in general, customization 1 results in only slightly better OAR sparing. For all patients for which the EMBRACE-II aims were not satisfied, this was also the case in the clinical plan.