ESTRO 2024 - Abstract Book

S2971

Interdiscplinary - Other

ESTRO 2024

1 Humanitas Research Hospital, Medical Physics Unit of Radiotherapy, Milan, Italy. 2 Humanitas University, Department of Biomedical Sciences, Milan, Italy. 3 Humanitas Research Hospital, Radiotherapy and Radiosurgery Department, Milan, Italy. 4 Università Degli Studi di Milano, Dipartimento di Fisica, Milan, Italy

Purpose/Objective:

Re-irradiation is becoming a frequent therapeutic strategy in modern radiation oncology. The evaluation of cumulative dose between plans with different fractionation could be critical. Recent studies have explored the α/β ratio's dependence on fractionation and on histological factors 1-8 , highlighting the limitations of the linear quadratic (L-Q) model (different radioresistant clonogens in tumors and derivation of α/β ratios from in vitro studies) 1-3 . With this study we developed a new tool for converting the entire dose matrix in EQD2 and BED in order to evaluate not only the maximum dose for serial organs but also dose-volume constraints in a cumulative plan sum. In this scenario the selection of the alpha-beta (α/β) ratio for each tissue becomes crucial.

Material/Methods:

This study focused on patients with prostate cancer who underwent initial radiotherapy treatment and later required a re-irradiation. Both the primary and the re-irradiation treatment plans were meticulously scaled voxel by voxel using EQD2 and BED formulas, employing custom Python scripts (v3.12.0). All scaled plans were subsequently imported into our Treatment Planning System, (Eclipse v15.6, Varian) 9 and all CT images were rigidly co-registered. Mean dose and D95%, for dose BED and EQD2 scaling were reported for the target using α/β ratios of 1.78 and 3.46 for conventional and hypofractionated treatments respectively, instead of the widely accepted α/β ratio of 1.1 for prostate cancer 3-10 . The EQD2 for primary plans was calculated only for two patients that received an hypofractionated tratment. OARs constraints for conventional schemes were reported and compared with a EQD2 calculated with α/β ratios of 3, hypofractionation, and 5 as the standard one 7-9 . A plan sum was genereted for both primary plans and re-irradiations, converting them into EQD2. This study involved five patients who were re-irradiated for local relapse of prostate cancer. A total of 54 scaled plans had been evaluated. The primary treatment had a median dose of 76 Gy over 28 fractions and a median Gleason score of 4+3. Three patients received conventional fractionation of 76 Gy in 38 fractions, while the remaining two received a total of 74.2 Gy in 28 fractions. The re-irradiation was delivered with a median dose of 25 Gy (range 25-30) delivered over 5 fractions, and the median time of relapse was 53 months. The two patients who received hypofractionated treatment experienced acute GU grade 2 toxicity, while the remaining three had only acute and late grade 1 toxicity. No acute and late GI toxicity was observed. We compared the α/β ratios of 1.1 (considered as reference value) with 1.78 for primary conventional treatment and 3.46 for hypofractionation both for primary and re-irradiation plans. Figure 1 displays median D95% results based on the α/β ratio for all treatment. Table 1 presents median volume-dose values and percentage variation for each OAR at various α/β ratios. Results: