ESTRO 2024 - Abstract Book

S2972

Interdiscplinary - Other

ESTRO 2024

Conclusion:

This study investigates the possibility to use a scaled dose matrix in clinical practice in case of plan sum for re treatment. The main limitations are linked to the rigid registration and how OAR have changed between treatments. Since the study was limited to only five patients, a clear correlation between tolerance constraints and GU/GI toxicities was not possible. Notably, for the rectum, despite dose-volume constraints not being met in the plan sum, no toxicities were observed. The most significant observation was the substantial differences in BED, with an α/β ratio of 1.78 for conventional treatment compared to the commonly used 1.1 for prostate. In case of hypofractionation, the differences were even more pronounced, with an α/β ratio of 3.46 vs. 1.1, showing differences exceeding more than 56%. Recent researches, such as Ming Cui et al. 3 , suggests that the α/β ratio may vary depending on the fractionation and the damage mechanisms involved, hinting at an increase in α/β with higher fractionation. This aspect, emphasized by Kirkpatrick et al. 2 in the context of radiosurgery, raises questions about the linear quadratic model's ability to account for vascular damage in stroma. Additionally C. M. van Leeuwen 1 suggests to consider not only the tumor site but also the histology of tumors when selecting the α/β showing that tumors in different sites have similar α/β ratioIn a scenario marked by an increasing need for retreatment, defining guidelines for the appropriate use of α/β ratios for tumors and OARs in terms of BED and toxicities is crucial. This could pave the way for personalized radiotherapy treatments, but further studies and data collection are essential to refine these guidelines.

Keywords: Radiotherapy, reirradiation, prostate cancer

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