ESTRO 2024 - Abstract Book

S29 ESTRO 2024 synergies, and where a combination of these two techniques could be implemented today or in the near future will be discussed. Invited Speaker

1. Prezado, Y., Divide and conquer: spatially fractionated radiation therapy. Expert Reviews in Molecular Medicine, 2022. 24 : p. 12. 2. Favaudon, V., et al., Ultrahigh dose-rate FLASH irradiation increases the differential response between normal and tumor tissue in mice. Sci Transl Med, 2014. 6 (245): p. 245ra93. 3. Vozenin, M.C., J.H. Hendry, and C.L. Limoli, Biological Benefits of Ultra-high Dose Rate FLASH Radiotherapy: Sleeping Beauty Awoken. Clin Oncol (R Coll Radiol), 2019. 31 (7): p. 407-415.

3320

Contrast enhancing brain lesions (CEBL): A new approach for clinical RBE analysis

Julia Bauer

Heidelberg Institute for Radiation Oncology (HIRO), National Center for Radiation Research in Oncology (NCRO), Heidelberg, Germany. University Hospital, Department of Radiation Oncology and Heidelberg Ion beam Therapy Center (HIT), Heidelberg, Germany. German Cancer Research Center (DKFZ), Clinical Cooperation Unit Radiation Oncology, Heidelberg, Germany

Abstract:

The relative biological effectiveness (RBE) of proton radiotherapy (PT) is still a controversial topic, even after decades of clinical PT treatment. Preclinical in-vitro studies support a variable proton RBE, but a constant increase in biological effectiveness of 10% as compared to photon irradiation is worldwide used as clinical standard. This concept allows for generally safe and efficient PT treatment, however, clinical evidence for localized dose effects that are attributed to variable proton RBE has been reported over the last years by different treatment centers [1]. Most groups have focussed their assessment on cerebral lesions and an association of the observed radiation-induced tissue changes with the linear energy transfer (LET) of the irradiation field. However, the obtained results were inconsistent regarding this correlation and the question whether or not it is time to consider a variable proton RBE in clinical practise is currently under debate [2]. Reliable clinical evidence is still rare and, although LET-based treatment planning (TP) is becoming available in clinical TP systems, broad knowledge on reasonable exploitation of such functionality is lacking. Clinical RBE analysis is generally challenged by the fact that RBE depends on various physical and biological parameters, which are usually well controlled in in-vitro and preclinical in-vivo experiments, but in particular the biological aspects can vary considerably in clinical RBE analyses. Furthermore, proton RBE effects are expected to be very localized at distinct regions of the treatment field and initially weak. The selection and precise definition of an appropriate endpoint that is sensitive to such spatially varying effect is therefore crucial for clinical RBE analysis. In this regard, late-occurring contrast enhancing brain lesions (CEBL) observed during regular T1-weighted follow-up MR imaging after PT in low-grade glioma patients have been suggested as endpoint to investigate dose effects in the irradiated healthy brain tissue [1].