ESTRO 2020 Abstract book

S827 ESTRO 2020

Material and Methods The NTCP model calculates a local probability-of-lesion- origin (POLO) depending on dose, LET d , proximity to the ventricles (VP), age and chemotherapy (cf Fig. 1). Dose planning should prevent excessively high POLO values and at the same time large VP volumes of elevated POLO. Means to achieve this in conventional spot scanning planning (CP) are multiple beam plans for simultaneous LET d reduction by averaging and ventricular sparing. In contrast, Risk Avoidance Dose Painting (RADP) employs the POLO directly as a cost function to the optimizer. RADP was implemented in a research version of RayStation © 9A (RaySearch Laboratories AB, Stockholm, Sweden). Patients of the modelling cohort were stratified into risk groups and unfavourable anatomic and plan features were identified. For those patients presenting with the highest risks, CP strategies were employed to counter the unfavourable anatomic features. RADP plans were generated with penalty on NTCP.

Conclusion Robustness of 5mm is required when planning HNC treatment with protons to ensure adequate target coverage throughout 6 weeks of treatment. This results in increased dose to contralateral parotid and larynx compared to RP or MCO. The dosimetric advantages for OAR sparing, previously demonstrated with proton therapy compared to photon therapy for HNC, are lost when robustness is added to ensure adequate target volume coverage throughout treatment. Advanced photon planning techniques such as MCO result in better OAR sparing while maintaining CTV coverage. PO-1456 Normal Tissue Risk Avoidance Dose Painting vs Conventional Planning for Proton Brain Irradiation J. Bauer 1,2,3 , E. Bahn 1,2,3,4 , S. Harrabi 1,2,3,5 , K. Herfarth 1,2,3,5 , E. Traneus 6 , J. Debus 1,2,3,4,5 , M. Alber 1,2,3 1 Department of Radiation Oncology, Heidelberg University Hospital, Heidelberg, Germany ; 2 National Center for Tumor diseases NCT, National Center for Tumor diseases NCT, Heidelberg, Germany ; 3 Heidelberg Institute of Radiation Oncology HIRO, Heidelberg Institute of Radiation Oncology HIRO, Heidelberg, Germany ; 4 Clinical Cooperation Unit Radiation Oncology, German Cancer Research Center DKFZ, Heidelberg, Germany ; 5 Heidelberg Ion-Beam Therapy Center HIT, Heidelberg University Hospital, Heidelberg, Germany ; 6 RaySearch Laboratories AB, RaySearch Laboratories AB, Stockholm, Sweden Purpose or Objective Contrast-enhancing brain lesions (CEBLs) are a frequent occurrence in proton irradiation for low-grade glioma (LGG) patients and a precursor to brain necrosis. Previously, we established an NTCP model built on a cohort of 110 patients, which comprises both a substantial increase in relative biological effectiveness (RBE) with dose-averaged linear-energy-transfer (LET d ) and a spatially variable radiosensitivity of the brain. Here, we investigate how this model can be utilized in treatment planning (TP) to limit the risk of CEBLs.

Results Lateral target volumes, predominantly seen for WHO °II histologies in an old patient group (median age 45 yrs), were identified as unfavourable in terms of VP load, with a median NTCP of 11% (range [0.06,62]%). The comparably young patient group (median age 16 yrs) with WHO °I tumours had mostly central target locations yielding a considerably lower median NTCP of 1% (range [0.06,78]%) (cf Fig. 2). Despite a maximum RBE of ~1.6 extracted at LET d =5 keV/µm in the patient cohort, the VP was found to be the main driver of high risks. Tested CP strategies were not able to effectively decrease NTCP. In fact, most scenarios yielded an increase due to an unfavourable redistribution of intermediate to high LET d into the sensitive VP. RADP succeeded in constraining the NTCP at the cost of some underdosage to the PTV and CTV edges overlapping with the sensitive VP. Depending on the overlap size, local dose reductions of up to 20% were required to achieve NTCP≤10%. As the required dose reduction depends on the local LET d , intricate patterns of sparing are obtained. Thus, RADP also re-distributes LET d away from the ventricles to some extent, leading to a small but noticeable reduction of POLO in the entire target volume.