ESTRO 2022 - Abstract Book

S1279

Abstract book

ESTRO 2022

cGy vs. 2675 cGy, respectively), highlighting the importance of plan evaluation using a voxel-wise worst case evaluation.

Conclusion Evaluation of ultra hypofractionated IMPT for PCa by a voxel wise worst-case approach using 5-mm setup and 3% range uncertainty is found to be robust for the anticipated target coverage and OAR dose for at least 95% of intra-fraction prostate movement found in clinical practice.

PO-1501 Evaluation of proton PBS treatment planning guidelines for brain tumours using a variable RBE model

A. Vestergaard 1 , J. Kallehauge 1 , P. Lægdsmand 2 , K. Seiersen 1 , B. Smulders 1,3 , P. Witt Nyström 1 , Y. Lassen-Ramshad 1 , M. Høyer 1 , O. Nørrevang 1 , S. Korreman 1 1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 2 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus N, Denmark; 3 Rigshospitalet, Department of Oncology, Copenhagen, Denmark Purpose or Objective Brain tumour patients have been treated with proton therapy for decades, and more recently with Pencil Beam Scanning (PBS). Proton treatment planning often uses planning guidelines to avoid high Linear Energy Transfer (LET) and thereby high variable Radiobiological Effect (RBE) in high-risk organs, especially in the brainstem. The aim of this study was to evaluate the safety of clinically applied planning guidelines with respect to brainstem doses, when taking into account a variable RBE model. Materials and Methods 15 consecutive brain tumour patients treated with proton therapy to 50.4 GyRBE and with brainstem near-max ≥ 40 Gy, and 18 patients treated to 54 or 59.4 GyRBE with brainstem near-max ≥ 52.5 GyRBE were included. All patients were planned according to in-house treatment planning guidelines. For patients with tumours close to the brainstem treated to a prescribed dose ≥ 54 GyRBE, the local guidelines imply that only one out of minimum three fields was allowed with distal edge in the brainstem. For patients with a prescribed dose below 54 GyRBE, there are no guidelines explicitly regarding the distal edge, as the RBE effects are not expected to be clinically relevant. Results The figure shows D 1cc and D 1cc,var of the brainstem for all 33 patients. The median and inter quartile range of D1cc, var was 53.9 GyRBE (43.5;57.4) for the 50.4 GyRBE group and 57.6 GyRBE (55.7;58.6) for the 54-59.4 GyRBE group. None of the patients had a D1cc,var of more than 60 GyRBEvar.