ESTRO 2021 Abstract Book

S1592

ESTRO 2021

Materials and Methods A CT of an adult male human diagnosed with a human glioblastoma at the centre of the brain was selected for this study. The multi-field pMBRT (i.e., combination of pMBRT and Arc therapy) treatment plan involved 13 equally-spaced fields forming a 120-degree arc around the occipital region of the patient head in the longitudinal axis (Figure 1). The single-field pMBRT plan consists of one beam in the coronal direction. The collimator utilized in both cases comprises 400 µm width slits separated by 4.0 mm centre-to-centre distance. The energies and weights of the spots conforming both PBS plans were calculated by the ECLIPSE treatment planning system and dose distributions were computed using the TOPAS toolkit.

Figure 1. Dose distributions in the lateral and posterior views (A,B) and field distribution (C). Results

The combination of pMBRT and Arc therapy leads to a 30-90 % reduction of both peak and valley doses to healthy tissues in comparison with the single-field pMBRT approach. Furthermore, the addition of different treatment fields does not modify the peak-to-valley dose ratios nor create hot spots in normal tissues. Conclusion The combination of Arc therapy and spatially fractionated radiation therapy does not reduce the dosimetrical benefits of pMBRT but increases its potential of tissue sparing by reducing the dose delivered to non-tumour volumes. PO-1869 Are distal edge planning guidelines needed in proton PBS treatment planning of brain tumours? A. Vestergaard 1 , J. Kallehauge 1 , P. Lægdsmand 2 , K. Seiersen 1 , B. Smulders 3 , P.W. Nyström 4 , 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, Denmark; 3 Rigshospitalet, Department of Oncology, Copenhagen, Denmark; 4 Aarhus University Hospital, Danish Centre for Particle Therapy, Aaehus, Denmark Purpose or Objective The clinical use of RBE=1.1 is currently the standard value in proton treatment planning. However, the RBE varies with different tissues, LET, α/β and endpoint. Proton treatment planning guidelines for brain tumours often include instructions to avoid beam angles placing the distal edge in the brainstem in order to avoid an increase in variable RBE. The aim of this study was to quantify the cost in mean dose to the normal brain (Brain-CTV) and to evaluate the potential increase in variable RBE to the brainstem if the distal edge planning guideline is omitted. Materials and Methods 18 consecutive brain tumour patients were included based on near max dose to the brainstem of more than 52.5 GyRBE. All patients were treated according to in-house treatment planning guidelines. The local guidelines imply that maximum one out of minimum three fields can have the distal edge in the brainstem, for patients with tumours close to the brainstem treated to at least 54 GyRBE. The patients were re-planned (Replan) without the distal edge rule to improve other OAR doses, such as mean dose to Brain-CTV. Monte Carlo simulations (TOPAS v3.5/Geant4 v.10.6) were used to calculate dose and dose averaged linear energy transfer (LET) for both plans. Variable RBE doses were calculated using the McNamara model, assuming α/β=2 for late reacting normal tissue (RBE var ). Mean dose to Brain-CTV, the brainstem V54GyRBE var and D 1cc for the brainstem, were compared between the two plans using Wilcoxon singed-rank test (on a 5% level). Results CTV coverage and brainstem maximum doses was comparable between the two plans using constant RBE. The mean dose to Brain-CTV had a statistically significant reduction of 10%[-20;26] when omitting the distal edge planning guideline. Brainstem V54GyRBE var increased by a mean of 1cc [-0.3;4.9] and D 1cc increased by a mean of 2 GyRBE var [-0.7;4.5] all differences were statistically significant.