ESTRO 37 Abstract book

S1065

ESTRO 37

EP-1957 Proton grid therapy: LET and variable RBE- weighted dose distributions for interlaced beams J. Odén 1,2 , T. Henry 1 1 Stockholm University, Medical Radiation Physics, Stockholm, Sweden 2 RaySearch Laboratories, Department of Research, Stockholm, Sweden Purpose or Objective Grid therapy uses small and spatially fractionated radiation beams to improve the outcome of radiotherapy treatments. It relies on the biological observations that using such geometries, with unirradiated parts in between small radiation beams, have the possibility to improve the radiation tolerance of healthy tissues. Our research group has explored the possibility to clinically use interlaced proton beam grids to achieve very heterogeneous dose in the normal tissue, while keeping a high homogeneity in the target dose (Henry et al. 2016). So far, this has been explored assuming a constant RBE of 1.1. The aim of this study was to compare distributions of the LET d and the RBE-weighted dose of proton grid plans with conventional IMPT plans, assuming a constant RBE=1.1 and several variable RBE models. Material and Methods Conventional IMPT and proton grid plans were generated for four prostate cases. The PTV was defined as an isotropic 5 mm expansion of the prostate, resulting in PTV volumes between 50-215 cm 3 . For both modalities, two lateral fields were used to deliver a homogenous dose of 78 Gy (RBE) in 39 fractions to the PTV, assuming RBE=1.1. The grid plans were optimized to achieve similar dose coverage as the IMPT plans. The clinical goals for the PTV was D 98% ≥74.1 Gy (RBE) and D 2% ≤83.5 Gy (RBE), whereas the normal tissue goals were adopted from QUANTEC. In addition to RBE=1.1, the three variable RBE models (Carabe et al. 2012, Wedenberg et al. 2013 & McNamara et al. 2015) were used for plan evaluation. An α/β of 1.5 Gy was assumed for the PTV, while 3 Gy was used for the normal tissues. The LET d distribution was Monte Carlo calculated for all plans. Results All IMPT and grid plans met the clinical goals assuming RBE=1.1. The conformity and homogeneity of the PTV was slightly better for the IMPT plans compared to the grid plans (Figure 1a & b). The LET d distribution of the grid plans followed the interlaced pattern of the beams, giving rise to localized high LET regions at the PTV border. This contrasted with a more spread-out high LET region around the PTV of the IMPT plans (Figure 1c & d). Yet, the different LET d maps did not produce significantly distinctive RBE-weighted dose distributions when considering the variable RBE models, as seen in Figure 2 assuming the Wedenberg RBE model for a representative case. The PTV coverage was sufficient for both techniques with similar rectum DVHs (Figure 2c). The mean dose to the femoral heads was kept similar between the two delivering techniques, although the dose pattern within the volume naturally differed (Figure 1 & 2). This resulted in the different shapes of the femoral heads DVHs in Figure 2c.

Conclusion This study showed that despite unconventional beams arrangement and high localized LET regions, proton grid plans deliver dose distributions that are similar to regular IMPT plans, independently of the RBE model considered (constant or variable). This goes in favor of a clinical implementation of proton grid therapy as presented here. EP-1958 LET evaluation for pediatric craniopharyngioma with cerebral vasculopathies after PBS proton therapy L. Placidi 1 , A. Pica 2 , F. Ahllhelm 3 , M. Walser 2 , A. Lomax 2 , A. Bolsi 2 , D. Weber 2 1 Fondazione Policlinico Universitario A. Gemelli, Medical Physics, Roma, Italy 2 Paul Scherrer Institute, Center for Proton Therapy, Villigen, Switzerland 3 Cantonal Hospital Baden, Department of Radiology, Baden, Switzerland Purpose or Objective The current study aims at analysing the dosimetric and Linear energy transfer (LET) correlation in paediatric craniopharyngioma (CP) patients with and without radiation-induced cerebral vasculopathies (RICVs) treated with pencil beam scanning (PBS) proton therapy (PT).