Abstract Book

S1064

ESTRO 37

surgical resection (n=13) or postoperatively after partial resection (n=3). Two (12.5%) patients presented RICVs 14 and 24 months (median,19) after PT. CTs and MRIs pre- and post-PT were used to contour organs at risks (OARs) as bilateral ICAs and circle of Willis. A detailed analysis of the vascular toxicity was performed for the patient’s cohort including number of fields, PTV volume, prescribed dose, dose per fraction and delivery technique (SFUD or IMPT). A preliminary quantitative analysis, in terms of dose volume histogram and dose metric, was performed computing the dose metric of each contoured volume of interests (VOIs: OARs and PTV) for all the patients. The LET distribution has been then computed for all the patients of this study, with in-house dedicated software. LET analysis was performed similarly to the dosimetric (DVH based) analysis, evaluating the LET metric (minimum, mean and maximum) of the PTV and the circle of Willis’structures (Tab. 1). Finally, evaluation of LET changes due to range uncertainties has been also computed, introducing a +/-3% variation on the nominal HU values of the planning CT.

Results Quantitative dosimetric results, based on the DVH metric related with the PTV and the circle of Willis’structures, did not reveal any evident dose metric/toxicity correlation. Both patients presenting with RICV toxicities presents max LET values up to 7.9 keV/µm in the vascular structures. The fields arrangement for those two patients did not include contralateral fields (asymmetric fields arrangement). In case of this fields arrangement, the LET hot region (>60%) is localized asymmetrically in the distal range of the fields, involving (based on the size of the PTV) the side of the Willis’ structure with toxicity. Instead, a symmetric fields arrangement, shows how the highest LET regions (smaller in comparison with the asymmetric field arrangement LET distribution) are localized in the anterior/posterior region outside the PTV (Fig.1). Despite the highest max LET values, no significant differences were found the RICV between patients with and without toxicities. LET variations due to range uncertainties was estimated and up to 1.2 keV/µm, especially for patients treated with asymmetric fields’ arrangement.

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). Material and Methods We reviewed a series of 16 CP patients treated with PT to a median dose of 54 GyRBE, for progressive disease after