ESTRO 2023 - Abstract Book

S1738

Digital Posters

ESTRO 2023

Materials and Methods Adult patients from two institutions with a primary RCC underwent renal SBRT planning using VMAT, CK and PBS PBT platforms. For VMAT and CK, a PTV margin of 5mm from ITV and CTV respectively was used. For PBS, the plans were evaluated under robustness for uncertainties of 5mm (geometric) and 3.5% (range) using ITV. A dose prescription of 42Gy in 3 fractions for tumours >4cm was used in line with FASTTRACK II (NCT02613819) clinical protocol. Cases were stratified to left and right-sided tumours. Dose constraints were in line with national clinical guidance. The aim was to achieve 95% coverage of the target volume with 42Gy, with a GTV Dmax of between 123-143%. Benefits and challenges of each technique in minimising dose to organs at risk were evaluated. Doses to skin, spinal cord, cauda equina, small bowel, duodenum, large bowel, stomach, liver, pancreas, spleen, ipsilateral normal and contralateral kidney were recorded. Means, range and SDs were calculated. Results Mean target volumes were 388.48cm3, 268.11cm3 and 388.73cm3 for VMAT, CK and PBT respectively. CK volumes were smaller, as it required a PTV expansion from CTV only. There was equivalent target coverage using all 3 platforms. However, regardless of technique, limitations were noted in achieving adequate dose to the target volume when bowel loops are immediately adjacent. PBT was able to achieve most sparing of the large bowel; mean D0.1cc of 14.03Gy (0-28.06) and 16.27Gy (5.04-27.5) for PBT and VMAT respectively. An overall trend towards superior OAR sparing was seen with PBT, but with higher skin doses. Conclusion Clinically acceptable target volume coverage was achievable and comparable with VMAT, CK and PBT. Bowel within or immediately adjacent to the target volume is a major limitation to achieving the prescription dose. Strategies that include an adaptive approach can be explored. Using PBT as a modality offers potential to decrease mean dose to OARs and may permit treatment in cases where the OAR constraints are challenging to meet. Further work will explore its utility with different patient cohorts in primary RCC. A. Edvardsson 1,2 , K. Andersson 3 , C. Vallhagen Dahlgren 4 , J. Gorgisyan 1,2 , A. Dasu 4,5 , T. Björk-Eriksson 6,7 , P. Munck af Rosenschöld 1,2,8 1 Department of Hematology, Oncology and Radiation Physics, Skåne University Hospital, Lund, Sweden; 2 Medical Radiation Physics, Department of Clinical Sciences Lund, Lund University, Lund, Sweden; 3 Department of Medical physics, The Skandion Clinic, Uppsala, Sweden; 4 Department of Medical physics, The Skandion Clinic, Uppsala, Sweden; 5 Medical Radiation Sciences, Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden; 6 Department of Oncology, Institute of Clinical Sciences, Sahlgrenska Academy at the University of Gothenburg, Gothenburg, Sweden; 7 Regional Cancer Centre West, Western Sweden Healthcare Region, Gothenburg, Sweden; 8 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Purpose or Objective Craniospinal (CS) radiotherapy (RT) is a part of the standard treatment for pediatric medulloblastoma patients. Protons reduce the dose to many organs-at-risk compared to photons, however, the whole brain still receives full prescription dose which results in a high risk of neurocognitive impairment. To reduce these side effects hippocampal-sparing (HS) CS proton pencil beam scanning (PBS) RT has been proposed, which uses complex treatment plans with sharp dose gradients. The aim of this study was to dosimetrically verify HS PBS proton plans. Materials and Methods Thirteen pediatric medulloblastoma patients previously treated with CS RT were included. In this study, the CTV included the cranial target. Prescribed dose was 23.4 Gy(RBE) in 13 fractions. Proton PBS plans with two lateral and one posterior field (fig 1a) were created in Eclipse (Varian Medical Systems). For each patient, one HS plan with a mean dose of 9 Gy(RBE) to the hippocampi and one conventional plan with homogeneous dose distribution were created. The plans were recalculated in a water phantom and measured field-by-field using the MatriXX PT detector (IBA Dosimetry) and solid water blocks. The measurements were performed at water equivalent depths (WED) corresponding to the central part of the hippocampi (fig 1a). Both the HS and conventional plans were measured for all patients with the standard resolution of the detector (7.6 mm), in total 130 measurements. The measured and planned dose distributions were compared using 2D global gamma evaluation (3%/2mm, threshold 5%) and the resulting pass rates were compared using a paired Wilcoxon test. Ten fields for three HS plans were measured with higher resolution (3.8 mm) by shifting the detector. The measured and planned dose distributions were compared using 2D global gamma evaluation (3%/1mm) within ROIs corresponding to the HS area (ROI_HS) and a surrounding ring (ROI_ring) (fig 2). PO-1971 Quality assurance of hippocampal-sparing craniospinal proton plans for pediatric medulloblastoma