ESTRO 2020 Abstract book

S508 ESTRO 2020

control (LC) and distant brain failure (DBF) were assessed using the Kaplan- Meier method and log-rank test (p value<0.05). A multivariate analysis was done using the Cox proportional hazard model for most important prognostic factors including age, tumour type, Karnofsky performance status (KPS), dose/fraction, tumour size at diagnosis, Her2 status, postoperative residual enhancing tissue at MRI, interval time between surgery and start of fSRT. Radionecrosis (RN) was assessed by perfusion MRI. Results Median age at diagnosis was 64 years (range 32-80 years). Median follow up was 16.5 months (range 1-48). One-year LC rate was 68,5% and 63,4% for the 7.7Gy and 6Gy group, respectively. One-year DBF rate was 47% and 41% for the 7.7Gy and 6Gy group, respectively. The Kaplan Meier analysis didn’t show any difference between the 2 groups of fractionation regimen for LC (p=0.9) and DBF (p=0.2). Cox regression analysis showed that significant factors were tumour dimension >3 cm (p=0.027) for LC and timing between surgery and start of fSRT (>30 days) for both LC (p=0.037) and DBF (p=0.001). Conclusion The one-year local failure rate might be explained by the low prescription schedules used. On multivariate analysis, tumour dimension (>3 cm) and a delay >30 days between surgery and start of fSRT have a significant impact on LC and DBF. A larger number of patients and a longer follow up are necessary to define the dose and PIL adequate to obtain a good LC in postoperative treatment of brain metastases with fSRT. PO-0860 Hippocampal dose reduction while treating brain gliomas using 3DCRT S. ElsharkawY 1 , A. Abo Gabal 1 , A. Hassouna 1 , M. Mokhtar 2 , M. Hassan 3 1 National Cancer Institute- Cairo University, Radiation Oncology, Cairo, Egypt ; 2 National Cancer Institute- Cairo University, Radiation Oncology Physics, Cairo, Egypt ; 3 Kasr Alaini- Cairo University, Clinical Oncology, Cairo, Egypt Purpose or Objective Routinely the hippocampus is not a contoured OAR when treating brain gliomas using 3D conformal radiotherapy (3DCRT) despite the expected radiation induced hippocampal damage. We proposed this dosimetric study to analyze the possibility of reducing the hippocampal dose using 3DCRT, rather than more advanced techniques like IMRT and VMAT proving feasibility in previous studies. Material and Methods This is a dosimetric study of 29 cerebral brain glioma patients who presented to the Radiation Oncology Department, National Cancer Institute, Cairo University from January 2018 to January 2019. Twenty three had high grade brain gliomas and 6 had low grade gliomas. For each patient, two plans were created. The first plan was done before contouring the hippocampus. The second plan was done after hippocampal contouring following the RTOG 0933 guidelines. The two plans were then compared in terms of dose reduction to the contralateral hippocampus.

Figure 1: Axial views of isodose coverage of conventional 3DCRT plan vs. hippocampal avoiding 3DCRT plan for one patient. Hippocampus: Orange, Hippocampal Avoidance Zone: White, PTV: Green. Left: Isodose coverage of conventional 3DCRT (RT Hippocampus D mean 29.33Gy, D max 33.76Gy). Right: Isodose coverage of hippocampal avoiding 3DCRT (RT Hippocampus D mean 18.72Gy, D max 23.31Gy). Results The hippocampal Dmean was reduced from 21.93Gy to 13.83Gy, with a 32.1% reduction (p value <0.0001). The hippocampal Dmax was reduced from 35.04Gy to 23.58Gy, with a 30.4% reduction (p value <0.0001). By further analysis a higher reduction was achieved in the low grade glioma subgroup with a 48.15% and 45.4% reduction of the Dmean and Dmax, respectively. PTV to hippocampus distance was not of a statistical significance in hippocampal Dmean reduction; however, there was a significant Dmax reduction with distances of ≥ 8 mm. PTV’s volume and tumor site had no significance on reduction feasibility.

Figure 2: Box plot graphs of hippocampal Dmean and Dmax in conventional 3DCRT plans vs. hippocampal avoiding 3DCRT plans. Right: hippocampal Dmean. Left: hippocampal Dmax. Conventional 3DCRT: Blue, Hippocampal Avoiding 3DCRT: Red. Conclusion Contralateral hippocampal sparing is feasible by 3DCRT. Routine contouring of the contralateral hippocampus as an OAR is advised especially in low grade gliomas, which are expected to have a long survival.A clinical trial is being designed at our institute to evaluate the clinical impact. PO-0861 Survival outcomes after upfront stereotactic radiosurgery for brain metastases from breast cancer Y. Wang 1 , R. An 1 , C. Gao 2 , A. Singareeka Raghavendra 2 , D. Amaya 1 , N. Ibrahim 2 , J. Li 1 1 MD Anderson Cancer Center- University of Texas, Radiation Oncology, Houston, USA ; 2 MD Anderson Cancer Center- University of Texas, Breast Medical Oncology, Houston, USA Purpose or Objective As systemic therapy for metastatic breast cancer (BC) improves, effective treatment for central nervous system involvement has become a major concern, as 10% ‒ 30% of