ESTRO 2021 Abstract Book

S471

ESTRO 2021

exclusively intra-hippocampal. Median time to recurrence was 7 months (range 0-23). After a median follow up of 6.3 months (range 0-23), 68 pts (57%) were dead, 32 pts (27%) had a relapse. The most common toxicities were headache, fatigue, and nausea. Overall, 10/120 (8%) pts experienced a G1 toxicity and 2/120 (2%) a G2 toxicity. In cohort 2, median dose to hippocampus was 7.41 Gy (range 6.4-9.1), median volume was 2.26 cc (range 0.74-8.38) and maximum dose was 14.54 Gy (range 11.59-20.4). One pt (3%) experienced extra- hippocampal brain disease. Conclusion Given the growing potential for prolonged survival with metastatic disease, identifying strategies to reduce treatment-associated side effects is of increasing significance. In the present study, according with published literature, HS-WBRT shows a durable intracranial disease control with tolerable side effects. A small risk of additional intra-hippocampal recurrence seems to be justified by memory preservation reported in numerous studies on HS-WBRT. PH-0605 Role of hippocampal location and radiation dose in glioblastoma patients with hippocampal atrophy N. Georges 1 , C. Le Fevre 2 , M. Loit 3 , A. Keller 4 , H. Cebula 5 , D. Antoni 4 , A. Thiery 6 , J. Constans 7 , F. Proust 5 1 ICANS, radioation oncology, Strasbourg, France; 2 ICANS, Radiation oncology, Strasbourg, France; 3 CHU Hautepierre, Neurosurgery department, Strasbourg, France; 4 ICANS, radiation department, Strasbourg, France; 5 CHU Hautepierre, Neurosurgey department, Strasbourg, France; 6 ICANS, Statistic department, Strasbourg, France; 7 CHU Amiens, neurology department, Amiens, France Purpose or Objective The hippocampus is a critical organ for irradiation. A decline in cognitive functions has been correlated with the radiotherapy dose delivered to both hippocampi. Thus, we wanted to explore the changes in hippocampal volume according to the dose delivered to each hippocampus and their location relative to the glioblastoma Materials and Methods All patients were treated for glioblastoma with surgery, concomitant radiotherapy, and temozolomide and adjuvant temozolomide. The irradiation dose was 60 Gy in 30 daily fractions. Hippocampi were retrospectively delineated on three MRIs, performed at baseline (i.e., just before radiotherapy), at the time of relapse, and in the last MRI available at the end of follow-up. A total of 98, 96, and 82 hippocampi, respectively, were measured in the 49 patients included in the study. The patients were stratified into three subgroups according to the dose delivered to 40% of the hippocampus. In the group 1 (n=6), the hippocampal D 40% was < 7.4 Gy, in the group 2 (n=13), only the H contra D 40% was < 7.4 Gy, and in the group 3 (n=30), the D 40% for both hippocampi was > 7.4 Gy. The hippocampi were further split into four subgroups: D 40% , < 7.4 Gy, between 7.4 Gy and <30 Gy, between <=30 Gy and <50 Gy, and <50 Gy. Results Regardless of the time of measurement, hippocampal volumes homolateral to the tumor were always significantly lower than those contralateral to the tumor. Regardless of the side, the volumes after the last dose were still significantly lower than those measured at baseline. There was a significant correlation among the decrease in hippocampal volume regardless of its side, and D max (p = 0.001), D 98% (p = 0.028) and D 40% (p = 0.0002). After adjustment for the time of MRI, these correlations remained significant. According to the D 40% and volume at MRI last , the hippocampi decreased by 4 mm 3 /Gy overall. However, these changes were not linear when the doses were stratified into the into four subgroups (<7.4 Gy, between >=7.4 Gy and <30 Gy, between >=30 Gy and <50 Gy, and >=50 Gy). The slopes were +94.3 mm 3 /Gy, -8.6 mm 3 /Gy, -44.5 mm 3 /Gy, and -112.2 mm 3 /Gy, respectively Conclusion Tumors seem to impact the hippocampal volume. There was a significant relationship between the radiotherapy dose and decrease in hippocampal volume. However, at the lowest doses, the hippocampi seem to exhibit an adaptive increase intheir volume, which could indicate a plasticity effect. Consequently, shielding at least one hippocampus by delivering the lowest possible dose is recommended so that cognitive function can be preserved. PH-0606 18F-FDG hippocampal metabolic preservation following hippocampal-sparing PCI in SCLC patients S. El Chammah 1 , G. Allenbach 2 , R. Jumeau 1 , S. Boughdad 2 , J.O. Prior 2 , M. Nicod Lalonde 2 , N. Schaefer 2 , M. Meyer 2 1 CHUV, Radiation Oncology, Lausanne, Switzerland; 2 CHUV, Nuclear Medicine, Lausanne, Switzerland Purpose or Objective Prophylactic cranial irradiation (PCI) in patients with small-cell lung cancer (SCLC) is associated with an increased survival. However, it is also associated with cognitive impairment although the underlying mechanisms remain poorly understood. Our study aims were to evaluate the impact of PCI and the potential benefit of hippocampal sparing (HS) on brain metabolism assessed by 18 F-Fluoro-Deoxy-Glucose Positron Emission Tomography/Computed Tomography ( 18 F-FDG PET/CT). Materials and Methods We retrospectively included 22 SCLC patients. Among these patients, 11 (50%) had hippocampal-sparing PCI (HS-PCI). All patients received a standard PCI dose of 25 Gy in 10 fractions. 18 F-FDG PET/CT was performed on average 145±73 days before and 383±451 days after PCI. Brain 18 F-FDG PET scans were automatically segmented in 12 regions using Combined-AAL Atlas from MI-Neurology Software (Syngo.Via, Siemens Healthineers). For all atlas regions, we computed SUV Ratio using brainstem as a reference (SUVR = SUVmean / Brainstem SUVmean) and compared SUVR before and after PCI, using a Wilcoxon test, with a level of significance of p<0.05.