MB-16 * BOOST VOLUME EFFECTS ON INCIDENTAL HIPPOCAMPAL DOSE IN PEDIATRIC MEDULLOBLASTOMA

Abstract

INTRODUCTION: Radiation treatment for medulloblastoma, which involves craniospinal radiation with posterior fossa boost, has been associated with neurocognitive sequelae, possibly due to impaired hippocampal neurogenesis. The posterior fossa has recently been shown to be radiation tolerant with respect to neurocognitive outcomes, though previous studies highlight effects of tumor bed vs. whole posterior fossa boost volumes. The incidental dose to the hippocampus may account for the boost volume effects, and despite traditional boost volume categories, the relative volume of posterior fossa treated may vary significantly. This study analyzes the association of hippocampal dose and boost volume as a continuous variable. METHODS: Bilateral hippocampi and posterior fossa were contoured on T1 axial images for 25 medulloblastoma patients. Minimum dose received by 100% of each hippocampus (D100) and percent volume of posterior fossa receiving 100% of boost dose (V100) were extracted from dose-volume histograms. Analysis of covariance was used to investigate the effect of V100 as a continuous variable while controlling for total craniospinal dose categorically (Low Dose 18-23.4Gy, High Dose 36Gy). RESULTS: Patient characteristics were as follows: age 3-21Y; 5 female, 20 male; 15 average risk (7 18Gy, 8 23.4Gy), 10 high risk (36Gy). Right and left total incidental hippocampal biologically effective doses were both greater for the high dose group by 13.9Gy (p < 0.0004) and 14.0Gy (p < 0.0001) respectively. Right and left D100 significantly varied with V100 by 0.18Gy (p < 0.0218) and 0.15Gy (p < .0322) per percent volume respectively. CONCLUSION: Incidental hippocampal doses are positively associated with boost volumes and may account for neurocognitive decline in medulloblastoma patients. An estimated 11-13% boost volume increase may further impair neurogenesis for a 2Gy threshold. Posterior fossa V100 can be a useful metric to more accurately describe boost volumes, and new hippocampal sparing techniques may allow for greater posterior fossa coverage.