GERM-19. DIAGNOSTIC EXPOSURE TO LOW-DOSE RADIATION AND SPONTANEOUS REGRESSION IN INTRACRANIAL GERM CELL TUMORS

BACKGROUND: Spontaneous regression in intracranial germ cell tumors has been reported in some literatures, but the mechanism has not been well known. We retrospectively measured the tumor volume before chemoradiotherapy and analyzed factors that influence shrinkage of tumor volume. PATIENTS AND METHODS: Thirty-nine cases with primary intracranial germinomas and HCG-producing germinomas were treated in our hospital from 1994 to 2017. In twenty-one of them, plural MRI scans were done before the first course of chemotherapy regimen. Their age ranged from 8 to 31 years. Three cases were bifocal type, one case had multiple lesions. Biopsies were performed in all regions. Tumor volume of twenty-four lesions was analyzed by volumetric assessment based on MRI. Ratio of volumetric change between the first MRI and the scan immediately before chemotherapy was defined as shrinking rate (%). Period between disease onset and the first chemotherapy was 20 to 47 days. Diagnostic radiation dose was calculated in each case. RESULTS: Initial tumor volume ranged from 0.962 to 72.356 cubic centimeters (mean: 9.67). Diagnostic radiation dose: 52.2 to 910.1 mGy. Shrinking rate ranged from -57.8 to 85.4% (mean: 28.7). In only 7 regions (29.1%), shrinking rate was within 30%. Shrinking rate was significant positively influenced by diagnostic radiation dose (p