RARE-35. MRI FINDINGS AT PROGRESSION IN ADULT PATIENTS WITH MEDULLOBLASTOMA

Progression (PD) in medulloblastoma (MB) represents a diagnostic challenge due to imaging heterogeneity among/within patients. SHH (Sonic hedgehog)-MB is thought to recur mostly within the tumor bed (TB). In children, DWI restriction (DWIr) is more sensitive than contrast enhancement(CE) for first PD. Whether this is applicable to adults is unknown. METHODS: Retrospective review of adults (age ≥18) with MB enrolled to Natural History study at NCI-NOB. Descriptive statistics of imaging at diagnosis and PD(CE, T2/FLAIR signal without CE, DWIr) and imaging patterns for each PD. RESULTS:14 adults with MB: 5 diagnosed in childhood (8-16 yrs), 9 as adults (18-45 yrs); Subtypes: 7 SHH, 3 non-WNT/non-SHH, 4 unknown. Eleven experienced ≥1 PD (6/7 SHH, 2/3 non-WNT/non-SHH, 3/4 unknown); median PD of 5 (range 1-9). Median age at first PD 31 years (range 10-46) with 5 first PDs >5 years after diagnosis. In 10 patients with available baseline MRI, 9 had CE, and 8 DWIr (2 without DWI sequences). Of 48 total PDs, the commonest patterns were: brain LMD alone (n=14), TB alone or distant brain parenchyma alone (each n=7), distant brain parenchyma with brain LMD (n=6), and TB with either distant brain parenchyma or LMD (n=3). Of the 82 PD lesions, 23% (n=14) of brain lesions lacked DWIr, and 37% (n=23) had T2/FLAIR signal without CE. PD tissue confirmation obtained at 18 time points:16 cases with confirmed recurrence had heterogeneous characteristics; in 12 with brain PD: CE in 5, T2/FLAIR without CE in 3 (unknown: 4). 2 CE lesions revealed meningioma (oneatypical meningioma- had DWIr). CONCLUSIONS: Imaging findings in adult patients with MB are highly heterogeneous. Despite high specificity of DWIr for PD in children, it failed for 23% of brain lesions across multiple patients. Most SHH-MB had PD outside the TB, unlike what is widely accepted in the literature.