Traditional MR methods for staging, pre-operative planning and post-therapy surveillance of brain tumors rely largely on qualitative comparisons, limiting accuracy in diagnosis and the measurement of longitudinal changes. Magnetic resonance fingerprinting (MRF) allows rapid, simultaneous mapping of multiple tissue properties, including relaxation times (T1 and T2) and proton density.[1] MRF has been shown to distinguish pathology and tumor grade in adults with primary and metastatic brain tumors.[2] However, brain tumors in children and young adults are often pathologically distinct and frequently rely on non-surgical treatments to forestall growth.[3] A fast, quantitative method to identify tumor pathology and measure the effect of treatment would be an important diagnostic and therapeutic tool. We examined children and young adults with primary brain tumors to determine whether MRF can measure differences in histologic grade and measure treatment effects.Subjects with primary brain tumors were scanned on a 3T scanner (Siemens, Skyra) using a 20 channel head coil in an IRB-approved study. A FISP based MRF sequence [4] was used to create T1 and T2 maps for 7-10 slices acquired in each subject. Longitudinal surveillance scans with or without chemotherapy were performed. Scan parameters included field of view 300x300 mm2, resolution 1.17x1.17 mm2, slice thickness 5 mm, flip angle 0 to 74 degrees, TR 12 to 15 ms, sinc pulse with duration of 2000 ms, and time-bandwidth product of 8. Total acquisition time was 41 seconds/slice. Regions of interest included tumor tissue, the peritumor region 1cm adjacent to tumor, and contralateral white matter (CWM) (Figure 1). In the absence of enhancement, T2 hyperintensity with mass effect was used to define the extent of tumor. Diagnosis was based on clinical presentation and MR spectroscopy when pathology was impractical (eg, for optic pathway gliomas). Wilcoxon tests were used to compare paired mean relaxometry values between tumor and CWM; Mann-Whitney tests were used to compare parameter distributions between histologic groups.6 subjects (3 male, 5 children (1-14years) and 1 young adult (34years)) underwent a total of 11 scans. Tumors included 3 low-grade tumors (2 optic pathway glioma, 1 thalamic glioma) and 3 high-grade tumors (primitive neuroectodermal tumor, atypical teratoid/rhabdoid tumor, anaplastic astrocytoma). Mean (±SEM) T1 and T2 for tumors differed significantly from CWM: T1 1586(±167)ms vs. 980(±76)ms (p=0.028) and T2 67.6(±10.2)ms, vs. 45.3(±3.2)ms ( p=0.046). T1 and T2 of peritumor regions also differed from CWM: T1 1669(±180)ms vs. 980(±76)ms (p=0.043) and T2 76.9(±17.2)ms vs. 45.3(±3.2)ms (p=0.043). T1 and T2 relaxation times were significantly different between histologic grade (high- vs low-): T1 2340 (±309)ms vs. 1255(±115)ms (p=0.049) and T2 156.0(±80.4)ms vs. 48.3(±1.8)ms (p=0.049) (Figure 2). 3 tumors (2 treated, 1 untreated) were followed longitudinally over time. Treatments included surgical decompression and chemotherapy. T1 in treated tumors changed 14% and 17% but only 6% in untreated tumors, similar to treatment effects shown in previous preclinical tumor models (Figure 3). [5,6]This study describes the first use of MRF in pediatric patients and the first description of treatment effects using MRF. Despite the small cohort, MRF-derived relaxation times distinguished tumor from CWM and differentiated tumor grade. Treated tumors appeared to have larger changes in T1 values than untreated tumors. Therapy-related changes in low- and high-grade tumors were in opposite directions, which may be related to treatment modality and will be further explored as subject accrual continues. MRF may be an important tool for the characterization of pediatric brain tumors and the evaluation of treatment effects. Future studies will determine whether treatment effect correlates with clinical outcome. This study continues to accrue pediatric, adolescent and young adult subjects and follow current subjects to expand our cohort and develop our longitudinal data.MRF was able to characterize pediatric and young adult brain tumors and measure the effect of treatment on tumor tissue. MRF may be an important tool in the early evaluation of treatment efficacy.