Some feeble research studies have ferreted out the good sides of marijuana. It has been said to be a drug for HIV for instance. But no strong clinical support.
I found nothing about cannabis and medulloblastoma, but there are several findings about cannabis and other brain tumors like glioblastoma and gliom as well as non-brain cancer. Maybe these references could be helpfull
Glioblastoma
Guzman M, Duarte MJ, Blazquez C, Ravina J, Rosa MC, Galve-Roperh I, Sanchez C, Velasco G, Gonzalez-Feria L. A pilot clinical study of Delta(9)-tetrahydrocannabinol in patients with recurrent glioblastoma multiforme. Br J Cancer, 27. Juni 2006
California Pacific Medical Center Research Institute, San Francisco, USA. Singer E, et al. Cell Death 2015;6:e1601. http://www.ncbi.nlm.nih.gov/pubmed/25590811
Marcu JP, Christian RT, Lau D, Zielinski AJ, Horowitz MP, Lee J, Pakdel A, Allison J, Limbad C, Moore DH, Yount GL, Desprez PY, McAllister SD.Cannabidiol enhances the inhibitory effects of Delta9-tetrahydrocannabinol on human GLIOBLASTOMa cell proliferation and survival. Mol Cancer Ther 2010;9(1):180-9.)
Schley M, et al. Brain Res Bull 2009;79(5):333-7.
GW commenced a Phase 1b/2a clinical trial for the treatment of Recurrent Glioblastoma Multiforme (GBM).