The following study documented cannabis inhalation to treat astrocytoma that could not be fully removed surgically, and no other treatment was used to shrink the tumors, so it has to be called "spontaneous regression" even though we're confident it was the cannabis inhalation, it can't be proven as cause in such a small sample size.

Other types of brain cancer like glioblastoma cannot currently be cured with either cannabinoid or standard therapy, but treatment breakthroughs are being made combining them (THC, CBD, and temozolomide) https://www.ncbi.nlm.nih.gov/pubmed/30195736

Inhalation as a route of administration should not be overlooked in brain cancer. Many people assume suppository and edible oil are best for treating cancer, which is true, it raises the blood levels, but whenever possible, direct contact is best like topicals for skin cancer, suppositories for below-the-belt cancers, and inhalation through nose and mouth for sinus, head, and brain cancers. Since smoke is also a carcinogen, luckily we have vapour for this route.