I believe a THC + CBD combo is the most effective way for treating cancer. CBD alone does have some effects however CBD is used for a lot of other things (like seizures for example), while THC is more often used for pure cancer.
I know of someone that used rso tabs as supositories for prostate cancer. Built tolerance up to a fairly large dose used daily. Cancer was in remission in 6 months. Completely clear in a year.