Yes THC is definitely one of the main cancer-fighting cannabinoids but CBD (famous even in mainstream circles lately) and CBG are important as well. Certain types of cancer respond to different combinations of cannabinoids. A good general starting point is equal parts CBD and THC. There's also a combined therapeutic effect when dozens of cannabinoids work together in various ways, along with the therapeutic effects of the terpenes (non-cannabinoids organic compounds found in cannabis, responsible for the aroma and flavour). It's called the entourage effect, and it's why some patients prefer a "whole plant extract", rather than specific preparations of pure THC, CBD, etc. For a good general overview, here is a presentation (with 200+ really good links to reputable universities and hospitals) on the topic I did with my wife @MediKatie.