Marijuana can significantly relieve cancer symptoms in those with the disease. This has been proven true, and it can relieve pain and nausea especially. Certain strains of the herb are especially good at getting rid of pain and nausea that cancer causes. It has even been reported to get rid of the cancer itself.
Although THC does have anti-carcinogenic properties, there are numerous carcinogens present in most strains, coupled with the damaging effects on lungs with the smoke. So you could say that THC does help, but you have to remember that weed still can cause cancer. I would say the two effects cancel out.
Marijuana is not the cure for cancer. Marijuana makes people high. People like being high. In order to say Marijuana relieves cancer symptoms you would have to accept beer as a medicine. This is because beer makes you drunk thus alleviating pain. Studies showing marijuana relieve cancer are just patients stating they like being high.