This was a topic I stumbled into, but have been researching for some time now. I have been motivated somewhat after losing my father to Lung Cancer several years ago after standard Chemotherapy failed.
What I have found has been rather fascinating. There is a special class of dietary supplements, known as 'medicinal mushrooms', which tend to come from the Far East.
In summary: almost all of them have shown potency in stopping various forms of cancer.
The reason is simpler than you think: they all heavily up regulate the body's immunity (specifically, the cellular immunity, which basically kills off all abnormal cells inside the body, ie. tumours).
Ones that have been cited the most: 1) Grifola Frondosa (Maitake), 2) Agaricus Blazei, 3) Lentinula Edodes (Shiitake)
Feel free to check these out on NCBI if you're skeptical. Literally just google ''Grifola frondosa cancer ncbi'' and see what comes up.
Important note: If you are a cancer sufferer, or a relative, I have no idea about how these supplements may interact with standard chemotherapy drugs. But if you're interested, feel free to consult with a doctor or research more online.
The supplements themselves are easy enough to get, they're quite common in standard health food stores.