I recently read an excellent book "The Emperor of All Maladies--A biography of Cancer" by the oncologist Siddhartha Mukherjee....it won the Pulitzer prize. My take was that the cause is pretty simple. That is there are routinely mutant cells being produced. Most are either unable to further mutate and cause problems or the immune system takes care of them. Some, however, continue to evolve in their subsequent generations and learn how to evade the immune system and find or develop environments in which they can thrive, grow, spread, and do damage.
We know that many factors have been shown to enable cancer to develop and thrive including physical and chemical damage like smoking and environmental conditions like agent orange, asbestoe, sunlight/UV rays, etc.; inflamatory conditions like dietary components, obesity, and even stress; and conditions like HIVAIDS that seriously compromise the immune system. From the book, I surmised that the longer the bad cells can continue to mutate and the less robust the immune system functions the more prevalent the cancer can become. This is seen in age related cancers like breast and prostate cancers and inflamatory cancers like that from smoking and sun exposure.
The outliers that are very frustrating are the cancers that seem to have no reason behind their development like my recently departed niece. Allthough healthy, non-smoker, non-drinker, rigorous exerciser and fairly smart eater; she found at age 41 that she was invaded by a very rare and aggressive form of appendix cancer. Over the next twenty months the cancer quickly and significantly mutated into new forms in direct response to each series of treatments. Initially she was debulked of several pounds of solid abdominal tumor resembling those from advanced colon or ovarian cancer. After significant chemotherapy and a brief remission with no evidence of cancer, it recurred in the form of thousands of small seed-like tumors that couldn't be removed, so more chemotherapy was used. The next round saw lesions developing in her spine for which radiotherapy was used. Then more chemotherapy. Finally during surgery to resection a small intestine blockage, the cancer was found to be a thick mucous that was throughout her abdomen and most organs. At that point treatments stopped and she was in hospice care for about six weeks. Throughout her ordeal, the only cancer that ever showed on a scan, MRI, or blood marker were the early solid tumors and the dpinal lesions. The seed tumors and growth of the malignant mucous were discovered only during the surgery.
Mukherjie reports that cancer is simply a biologic condition of living. Our own body produces the very cells that eventually turn against us. As a parasite, malignancies can be formidable in the ways they can evade our immune system and fool our body (tricking the body to develop new blood supplies), but not smart enough to know when to stop and allow the host to survive indefinately so that the cancer can continue to survive. Overall, he reported the realism that cancer prevalance has not reduced, but that some cancers have become mostly curable (like testicular cancer, some blood and lymphatic cancers, and many skin cancers), and most significantly that many more cancers are far more manageable and less life threathening like prostate, breast, colon, and lung cancers. It remains that many cancers including pancreatic, appendix, and many head and neck cancers continue to be quicky deadly. I found the book informative, entertaing, optimistic, and depressing as it walked through thousands of years of cancer and thetreatments and research. I highly recommend it.