While I lost my grandfather recently due to pancreatic cancer, and it was absolutely depressing, the byproduct of killing the host in hopes of killing the cancer is the worst part of it, not the diagnosis. Do not take me wrong, I certainly believe the study of medicine is a blessing in this modern era, but I believe in the older days, I sure many people who were diagnosed at death: "died of natural causes," were actually victims of cancer, and went away without the toxins and agony.
However, I've always felt that drowning and suffocating would be the worst in terms of physical pain and panic to the victim, but stands nowhere near cancer in terms of emotional pain and agony.