Think long term diseases like that are outside the scope of the game for the most part.
Could one even resist cancer?
Why not? I can't think of any real-world examples of someone "getting over" rabies without immediate and serious medical intervention, but isn't it listed as a disease in PF with a number of successful saves curing you?
You have to remember that even a 1st level PC is WORLDS above you and I. And once you hit around 3rd level or so, the list of extraordinary feats they are capable of only grows at exponential rates. If a character can survive the very touch of death itself in the form of negative energy attacks, why shouldn't their bodies be able to resist something like cancer which, to them, would be relatively "mundane," so to speak.
As far as a save for it? This is a circumstance where I would probably base it off the character themselves. Cancer is a part of your own body that has grown and mutated out of control; from a balance and "in-game" perspective, it makes a certain amount of sense to me that the healthier you are normally, the healthier your cells are, the hardier they are even after they mutate.
Maybe 10 + 1/2 your HD + your Constitution modifier? I add the CON modifier as well as HD to reflect the idea that no matter how healthy you are, it can bite you in the ass w/ this rendition of cancer.
This is a really touchy subject, and I wouldn't recommend dropping this on your group. A lot of people are in fear of losing relatives to cancer or have lost them already, and I can easily imagine one person at the table sitting quietly while the group makes jokes about just needing another +2 to make the save, or the paladin boasting that everything's alright because she can cure cancer.
Although mechanically, there's nothing preventing long-term diseases existing. Rabies can go undetected for years. But it's not something that most groups will see used for or against PCs, given the short lifespans of many campaigns. My last 1-17 campaign elapsed in an in-game space of several months, so my character could have had rabies and never known.
Then there's the difficulties and paperwork of remembering to make the save once every [very long time].
Long term diseases such as cancer should probably be treated as a form of aging -- basically incurable without powerful magic. One way to warn a player that his venerable character has only a year or two left would be to inform him that he has come down with cancer. A younger character should not come down with cancer unless there is a very good in-game reason to make him terminally ill.
There were rules on diseases in the 1e DMG. Perhaps, if you wanted to go down this route you could amalgamate the old rules with the PF mechanisms. At the end of the day though, a cure disease would easily solve it in game before it had any impact and Troubleshooter's point about it being a touchy subject is well made.
