That's... something I'd been thinking about, too.
We know parasites and poisons don't mind the dirt. But since we've added so much BS about the supercharged immune system, we gotta admit, that under the current explanation of the healing effect, it would be unlikely, unless the dirt's ability to enhance the immune system also included making it capable of identifying cancerous cells.
http://www.cancer.org/treatment/treatmentsandsideeffects/treatmenttypes/immunotherapy/immunotherapy-immune-system
So if you think the dirt boosts your immune system, there's a shot it also helps fight cancer. Given the risk of cancer increases with age (simply because the older you are, the more times the cells in your body have divided, risking cancer every time), and we've never heard of predators getting cancer, it wouldn't be unfounded to assume that the Felaryan dirt does fight cancer.
Then again we could also assume predators do get cancer and never notice, because they don't have a healthcare system, and only notice anything is wrong when they die because they don't have enough energy to fight off a kensha beast.
If on the other hand you think the Felarya health benefits are magical and just a blanket term, then since cancer's a "disease", it could be prevented, fought, and even beat by the dirt. It could also be that if you fight it too late, cancer still wins.
But being trapped in this living hell, still ravaged? No, not likely, unless you consider the dirt would stabilize the progress of the disease. Which would be rather odd, it only happens with old age and missing bits far as I know. That's not little- old age can be a world of pain, even without cancer.
Alternatively, you could say cancer is not different enough from healthy cells for the dirt to do anything about it. In that case, you'd have a very healthy person with a very healthy cancer that would kill them like a predator does to its prey. And that's the circle of life.
If it works, I'd expect something on the level of a complete remission, rather than a partial remission, but again, this is uncharted territory, Nick.
http://www.webmd.com/cancer/remission-what-does-it-mean
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If someone were to arrive in Felarya and found the dirt doesn't do it all, though, I'd wager they could always look for local healers. There may be more advanced medical doctors (or priests of Pelor
) in Felarya, with different resources to deal with what ails you, and if not, magic can do weird stuff. Could be worth a shot, in spite of the numerous snake oil salesmen, acupunturists, homeopaths, swindlers and quacks you'd obviously find.