When my FIL got prostate cancer 18 months ago, my husband asked his doctor that very question. Dad was hale and hearty, no vices, worked out (at 85!!) and was more fit than most 30 year olds. The doctor told my husband that cancer seems to prefer a healthy body for some reason. We lost his Dad last November - devastating to watch him go from a specimen of health to a frail man in a matter of months.
I’m so sorry to hear about your father in law. We lost my husband’s mom and dad within a 47 hour period a couple of years ago. It’s a hard thing.
susie