I was raised with well water, and I didn't even use fluoride toothpaste until I was around 6 or 7.  I had 3 cavities, which they filled, and that was it.  I went to the dentist yearly for my cleanings, until I was 45, then went twice a year for cleanings.  I think they replaced my fillings twice in my lifetime.  The problems I'm having now, are not due to lack of proper care. 
The first time I went through chemo, I went to the dentist just prior to treatment, and made sure my mouth was in tip top shape.  My dentist warned that sooner, or later I would most likely have issues.  It affected my salivary glands, so I don't produce enough saliva.  This time, I went and got everything done, but the dentist warned me again, that I would have more issues.  I did.  It's just the nature of the beast, not a lack of good dental hygiene, or anything genetic.