Heightened risk

The body has some built-in cancer fighting capabilities. These have some fairly high probability of destroying mutant cells--though not 100 percent, so if your lifestyle exposes you to lots of mutagens something is more likely to get past the defenses. And presumably these defenses get weaker with aging and other bad habits. Once something gets past, ouch.

How likely is it that more than one type of cancer (not metastasis of the same!) develops?

Estimates: 1 in 20 within 6 months; 1 in 5 lifetime.

Time for a little back-of-the-enveloping.

For someone in the 70-74 range (averaging the sexes; men have higher rates), the population average for developing cancer is 2%: 1 in 50. That range includes 10 6-month periods, so estimate 0.2% chance per 6 month period, just from the normal incidences. The 1 in 20 cited above is 25 times bigger than normal.

Some of that may be due to the insults to the body from radiation or chemo, both as mutagens and as reducing the body's capacity to fight off mutants.

If I assume a 70-year-old, and if 13 years is the residual lifespan, and I use the same numbers as before, ignoring the rise in rate, then the expected lifetime risk is about .05, or 1 in 20. So having cancer increases the lifetime risk of a different cancer by about a factor of 4--better than the 25 above.

That seems consistent with a temporarily increased risk (e.g. chemo weakening the body) on top of a more generic increased risk.

Remember the rule: Where you see one problem, look for another.