Of 135 shuttle missions, 2 resulted in the death of all crew, giving a confidence interval of something like 1-4% for the mortality risk of each flight. Any sane assessment of the first Apollo landing missions would put the subjective mortality risk at 10% at the very least. (Later landings were somewhat safer, subjectively, since the equipment was proven.)

I can't imagine the in-mission mortality risk of a Mars mission would realistically be less than 5%, and 15% would not be unacceptable to me. A 10% in-mission risk equates to something like 5 years of lost life in expectation.

In comparison to this, the risk of cancer seem small or comparable. Yes, you'll need to provide sufficient shielding to prevent aggressive cancers from probably developing during the mission, or ones that will reliably lead to death before 60. But I'm not at all surprised that we may need to accept a much higher cancer risk than we would for a normal occupation. This doesn't strike me as one of the key challenges.