Yeah, I don't buy that. While it HAS been established that Peter has at least a small radioactive element in his blood, it makes NO sense that he'd have enough of it in him to give someone cancer. If he had that much radioactivity in him, he wouldn't have survived his teenage years - he would've dropped dead years ago. Besides, Peter has worked as a scientist at least two or three times in his life so far, often in laboratories where there would be all kinds of radiation-monitoring devices - geiger counters, things like that. Those things would have been going off like popcorn if he'd had that much of it inside him. The way I look at it is as follows: Peter was GIVEN his powers through a radioactive spider, but that doesn't mean his powers themselves are fueled through radioactivity. The radiation somehow acted as a transformative agent to briefly turn the spider into a sort of power-transferrer. "Somehow - in some miraculous way, his bite has transferred his own power... to ME!", remember? In short, the radiation may have passed on the powers, but effectively, the powers are the result of a mutation brought on by the combination of radiation and spider-bite. Any radiation in his blood is there as a slight, lingering residue from the bite, nothing more. It may have been a bit stronger earlier in his career, back when Aunt May got sick from it, but surely it would have all but dissappeared by the time he reached his sixth decade. I refuse to believe it was ever strong enough to give anyone cancer.