Leukemia doesn't affect many enough people in reproductive age to cause a population explosion. Basically, no medical advance can cause population explosion in the rich world: there are very few people dying from disease in reproductive ages there. Making people who would otherwise die at 55 live through 95 does not make a population explosion because they won't have any kids anyways.

What sudden population explosion? I'm fairly sure that leukemia isn't playing a huge role in demographics.