This is not how population growth works, we have population growth because we have enough resources to maintain it, once we no longer have resources the population will begin to either A: die or B:stop reproducing as much, either due to famine, plague, war, or the inability to sustain a family. Let's look at some crazy stuff in history where this happened. The black plague killed off a large chunk of the european population, this benefitted the society by decreasing its population, thus decreasing resource usage, which created an abundance of resources. This abundance of resources made it possible for there to be more artisans, meaning more technological advancement.
Besides, most of the people who get cancer are old and are going to die soon anyway.