I agree with having a female spayed before the first heat if you do not intend to breed. I have seen to many mam tumors in unaltered females as they get older. Boxers are prone to cancer, but other dogs are as well. I lost my last dog to cancer and she was a lab.
I feel it is being aware and feeling all over them to find any lumps and bumps at a early stage. Have them removed or watch them for growth very carefully.
Finding one and it not being cancer does not mean the next one could not be. There is also having one removed that is cancer, it could come back in the same place or pop up in another place.
I agree with Hanna that having a pup from tested parents is always a plus, taking care of them, feeding a good food, having vaccinations done at an early age for protection against Parvo....all of these things are steps you take to protect your babies.
In the end the truth is that cancer or not cancer prone Boxers are wonderful dogs and they just do not have a long enough normal life span.