Study: Lean mice fight cancer
This AP article details the results of a recent study involving mice that were studied for their ability to fight off tumors based on their relative leaness. The leaner mice were more successful in this than the fatter mice. Leaner, exercising mice had better anti tumor defenses. Of course mice are not people and researchers are a long way from saying that lean body mass and exercise will definitively help fight off tumors.
However, as the article says, "this particular study certainly provides a biologic rationale or explanation about why weight loss may be helpful (in fighting cancer.)"