I can see what you're saying, but self examination of breasts or testicles is usually the only way you will know if you have cancer. Most lumps are nothing to worry about, and I have never heard of any information to suggest that you can get cancer by worrying about it. There could be something in it though, no one really knows why or how it starts.
Well, it's been known for people who have unshakable faith to rid themselves of the disease (and it does happen). I wouldnt be surprised if it worked the opposite way and so, people who are paranoid, could end up getting it.