Sounds like you are suffering from a phobia of cancer. I don't think your doctor nor any clinical psychologist could do much to extinguish that phobia, as they don't have a clue how this phobia came about in the first place. The best they can do is telling you that you don't have cancer, and you are not likely to have cancer.
This might sound strange to you. Most likely, this phobia has its root in one of your past lives. It was something that you did in a previous incarnation and your soul knows that you will get cancer
as a result of what you did. This fear will surface when you do something that mimics the very act you did in the past life.
To get rid of the phobia for good, you need to reprogram or heal your soul.
You can ask a hypnotherapist to do a past life regression for you. You need not tell him or her your problem. Just ask him to do a regression and see what issues you have that need your attention. Once you found the source of the problem, he or she could lead you though a healing session.
Alternatively, you can do a spiritual healing. This is the ultimate cure for all diseases. If you feel an inclination to become spiritual, this is the path that you should take. For a deeper understanding of spiritual healing and past lives issues, you might want to check out www.blunt.fm or www.onlygodheals.com.
Even if you don't buy my ideas, please remember what I said. After you have exhausted all other possible remedies, you might want to reconsider my suggestions. Some remedies might work for a while. But the phobia could come back after a few years, or even a few decades.
Hope you get well soon.