I've got a (so far benign but growing) tumour on my spine in a place where it would be a very difficult and major surgery if it were to have to be removed. What I do and what works for me, is that when I notice worrying thoughts about it come up, I tell my brain "no thank you" and I keep doing that until I can entirely focus on something else that I'm doing or that's going on in my life that's good or okay (it takes determination to do this - as the brain likes to stick on to fearful thoughts like velcro - but I find it usually doesn't take too long, if you're persistent). Something I also have done regarding my tumour is to read up about it as much as possible so that nothing much has come out of left field to surprise me, and to write down my questions for my appointments with specialists in advance, which gets them off my mind, rather than them going around and around and around in my head...I do appreciate though that this is not for everyone - particularly for those who tend to dwell on worst case scenarios - but for me it has worked to help me cope.