Once you are diagnosed with cancer, the doctors usually do a few other tests to make sure there are signs of cancer anywhere else. Then, if you are like most cancer patient, you research the crap out of your diagnosis and start finding out about metastases and recurrences.
Then you get paranoid and decide you are going to die. Cancer is going to get you. You are doomed! Eventually you start to calm down and decide it was all in your head. But its very easy to start going down that road again - especially if your oncologist says you need a few more tests or something.
With most cancers recurrences head for major organs such as liver and lungs and even your brain. If you get brain metastases, the outlook is usually not very good. It is the one that is feared the most I think. But also every little ache and pain makes you think is that a cancer symptom? Over reactions to every little ache or pain are very common. Its not a head ache its a brain tumor. Its not a stomach ache, its stomach cancer. Every pain becomes a cancer symptom. That paranoia again.
But sometimes a head ache can be a brain tumor. This article talks about finding a brain tumor and their symptoms. I personally know someone who is a brain tumor survivor for over 20 years so I can rationally say it can be beaten. But that little paranoid side of me always cringes at the first sign of a headache.