I have swelling on one side of my posterior skull. i had a head injury many years back. i had c-t scan also. can this swelling develop into tumor?

Unlikely: It is unlikely. If the swelling is getting larger or is getting more painful then it needs to be re-evaluated to make sure that it does not represent a sarcoma or any other kind of soft tissue tumor.

Unlikely: Usually not. Here is some additional reading on risk factors and brain tumors. http://www.cancer.net/cancer-types/brain-tumor/risk-factors.

Sounds unlikely: Soft tissue swelling doesn't usually develop into a malignancy but need to know why you have the swelling. Is it new? You should be examined carefully.