I am sorry to hear that you have a brain tumor, and hope that everything goes well. The word tumor means that you have a mass, or swelling, or some lump or bump. It does not necessarily mean that it is bad, such as a cancer, but it is implied that it is at least somewhat abnormal. Because of this, your doctor
is pursuing additional testing, which appears to be warranted from the limited information you have provided.
A CT scan is a series of x-rays that are taken very close to each other. They provide excellent detail of many parts of the anatomy of your brain, including, when used with certain contrast dyes, the vasculature. This will allow your surgeon
to determine more information about your tumor and decide if there is anything that needs to be done. At this point, it would be impossible to say if one test will be enough, although your doctor would be better able to answer that and other questions.
Many times, a single CT scan is sufficient to give the detail with regards to what the tumor might be. Sometimes, the doctor will order further imaging, either with an MRI or possibly just repeating the CT scan later to see if the tumor has grown or changed. As for the prognosis of your tumor, that will depend on what it is, and that question can also be addressed to the doctor who is evaluating you.