If a doctor suspects a brain tumor, one of the first steps in getting an accurate diagnosis is through magnetic resonance imaging (MRI). This imaging test gives physicians an extraordinary view of the brain and this is often the only test needed to identify the possible presence of a brain tumor. In some limited cases, a CT scan may be used. PET scans, which help doctors see the activity of the brain, may help diagnose primary brain cancer but their use is less certain with a metastatic disease.Ultimately, it is a brain biopsy that confirms the malignancy and type of brain tumor present. If tumors are present as shown on an MRI and a person suffers from a type of cancer that is known to metastasize, then a biopsy may not be necessary. However, with types of cancer that don't often spread to the brain, a biopsy is a vital diagnostic tool. Primary brain tumors most always require a biopsy.
Brain biopsies can be done during times of surgical exploration or open surgery. The sample tissue can be examined in the operating room, allowing the surgeon to make a decision about whether to proceed with surgical treatment or not. More extensive evaluation of the tumor specimen will also be done by a pathologist. It may take several days to receive results.
In some cases, a closed biopsy, also called a stereotactic biopsy, is performed when the tumor is located in a region of the brain that is difficult to reach. It is the least invasive type of biopsy, but does carry risks.
source : cancer.about.com
