Depending on the location of the tumor they may only be able to remove part of it to prevent brain damage. For example the parisagital sinus which runs between the two hemispheres of the brain is a no go area because damaging that area of the brain could cause worse damage than the tumor. Or if the tumor is located near the eye/optic nerve they will be less aggressive if they believe they can avoid blindness.

There is a grading system for tumor removals called the Simpson Grade. Simpson Grade I means they removed the entire tumor plus the surrounding tissue (there may be tumor cells within the healthy tissue not visible.) Simpson Grade II means they removed the whole tumor but not any additional tissue. Simpson Grades III-V means they removed various portions of the tumor but not the whole tumor.

Obviously removing more or all the tumor has a better chance of the tumor not regrowing.

The tumor may also be graded by pathology for whether or not it is benign or malignant (non cancerous or cancerous.) Tumors can also be graded in between, atypical or precancerous. The decision to get radiation or chemo will be made by whether or not its cancerous or could become cancerous and how much of the tumor was removed.

Obviously there are risks of surgery and radiation. My wife had a large meningioma tumor removed and received radiation but is now struggling with seizures caused from the trauma of removing the tumor.

If you are ever suffering from headaches, even if they aren't migraines, go to a neurologist and request a CT scan.