PHOENIX - It has been almost two weeks since it was announced Sen John McCain was diagnosed with a brain cancer called glioblastoma.

A week after the brain tumor was removed, the senator was back in Washington D.C. voting on the health care bill. His vote proved to be critical in stopping the repeal of Obamacare.

Now McCain is back in Arizona undergoing more treatment for the cancer at the Mayo Clinic.

We spoke with Dr. Nader Sanai, a neurosurgical oncologist at the Barrow Neurological Institute about what the typical treatment course is for patients who are battling glioblastoma. Neither Barrow or Dr. Nader are treating the senator.

PREVIOUS: McCain's office says he has cancerous brain tumor

“The treatment has three phases. It starts with surgery. The goal is to remove everything you can see on the scan," Sanai said. "We know for these patients there are cells that you can’t see on the scan. That’s why you have the second two phases of treatment, which is a combination of chemotherapy -- typically a pill -- and radiation therapy, which is a six-week course of doses of radiation every day."

Fatigue is the most common side effect of the cancer treatment, peaking during the weeks of radiation. Sanai said many patients will regain their energy levels several weeks to months after the radiation therapy is completed.

“The chemotherapy itself can also decrease your white blood cell count. So, it’s important that patients get treated while also getting blood tests to make sure it doesn’t go too low as that can affect your immune system,” said Sanai.

How the tumor reacts to treatment varies from patient to patient.

“Really what differs from patient to patient is the biology of the tumor. Even though two patients may have the same tumor with the same diagnosis, the biology is different," Sanai said.So, they respond differently to these therapies. That’s why there is a fairly wide spectrum of responses for glioblastoma patients."