Sen. John McCain is battling a glioblastoma, which is the deadliest form of brain cancer, top experts say.

McCain’s office disclosed on Wednesday that the 80-year-old senator had been diagnosed with this form of primary brain tumor.

“A glioblastoma is the most aggressive form of brain cancer, which usually kills its victims between 12 and 14 months,” top neurosurgeon Dr. Lee Tessler tells Newsmax Health.

“At his age, the prognosis is extremely poor,” agreed Dr. Russell Blaylock, a renowned neurosurgeon and Newsmax contributor, who writes the Blaylock Wellness Report.

A glioblastoma is a grade IV primary brain cancer. Unlike other cancers, brain tumors are not staged, but categorized according to “grades,” which denote their aggressiveness, says Dr. Tessler, a neurosurgeon at NYU Winthrop Hospital in Mineola, N.Y.

Glioblastoma represents 14.9 percent of all primary brain tumors and is the most common of all malignant ones, with an estimated 12, 390 new cases predicted in 2017, the American Brain Tumor Association says.

A glioblastoma is, by definition, a grade IV astrocytoma, a kind of tumor that arises from the glial, which are star-shaped cells that make up the supportive tissue of the brain, according to the association.

For adults with glioblastomas, the median survival rate is 14.6 months, although some patients have been known to live longer, the association says. Still, the five-year survival rate for such a tumor is only 5.1 percent, says the National Brain Tumor Society.

McCain’s treatment is under consideration but will likely be a combination of chemotherapy and radiation, his office says.

Such a combination is standard treatment for glioblastoma, says Tessler.

It is unlikely that McCain would receive immunotherapy as a first-line, or preferred treatment, says Tessler.

Immunotherapy is credited for saving the life of former President Jimmy Carter, who was diagnosed with brain cancer in 2015 at the age of 90, and remains cancer-free.

“The difference is that Jimmy Carter had cancer in his brain that had spread from melanoma, not primary brain cancer like John McCain has,” says Tessler.

There is research underway on different cancer therapies, including a widely publicized treatment pioneered at Duke University, which uses the polio virus.

McCain might get experimental treatments, but not as a clinical trial due to his age, Tessler says.

Such newer treatments could be used later on in McCain’s treatment as a means of extending his life, but they are unlikely to result in a cure, Blaylock agreed.

“He could do pretty well for a few months but these types of cancers almost always come back and are invariably fatal,” says Blaylock.

Both neurosurgeons agreed that it is likely the existence of the then-undiagnosed brain tumor that may have been responsible for McCain’s confused performance at the Capital Hill hearing last month focusing on former FBI head James Comey.

“At the time, I thought he might be suffering from dementia,” says Blaylock.

Tessler also says he wondered about McCain’s performance at the time, and that the existence of the brain tumor could explain it.

“That could have been the tipoff,” he said.