There have been plenty of tributes to and reflections on the Arizona Republican’s political career and record of public service. Those will, no doubt, continue to proliferate. But it’s also important to home in on the insidious disease that claimed McCain’s life—an aggressive form of brain cancer called glioblastoma multiforme (GBM, or just glioblastoma, for short).

A glioblastoma diagnosis, statistically speaking, amounts to a death sentence. Brain and nervous system cancers in general are deadly. In fact, just about one in three of the approximately 24,000 Americans diagnosed with such cancers in 2018 are likely to still be alive five years later, according to the National Cancer Institute (NCI).

Glioblastoma specifically makes up 16% of all brain and nervous system cancers—and the numbers are even more dire in these cases, says the American Cancer Society. If the disease manifests between the ages of 20 and 44 (a relatively rare occurrence), there’s a 19% chance of survival five years after diagnosis; among those 45 to 54, that drops to 8%; and for older Americans aged 55 to 64, the five-year relative survival rate is a dismal 5%. McCain was in his 80s when his diagnosis was publicly revealed. (Beau Biden, son of former Vice President Joe Biden, also died of the disease, but at the age of 46.)

Just why is the prognosis so dire? A lot of it has to do with just how aggressive this specific kind of tumor is, and the limitation of buzzsaw approaches such as surgery, chemotherapy, and radiation.

Dr. Duane Mitchell, a physician and professor of neurosurgery at the University of Florida, had this to say in The Conversation on Monday:

“An additional characteristic of GBM is the invasive nature of the disease. GBM tumor cells essentially crawl away from the main tumor mass and embed themselves deep within the normal brain, often hidden behind a protective barrier known at the blood-brain barrier,” Mitchell writes. “This invasive feature means that while neurosurgeons can often remove the main central tumor mass of a GBM, the invasive finger-like projections protrude into other areas of the brain. The distant islands of tumor cells that have migrated away cannot be effectively removed by surgery.”

Thus, the normal courses of treatment for other kinds of cancers may not prove effective against such a clever and malevolent foe. At the same time, the complexities of the brain’s structure—and its critical role in, well, everything—have made it so that we have yet to see the same kind of progress against glioblastoma that we have in so many cancer, such as immunotherapies and gene-based treatments for skin, lung, and blood cancers.

But scientists have sounded some (very, very early) notes of optimism in recent years. “[G]ene therapy, highly focused radiation therapy, immunotherapy, and chemotherapies utilized in conjunction with vaccines” are being tested in experimental glioblastoma trials, the American Association of Neurological Surgeons notes—while adding the sobering note that these early-stage technologies have only boosted patients’ survival rates by a median of three months.

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