Jason Collins Battling Stage 4 Glioblastoma, 'One Of Deadliest Forms' Of Brain Cancer

Ex-NBA player Jason Collins -- the league's first openly gay player -- just opened up on his serious health battle ... revealing he was diagnosed with Stage 4 glioblastoma, "one of the deadliest forms" of brain cancer.

Collins, who announced he had a brain tumor earlier this year, detailed his journey in an interview with ESPN's Ramona Shelburne ... saying he first noticed symptoms back in August.

The biggest issues he was dealing with included short-term memory loss and depleted comprehension ... and after testing, he learned he had glio -- an incredibly aggressive form of the disease.

"Imagine a monster with tentacles spreading across the underside of my brain the width of a baseball," Collins said ... adding it's impossible to fully resect the mass without coming out of the procedure "different."

"My glio is extraordinary for all the wrong reasons, and is 'wild type' -- it has all these mutations that make it even more deadly and difficult to treat."

But Collins fought -- starting with medication, then radiation and chemotherapy ... and it worked. He said he slowly started to come out of his fog.

Even with all the treatment, Collins said the average prognosis is 11-14 months.

"If that's all the time I have left, I'd rather spend it trying a course of treatment that might one day be a new standard of care for everyone."