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.

“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.”