Former pro-basketball player Jason Collins, the first active male athlete in a major American professional team sport to come out as gay, has announced that he has been diagnosed with an aggressive form of brain cancer.

Collins was diagnosed with stage four glioblastoma, he wrote in an article for sport broadcaster ESPN on Thursday, and is currently undergoing treatment to stop the spread of his inoperable disease.

The brain tumour, he says, is like “a monster with tentacles spreading across the underside of my brain the width of a baseball”.

Without treatment, he would be dead within three months, doctors told Collins.

Collins, who played for 13 seasons in the NBA, wrote that the challenges of treatment were somewhat similar to those he faced on the court.

“As an athlete you learn not to panic in moments like this,” he said, comparing treatment to going up against basketball star Shaquille O’Neal, or his decision to come out as gay.

“To me it’s like, ‘Shut up and go play against Shaq.’ You want the challenge? This is the challenge,” he wrote.

“And there is no bigger challenge in basketball than going up against prime Shaquille O’Neal, and I’ve done that.”

Collins family had released a short statement in September saying that he had a brain tumour, but in his article he says “it’s time for people to hear from me directly”.

The 47-year-old sports veteran writes that the cancer was discovered after he was struggling to focus.

He describes how shortly after marrying his husband in May, he missed a flight because he was unable to pack his luggage.

A brain scan later revealed the source of his issues with focus and memory.

“My mental clarity, short-term memory and comprehension disappeared – turning into an NBA player’s version of ‘Dory’ from ‘Finding Nemo’,” he jokes about the forgetful fish from the Disney film.

Revealing his diagnosis to the world reminds him of his decision to come out, he says. The years since coming out “have been the best of my life”.

“Your life is so much better when you just show up as your true self, unafraid to be your true self, in public or private. This is me. This is what I’m dealing with.”

Collins is currently being treated with a drug called Avastin to slow the tumour’s growth, and has been travelling to Singapore for a targeted form of chemotherapy.

He writes that he hopes that his treatment will help develop better methods to fight the disease, and that he can lead the way similar to how he did as the NBA’s first openly-gay player.

“After I came out, someone I really respect told me that my choice to live openly could help someone who I might never meet,” he says.

“I’ve held onto that for years. And if I can do that again now, then that matters.”

The California native played for six teams in his 13 seasons in the NBA. He had previously been featured on Time Magazine’s 100 most influential people list. He retired in 2014.