Dave Nitsche was a healthy triathlete — until a bizarre vision problem in 2019 stopped him in his tracks.

“I would look at telephone poles … they seemed kind of wavy,” Nitsche, a father of two and grandfather of one, told The Post.

He went to an optometrist, who suspected a detached retina and urged Nitsche to go to the hospital. Fluid had built up behind his left eye, and the eye was removed two days before his 50th birthday.

The fluid turned out to be cancerous. Then, over a liter of fluid was discovered in Nitsche’s left lung — he was stunned to learn he had Stage 4 lung cancer that had spread to his eye.

“It was very shocking because I was still running and being active,” Nitsche said, “and to go fourth stage right away, it’s pretty, pretty devastating.”

The Calgary project manager, a non-smoker, was told he only had a year or two to live.

Lung cancer is the deadliest cancer worldwide, responsible for roughly 1 in 5 cancer deaths. Its high mortality rate is often due to symptoms appearing only after the cancer has spread, like in Nitsche’s case.

Nitsche was divorced at the time of his diagnosis, but his ex-wife has been supporting him as a “best friend” and roommate.

He needed the assistance. He was quickly put on the lung cancer drug afatinib — and taken off just as quickly — when he developed brain lesions.

“The side effects with that one were pretty bad,” Nitsche said, “lot of fatigue, lot of headaches, body function issues.”

He moved on to the targeted cancer therapy osimertinib, which wasn’t available when he was diagnosed. He spent an astonishing six years on the medicine, far exceeding his two-year life sentence.

But then the osimertinib stopped working, so he switched to Rybrevant, an antibody that blocks two proteins that help cancer cells grow and spread. It works in conjunction with chemotherapy.

Nitsche, now 57, is among the first patients in Canada to be on Rybrevant.

“It was quite humorous, because I went to the hospital to get the chemo, and the nurses were like, ‘Oh, what’s this new one?’ I was very fortunate in my timing,” Nitsche said.

He gets infusions every three weeks and finds the drug works “quite well.”

His cancer battle has slowed him down, but hasn’t sidelined him from the activities he loves, like spending time with his dog, Indie, and taking cooking classes.

He’s also signed up for the BC Epic 1000, a 1,000-kilometer bike ride that traverses the Trans Canada Trail. He’s using the June event, which should take him 12 to 14 days to complete with friends, to raise awareness for lung cancer.

The Ironman finisher works with the Man Up To Cancer support group and Lung Cancer Canada to spread the word that you don’t have to be a smoker to get lung cancer.

There are things around your home that can increase your risk, such as radon gas, asbestos and volatile organic compounds like benzene and formaldehyde.

“There’s a saying that goes, ‘If you have lungs, you can get lung cancer,’ and pre-screening is very important,” Nitsche said.

“If you get cancer of any type, it’s not a death sentence. There’s so much research going on out there.”

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