Treating a couple at the same time is even more unlikely.
Of course, there is something to be said for growing up in the same place: both Ken and Jane are born and raised in Amherst so their life exposures and gene pool will be similar. But still, that is only part of the story.
Ken, 75, was diagnosed first, in late 2017. He was in the shower when he felt a lump, and went straight to his primary care doctor to get it checked out.
Ken was mildly aware the risk wasn't just for women: he'd been told about the small risk in the late 80s when he was briefly put on a blood pressure drug after fainting at the sound of a car horn, and a side effect of that drug was that it increased the size of his breasts. Soon after, he was switched to another, but he remembers being told to check himself as a woman does.
"I can't say in all those that I was checking, but I was [aware] so when I felt a lump, I went to my doctor," Ken told the DailyMail.com.
He says he wasn't that concerned, and neither was his doctor, who ordered a biopsy but insisted it seemed fine.
According to the biopsy results, it was: it came back clean and Ken was told to get on with his life.
But that night he noticed bleeding from his nipple.
"I called my doctor, and he said 'oh I left a mark there to help me find it because we're going to have to be watching it, give it another week'.
"I went back to him a week later and said "it's still bleeding, it's ruining all of my shirts!'"
It was enough for the doctor to schedule surgery to remove the lump - and it turned out to be cancerous.
A cancer diagnosis is a blow to anyone, no matter what the type or who the patient.
But according to Dr Jame Abraham, director of the Breast Oncology Programme at the Taussig Cancer Institute, that existential reaction is often particularly notable in men diagnosed with breast cancer.
To his surprise, Ken was nonchalant.
"When men get a diagnosis of breast cancer they are all shocked and for them, sitting in the women's clinic, it's really shocking," Dr Abraham, who treated both Ken and Jane at Cleveland Clinic, explained.
"They think 'I'm a man, I don't have breasts, what am I doing here? Why me?'
"But Ken ... he just didn't complain. He didn't ask any of those questions, he just took it in his stride. He was more interested in talking about other things than about his cancer - he told me how he met and married his wife, his whole life story."
Jane, 66, says it was this gung-ho attitude that calmed her nerves when, six months later, she went in for the annual mammogram she's been getting since she was 30, and it revealed a tumour, which was swiftly determined to be cancer.
Ken coached her through it, giving her a heads-up about what to expect from each test and treatment.
"It was good for me that Ken went through it first because he's always 'well, whatever's going to happen is going to happen, let get on with it'," Jane explained.
"I thought, it's got to be okay - it was ok for him, so it's got to be okay for me.'
The only person who was stressed out was their son, she says, and he couldn't believe how relaxed his parents were. "I was just joking around about it," Jane laughs.
Ken found out about Jane's diagnosis from his surgeon while he was sat in the surgeon's office having a check-up, and the landline rang.
"They asked me if I had a surgeon in mind and I said, 'well yes I do!'" Jane said. "So they called up and Ken was there."
By that point, Jane was already a familiar face at Cleveland Clinic, Dr Zahraa AlHilli, the breast cancer surgeon who treated both the Gremlings, told the DailyMail.com.
"She came to all the appointments, the surgery ... to everything. Everyone knew her. And for Jane, Ken did the same. They are both so supportive of each other and it was so nice to see that every step of the way, always together," Dr AlHilli said.
Their up-and-at-'em attitude has been at the heart of their relationship since they got married.
They met in 1970 when 27-year-old Ken hired 18-year-old Jane straight out of high school to work in the company's payroll department. Not long after, Jane was fired (it's unclear why), but before she left she looked Ken up to find out if he was married, and saw that he wasn't.
A year later, Jane bumped into Ken at the gas station. He, too, had just been fired from the same firm, and was trying to figure out what to do. Jane had an idea for the immediate future: he could come with her to drop off her car at her mother's house, then give her a ride home.
"That turned out to be our first date," Jane said. "I was quite forward!"
A couple of weeks later, they got engaged, then went to buy a ring, then got married within a month of dating, on November 27, 1971.
Over the years, they say, they've faced worse than a treatable bout of cancer - for example, when Jane was diagnosed with MS in 1993, an incurable condition with little to no treatment options.
"It was more dismal than this," Ken says. "It wasn't a matter of going in for check-ups and surgery. There's nothing you could do."
Jane's mastectomy triggered a flare-up of her MS so bad that they thought she'd had a stroke ("she was talking gibberish," Ken said). But she brushes it off: "We made it through that. I feel pretty good with the MS. I'm not in a wheelchair, I walk with a cane. I'm good."
Now, they are almost out of the woods, with Jane's breast reconstruction the final step. Ken said he's foregoing that step because"'I wasn't Mr America before the surgery, so I sure wasn't going to be Mr America afterwards", but adds that he's ready to support Jane, though he's upset that he won't be able to empathise with her as he did with everything else - "I just don't know what it's like."
But according to Dr AlHilli, just having each other is most of the battle.
"We try as much as we can to tell [our patients] about what to expect but when they have support, it's different. They feel in control, they're able to make joint decisions, stay active, discuss what they have. It makes a difference."
