Beyoncé's dad, Mathew Knowles, diagnosed with breast cancer

The father of Beyoncé and Solange Knowles recently revealed a shocking medical diagnosis: he has breast cancer. Mathew Knowles now wants to shine a light on male breast cancer, and is urging other men with the disease to speak out.

The 67-year-old appeared on "Good Morning America" on Wednesday to talk about his diagnosis. He said he first noticed something was wrong when he found blood on his white T-shirt.

"When I had the blood on my T-shirt initially I didn't think it was breast cancer," Knowles told GMA. "My mind went a lot of places. My mind went to what medication I was on, because different medications might have caused some sort of discharge ... and then I thought, just because of the risk factor, that it could be breast cancer and I would go get a mammogram."

That risk factor was from Knowles' past career at Xerox. "For context, in 1980 I worked in the medical division of Xerox," he said. "I worked there for eight years, selling Xeroradiography, which was at that point the leading modality for breast cancer."

Knowles went to the doctor and got a mammogram and confirmed he did, in fact, have breast cancer. He immediately got surgery and a BRCA test, a genetic test which shows if a person has a higher risk of developing breast cancer.

"I am going to get the second breast removed in January, because I want to do anything I can to reduce the risk," Knowles said. 

He also revealed a family history of the disease: his maternal aunt died of breast cancer, as did two cousins. "My kids have a 50- to 70-percent chance of getting the BRCA mutation and breast cancer. That's male or female," he said. "We used to think this was only an issue for women, but this is male or female."

Knowles said he has been surrounded by cancer on both sides — his wife Gena's mother has breast cancer, too. (Gena is not the mother of Beyoncé and Solange.)

He is hoping that sharing his story as man with breast cancer will shine a light on the risk men can face.

Breast cancer is very rare in men. According to the American Cancer Society, men have a 1 in 833 risk of developing breast cancer in their lifetime — compared to about a 1 in 8 lifetime risk for women. It is 100 times less common among white men than white women and 70 times less common among black men than black women.

Knowles follows a number of other men in deciding to publicly come forward about his breast cancer diagnosis. Last year, a New York City law firm said it was representing 15 male clients who are suffering from breast cancer believed to be linked to the toxins from ground zero.

Two of those clients spoke to CBS News last year. "We want to get the word out to other men because it is very rare and men tend to ignore it and don't see a physician," Jeff Flynn said.

"We know breast cancer is a popular disease, unfortunately, but for men, it's very rare. And I had it in both breasts, believe it or not," John Mormando said.

Those men, too, said they wanted to raise awareness about male breast cancer. Mormando recommends men talk to their doctor about breast cancer risk. "When you go in for your yearly physical, most men are programmed to get certain tests like prostate exam. … Just get a breast exam, too, especially if you were in the 9/11 area because you're at such a higher risk," he said.

Organizations like the Male Breast Cancer Coalition's Tops Off NYC project and the SCAR Project are also working to raise awareness for male breast cancer, by photographing real men who have battled or are battling the disease.