The first time Mathew Knowles spotted a pinprick-size red dot on his shirt, in July 2019, he didn’t think much of it. He found another spot on his shirt the next day, but again, the music producer and father to superstars Beyoncé and Solange let it go. When it happened a third time, though, Knowles, 68, brought it up with his wife of seven years, Gena Avery, who said she had noticed similar spots on their bedsheets. “I said, ‘OK, I need to go to the doctor’,” recalls Knowles, who had learned to be mindful of his health after years of selling medical devices, including mammography equipment. Though he hadn’t felt like anything was wrong, he knew nipple discharge is one of the signs of breast cancer for men. After…