The 43-year-old actress, who played Topanga Lawrence on the beloved coming-of-age sitcom, revealed the news on the Aug. 19 episode of "Pod Meets World," the podcast she hosts with fellow "Boy Meets World" co-stars Rider Strong and Will Friedle.
"So, I would like to share something with our listeners, something that Rider and Will were actually two of the first people I told the news to," Fishel said at the top of the episode. "I was recently diagnosed with DCIS, which stands for ductal carcinoma in situ, which is a form of breast cancer."
"It is very, very, very early. It's technically stage 0," she continued. "To be specific, just because I like too much information all the time, I was diagnosed with high-grade DCIS with microinvasion."
According to the American Cancer Society, ductal carcinoma in situ, or DCIS, is a non-invasive or pre-invasive breast cancer which is also referred to as intraductal carcinoma or stage 0 breast cancer. About 1 in 5 new breast cancers will be DCIS and nearly all women with DCIS can be cured, the organization states.
The high-grade nature of Fishel's DCIS diagnosis, according to the National Cancer Institute, means the DCIS tends to grow more quickly than other versions and is more likely to come back or become invasive breast cancer and spread to other tissues than other grades.
The microinvasion aspect of Fishel's DCIS diagnosis means that, according to the National Institute of Health, it is less than or equal to 1 millimeter in size.
"I'm going to be fine. I'm having surgery to remove it," Fishel reassured listeners. "I'm going to be on some follow-up treatment. I've had to make a lot of decisions over the last couple of days."
Fishel said she shared the news in the hopes it would motivate others to get checked -- and get checked regularly.
"The only reason I caught this cancer when it is still stage 0 is because the day I got my text message that my yearly mammogram had come up, I made the appointment," she explained. "They found it so, so, so early that I'm going to be fine. And so, I want to share this because I hope that it will encourage anyone to get in there. If it's time for your appointment, if you've never had an appointment before, get in there."
Fishel, who admitted she still has "some big decisions ahead" regarding treatment and doesn't have all the answers yet, added, "If you have to find out you have cancer, find out when it's at stage 0, if possible."
Strong and Friedle offered words of encouragement and support to their former former castmate and co-host.
Fishel shares sons Adler and Keaton with husband Jensen Karp.