(USA TODAY) -- Martina Navratilova, who won 18 Grand Slam tournament singles titles and competed on the women's pro tennis tour until she was nearly 50, is being treated for breast cancer.

Navratilova, 53, went public with the news Wednesday on Good Morning America and online at People.com.

"It was a total shock, because I've been so healthy," Navratilova told USA TODAY in a phone call from New York.

Navratilova said the cancer in her left breast -- diagnosed as ductal carcinoma in situ (DCIS) -- was detected during a mammogram in early January.

DCIS is the most common type of non-invasive breast cancer and is sometimes referred to as Stage 0 or precancer.

Though DCIS is hardly ever life-threatening, it carries an increased risk of invasive cancer down the road.

Navratilova, known as a pioneering paragon of fitness, partly blames herself for allowing four years to pass between mammogram screenings.

"I let it slip by," said the nine-time Wimbledon singles champion, who holds the most singles (167) and doubles (177) titles in history.

"I was a bad girl."

On March 15, the Prague-born American had a lumpectomy to remove cancerous tissue and will go through four to six weeks of radiation treatment starting in May.

She hopes to begin the treatment in Paris so she can continue her commentary duties with Tennis Channel during the French Open.

Navratilova said she first intended to keep the news quiet but changed her mind when she thought of other women who might face a more dire situation by skipping a routine mammogram.

"The sooner you catch it, the better," she said.

Despite recent medical controversy about when and how often to submit to mammography screening, Navratilova had this advice: "Get the bloody mammogram."

Navratilova said her first thoughts after the diagnosis were not about death but disfigurement.

"I thought, 'I'm going to lose my boob and then my hair, and I don't have that much,' " she said, with a laugh.

She emphasized that she is cancer-free and has not curbed her activities since the discovery.

She participated in last month's "Hit for Haiti" fundraiser in Indian Wells, Calif., three days before her lumpectomy.

She also recently competed in a 25-mile bike portion of a triathlon race in Hawaii.

"There's a good chance it won't come back," Navratilova said, adding her radiation treatment would "lower the risk."

"The bad news is it's cancer," she said. "The good news is that it hasn't spread."

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