When Zunilda Guzman first felt the lump on her left breast earlier this year, she dismissed it as an annoying byproduct of her menstrual cycle. She had no family history of breast cancer and a mammogram had given her a clean bill of health. A month later, however, the biopsy told a different story.

Since June, the Miami Lakes mother has endured a double mastectomy, prophylactic removal of her ovaries and uterus and is now in the middle of a six-part chemotherapy regimen. Next: radiation therapy and reconstruction surgery.

She is 39.

''People don't believe my age,'' says Guzman, who has continued to work in accounts receivable for a woman's golf apparel manufacturer in Hialeah, ``but I keep telling everybody, it doesn't matter how old you are. It can still happen to you.''

Indeed, breast cancer does not discriminate by age. Though it is uncommon -- fewer than 10 percent of all breast cancer cases occur in women under age 45 -- it tends to be more aggressive and harder to treat in younger women, studies show. Various cancer organizations estimate that more than 250,000 younger women are living with the disease and 11,000 will be diagnosed next year.

Dr. Stefan Glück, clinical director of the Braman Family Breast Cancer Institute at Sylvester Comprehensive Cancer Center, believes the proportion of young breast patients may be higher in South Florida. He bases his estimate on his own practice and that the area is home to more South and Central American women, who tend to develop the disease earlier. Some of those cases are due to genetic mutations.

''We don't know why, but we think the [local] rate is a reflection of our population,'' Gluck says.

There is some good news, however. Even with more testing, the incidence rate of invasive breast cancer -- cancer that has broken through normal breast tissue barriers and invaded surrounding areas -- is not increasing for American women, according to a study published in the journal Epidemiology in 2006.

This may explain why young patients say their doctors often dismiss their lumps or pains because they're not old enough. The average age of a women contracting breast cancer is about 62.

MISDIAGNOSIS

'It's amazing how many women are told, `You're too young. It isn't cancer,' '' says Gina Castronovo, a Lighthouse Point cancer survivor. ``Well, that isn't always true. Even in your 20s and 30s, women need to be vigilant.''

Castronovo knows of what she speaks. She thought she was having a cyst drained when doctors delivered the life-changing news three years ago. She was 42. She and five other South Florida women have written a book, Just a Lump in the Road (iUniverse, $16.95), recounting their ordeals -- and triumphs -- as young survivors. ``We realized there wasn't that much out there for women like us.''

Medical researchers agree that youth can hinder early detection. ''They tend to be diagnosed later because of misconceptions,'' admits Dr. Kimberly Blackwell, a breast oncologist at the Duke Comprehensive Cancer Center studying the traits exhibited by younger women's breast cancers. ``Yet, it's important to promote the awareness that early detection is vital. You can beat this disease.''

Women who are diagnosed at a younger age are more likely to have a mutated BRCA1 or BRCA2 gene. In normal cells, these genes help prevent cancer by making special proteins that keep cells from growing abnormally. A mutated version, however, increases the risk of developing breast cancer by up to 85 percent. Women with these mutations also have an increased risk of developing ovarian cancer -- the reason why Guzman had her ovaries removed.

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