Q. Because my 40th birthday is coming up, I decided to have a baseline mammogram. I had no symptoms, so I was surprised when the doctors said I needed special magnified pictures done. They said I had "micro-calcifications" in one of my breasts.

What are micro-calcifications? What causes them? Can anything be done to prevent them? Do they usually turn into cancer or stay benign?

I'm supposed to have a repeat mammogram in six months. How can I be sure I'm getting an accurate mammogram and proper interpretation?

A. Micro-calcifications are small deposits of calcium. The concern about them is that they are sometimes a sign of breast cancer.

Making calcium deposits is one of the ways your body reacts to injury. In some cases, the "injury" may be a malignant growth invading normal breast tissue. Tiny clusters of micro-calcifications are a possible sign of a very small, early breast cancer. But other things, such as prior injury, or even the aging process itself, may have caused calcium deposits to form. In these cases, the micro-calcifications are usually scattered, not clumped together.

The radiology physician interpreting your mammogram looks for clues like these to decide whether the calcium deposits are suspicious for cancer or not. If so, he or she will recommend that you have a breast biopsy to examine the tissue around them.

You may then need to have a small wire inserted into the breast to pinpoint the area of concern, because it may feel normal to the surgeon, who will otherwise have no way of telling where exactly to take the biopsy from.

The micro-calcifications don't turn into cancer, and there's really nothing you can do to prevent them. In a sense, they're either cancer or they're not. Undoubtedly, your mammogram wasn't suspicious enough to lead to biopsy, but to be on the safe side, the radiologist recommended that you have a repeat breast X-ray to make sure none of your micro-calcifications takes on a more worrisome appearance.

If you have any questions about the accuracy of your mammogram, I'd recommend asking the physician who originally ordered it. He or she might then review the mammogram with the radiologist, and, if any doubt remains, ask another radiologist for a second opinion.

In most cases, however, it's safe to wait until a follow-up mammogram is done. If anything suspicious develops, it would still be very early in its course and usually highly treatable.

The other important part of all this is that you should be sure to have a thorough breast exam each time you get a mammogram. Occasionally, your doctor may find a suspicious lump that won't show up on breast X-ray. And if your mammogram is questionable, your doctor may confirm that there's an abnormal-feeling spot in the area of concern. Mammography and breast exams are complementary; you can't substitute one for the other.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.