Q: After getting a mammogram last year, it was recommended I go for a breast ultrasound. According to my doctor, my breast tissue is very dense, making it more difficult to detect abnormalities. I went for the test, and everything came back normal. Next time, can't I just skip right to the breast ultrasound? And what about an MRI'should I also get one of those?

A: Generally, in this country, mammogram is the first line of screening to detect breast cancer. Overall, it has a very good detection rate, and the false positive rate (patients needing a biopsy that then turns out to be benign) is acceptable. Mammograms detect abnormalities by identifying masses, distortions, calcifications, and so forth. However, mammogram has its limitations.

In very dense breasts, there may not be enough background fat for these abnormalities to show themselves. The abnormalities get "lost" in the white, thick, dense background pattern. Dense breasts are particularly found in younger women or in women who have never had children, and digital mammography may help to resolve some of dense breast issues.

Ultrasound is generally ordered when a mammogram detects a mass that can't fully be qualified or when the patient or her doctor feels a lump or mass. Ultrasound is generally a diagnostic tool to specifically examine the area of concern and will tell us that the mass is either cystic, solid, or if nothing at all is seen. Ultrasound is extremely reliable when used in this setting, and if it does show a solid mass, we know the patient needs a biopsy.

There are some attempts to try to predict whether the solid mass is benign or malignant but those are not always reliable. So, usually all solid masses are biopsies. If the ultrasound shows nothing, it was probably just an overlap of tissue or a benign undulation in the breast.

There are some centers across the world that are experimenting with "whole breast ultrasound" as a screening tool. The problem with this, however, is that the false positive rate is very high, meaning that many women then get unnecessary biopsies. Along these lines, just skipping to the ultrasound without the mammogram wouldn't really make sense for you because the ultrasound was ordered to examine a specific finding.

When a patient being screened is considered more high risk, we will consider ordering an MRI of the breast. This involves the injection of gadolinium, which acts as an imaging agent for the MRI. This agent is taken up differently by normal breast cells, benign breast cells, and cancerous breast cells so it doesn't rely on background.

Breast MRI will discover cancers that mammogram can't see, but it does not always pick up abnormalities such as low-grade ductal carcinoma-in-situ. So, it is not a substitute for mammogram. In addition, again, the false positive rate is very high, leading to follow-up ultrasounds and biopsies in a large number of women screened. Finally, MRI is very expensive, and criteria for reading abnormalities is not as standardized as it is for mammogram.

Each institution, of course, uses different technologies and has its own criteria for ordering MRIs. Generally, MRI currently is recommended for BRCA gene positive patients who have a huge risk of developing breast cancer, as well as for new breast cancer patients who have dense breasts, a family history of breast cancer, or other criteria.

At UCSD, we also use MRI to follow pre-surgical chemotherapy, in patients with breast implants, in patients with histories of atypical biopsies, and to assess margin planning. The radiologist may order MRI after a screening mammogram if the patients breast density obscures his or her ability to properly read the mammogram. This usually occurs if there is something seen on the mammogram that cannot be better qualified.

Anne Wallace, M.D., F.A.C.S., is a professor of clinical surgery and director of UCSD Moore's Cancer Center Breast Care Unit. She has been the leader of the breast program since 1995 and is board certified in both general surgery and plastic surgery. Wallace has extensive experience in all aspects of breast health, specifically breast cancer and breast reconstruction, and has served as chairman of the California Breast Cancer Research Council and vice president of Cancer Control for the Golden Triangle division of the American Cancer Society.