Improvements in mammographic techniques have enabled radiologists to better distinguish benign from malignant soft tissue in the breast. In the past, asymmetric breast tissue was typically regarded as a sign of malignancy, whereas now it is nearly always regarded as benign. The ability to make finer distinctions on mammograms has also allowed for the development of more specific criteria for ordering additional views. Even though soft tissue asymmetry is considered benign, increasing density or other changes over time may be of more concern. Piccoli and associates studied serial mammograms of women with asymmetric breast tissue but negative physical examinations to determine the nature of soft tissue changes over time.

Twenty-eight women with asymmetric breast tissue, as seen on either routine screening or diagnostic mammograms, were eligible for the study. Of these, 16 underwent biopsy (four core and 12 excisional), five were followed clinically for 13 to 84 months and seven did not have follow-up examination. Mean patient age was 44.2 years, and none of the patients had a history of or was currently receiving hormone replacement therapy. In addition to mammography, eight patients underwent ultrasonography (US), three received contrast material–enhanced magnetic resonance imaging (MRI) and two had both US and MRI. Thirteen histologic specimens were available for review.

Asymmetric tissue was either absent or no greater than 0.5 cm on the baseline mammogram in 12 patients. Twenty patients demonstrated a change in asymmetric tissue size, most commonly in the upper outer quadrant, followed by the axillary tail, the 12 o'clock position and the inner part of the breast. Among these patients, the mean increase in size from baseline to biopsy, or follow-up mammography, was 2.7 cm and ranged from 1.0 cm during two years to 6.5 cm during five years. Several patients who experienced an initial increase in tissue size showed a negligible change or a decrease in size over one to three years. Of the nine patients who underwent US, only five showed abnormalities. In the three patients who had MRI, the focal asymmetry was interpreted as benign.

All 16 biopsy specimens were reported as benign. Of the 13 available for review, all showed evidence of fibrocystic changes but no microcalcifications or carcinoma. A prominent benign stromal change, referred to as pseudoangiomatous stromal hyperplasia, was identified in all specimens and reported as extensive in 12. This finding has been reported as an incidental histologic finding on breast biopsy for either benign or malignant disease, although the authors review of the literature found no evidence to suggest that it is a premalignant entity or high-risk marker for malignancy.

In the five patients who were followed, additional imaging studies were negative. No malignancies were reported, although in one patient the asymmetric breast tissue continued to enlarge.

The authors conclude that radiographic evaluation of patients with increasingly asymmetric breast tissue should focus on differentiating benign tissue from more ominous focal asymmetric density. Observation can be considered as a management option if benign imaging and clinical criteria are met. Fibrocystic changes, dense stromal fibrosis or pseudoangiomatous stromal hyperplasia can cause asymmetric breast tissue. With the correct imaging studies and clinical settings, these findings may be considered diagnostic.