Small masses on breast MR: is biopsy necessary?

To evaluate outcome of magnetic resonance (MR)-detected biopsied breast lesions ≤5 mm by correlating imaging characteristics with pathology.

Institutional review board-approved retrospective review of 565 lesions biopsied with MR guidance between March 2004 and February 2009 found 68 lesions ≤5 mm in 61 patients. Lesions evaluated were those prospectively recommended for biopsy based on clinical setting, suspicious lesion morphology, and kinetics. Two study radiologists, blinded to final pathology, reviewed MR exams recording patient age, exam indication (staging, surveillance, diagnostic, or follow-up), mass location, size, morphology, T2-weighted signal, and kinetics. Chart review provided final pathology.

Of 68 masses ≤5 mm, 14 (20.6%) were malignant. Of 32 <5 mm, 32 (28.1%) were malignant. Of 14 malignancies, 7 (50%) were in patients with recently diagnosed breast cancer, 6 in the same breast, of which 4 (66.7%) were in same quadrant. Higher likelihood of malignancy based on proximity to known cancer was statistically significant (P = .01). No significant difference in proportion of malignancies was found based on age, T2-weighted signal, morphology, or kinetics.

For MR-detected biopsied masses, the positive predictive value for malignancy of those ≤5 mm was 20.6%. The highest prevalence of cancers was in the same quadrant as a newly diagnosed breast cancer. The decision to biopsy small masses should be based on carefully assessed MR features, and in the context of exam indication, not solely on size.