Mucocele-like Lesion of the Breast Diagnosed on Core Biopsy.

CONTEXT.­: Mucocele-like lesion of the breast (MLL) is an uncommon entity, and recent studies show low rates of upgrade from core needle biopsy (CNB) to excision. OBJECTIVE.­: To evaluate features associated with upgrade of MLLs diagnosed on CNB. DESIGN.­: Seventy-eight MLLs diagnosed on CNB from 1998-2019 and subsequent excisions were reviewed. Histologic parameters evaluated included the presence of atypia, presence and morphology of calcifications, and morphologic variant (classic [C-MLL], duct ectasia-like [DEL-MLL], or cystic mastopathy-like [CML-MLL]). RESULTS.­: Overall, 45 MLLs lacked atypia and 33 were associated with atypia (atypical ductal hyperplasia, 32; atypical lobular hyperplasia, 1). Most were C-MLLs (61) with fewer DEL-MLLs (14) and CML-MLLs (3). Half showed both coarse and fine calcifications, with fewer showing only coarse or fine calcifications, and some showing none. Subsequent excision or clinical follow-up was available for 25 MLLs without atypia-of which 2 (8.0%) were upgraded to ductal carcinoma in situ (DCIS)-and 23 with atypia, of which 4 (17.4%) were upgraded to DCIS. No cases were upgraded to invasive carcinoma. All upgraded cases showed coarse calcifications on CNB, and all upgraded cases were associated with residual calcifications on post-CNB imaging. CONCLUSIONS.­: Most MLLs present as calcifications and nearly half are associated with atypia. Upgrade to DCIS is twice as frequent in MLLs with atypia versus those without. A predominance of coarse calcifications and the presence of residual targeted calcifications following core biopsy may be associated with higher upgrade rates.