Mucinous breast carcinoma has characteristic cytologic features, but few studies exist that analyze the reproducibility of this diagnosis.
To analyze participants' diagnosis of mucinous carcinoma in breast fine-needle aspiration (FNA) slides distributed in an educational interlaboratory peer comparison program.
Participant responses for FNA slides with a reference diagnosis of mucinous carcinoma, distributed between 2001-2008 in the College of American Pathologists Interlaboratory Comparison Program in Nongynecologic Cytopathology, were evaluated for concordance with the general category and reference diagnosis of mucinous carcinoma.
Of 8061 responses, 6353 (78.8%) were categorized as malignant; 775 (9.6%) as suspicious; and 933 (11.6%) as negative. The most frequent incorrect responses for the benign category included fibroadenoma (51.7%), nonspecified benign lesion (12%), fibrocystic changes (7.8%), and fat necrosis/granulomatosis/foreign body reaction (6.9%). Conventional Papanicolaou-stained preparations were reviewed for 58.7% (4732) of responses; of these, 39.4% (3177) were from modified Giemsa-stained smears and 1.9% (152) from ThinPrep slides. Papanicolaou-stained conventional smears had the lowest concordance (86.5%) when compared to modified Giemsa-stained smears (91.2%) and ThinPrep challenges (92.1%) (P < .001). Participants specifically diagnosed mucinous carcinoma 37.3% of the time, and modified Giemsa-stained challenges performed best (43.1%, P < .001). There was no significant difference between cytotechnologists' and pathologists' responses (87.9% versus 88.2%; P  =  .69).
Mucinous carcinoma in FNA was not accurately identified in a glass slide interlaboratory comparison program. We observed better performance with modified Giemsa-stained and ThinPrep slides than with Papanicolaou-stained preparations. The most common response for the benign category of mucinous carcinoma was fibroadenoma. Increased awareness of the cytologic features of mucinous carcinoma may improve accuracy in breast FNA.