Context: Mucinous breast carcinoma has characteristic cytologic features, but few studies exist that analyze the reproducibility of this diagnosis.
Objective: To analyze participants' diagnosis of mucinous carcinoma in breast fine-needle aspiration (FNA) slides distributed in an educational interlaboratory peer comparison program.
Design: 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.
Results: 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).
Conclusions: 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.