Sources of diagnostic error in the fine-needle aspiration diagnosis of Warthin's tumor and clues to a correct diagnosis.

Squamous metaplasia and cystic degeneration in Warthin's tumor (WT) are not uncommon. A recent case of WT misdiagnosed as metastatic squamous carcinoma with cystic change prompted us to review our experience with fine-needle aspiration (FNA) of WT, with special attention given to potential sources of diagnostic pitfalls. Aspirations from 16 cases of histologically confirmed WT were retrospectively evaluated for cellularity, cell composition, and background. The FNA review diagnosis was compared with the previous FNA and corresponding tissue findings. All tumors presented in the parotid gland. The initial cytologic diagnoses were: WT in 13 cases, oncocytoma vs. low-grade mucoepidermoid carcinoma in 1, squamous carcinoma vs. branchial cleft cyst in 1, and squamous carcinoma in 1. On review, 13 cases (81%) showed typical features associated with WT: Moderate to abundant oncocytic epithelium, lymphoid stroma, background debris, and mild squamous metaplasia. In the remaining cases, one lacked a lymphoid stroma and could not be further classified (initially called "oncocytoma vs. low-grade mucoepidermoid carcinoma"). The other 2 cases contained moderate to abundant atypical squamous cells and extensive necrotic/mucoid debris, and review diagnoses were consistent with the initial cytologic diagnoses (squamous carcinoma in one, and branchial cleft cyst vs. squamous carcinoma in the other). In our series, typical features of WT were seen in 81% of cases. Atypical features are largely present as individual metaplastic squamous cells. Diagnostic errors are caused by a lack of typical features and the presence of individual atypical squamous cells in a necrotic background mimicking carcinoma. An awareness of the morphologic variation present on cytologic preparations and correlation with clinical findings should prevent erroneous interpretation in the FNA setting.