Cytological findings for the diagnosis of primary thyroid mucosa-associated lymphoid tissue lymphoma by fine needle aspiration.

OBJECTIVE We examined cytological findings for the diagnosis of primary thyroid mucosa-associated lymphoid tissue (MALT) lymphoma by fine needle aspiration. STUDY DESIGN During the study period of 4 years, a total of 101 cases including 51 MALT lymphomas, 20 Hashimoto's thyroiditis (HT), and 30 diffuse large-cell B-cell lymphomas were cytologically examined. MALT lymphomas were divided into 44 common MALT and 7 MALT lymphomas with extreme plasmacytic differentiation (MALT-EPCD). RESULTS (1) Small- to medium-sized cells displaying irregularly shaped nuclei with prominent nucleoli (ISN-PN) were neoplastic cells. (2) In the case of a frequency of plasma cells (PC) below 15%, the accuracy rate for distinguishing common MALT from HT was 97% for ISN-PN cell frequencies above 20% in combination with the presence of lymphoepithelial lesion clusters (LELC) and mountain range-like clusters (MRLC). The frequency of large-sized cells was below 15% in common MALT. (3) In the case of a frequency of PC above 15%, cases with a sum of PC and ISN-PN cells above 30% were MALT-EPCD. (4) MRLC were cell clusters derived from regions of follicular colonization, and LELC were cell clusters from lymphoepithelial lesions of MALT lymphomas. CONCLUSION Useful cytological criteria for the diagnosis of thyroid primary MALT lymphoma, such as neoplastic cells and cell clusters, were defined.