[541] Napsin A Expression in Anaplastic, Poorly Differentiated and Micropapillary Pattern Thyroid Carcinomas

Rebecca Chernock, Samir El-Mofty, Nils Becker, James Lewis, Jr.. Washington University School of Medicine, St Louis, MO

Background: Napsin A is more sensitive and specific for pulmonary adenocarcinoma versus squamous cell carcinoma than thyroid transcription factor-1 (TTF-1). TTF-1 is also a recognized marker of thyroid carcinomas. Preliminary studies have shown that napsin A is positive in ∼5% of papillary thyroid carcinomas as well. The prevalence of napsin A in anaplastic (ATC) and poorly differentiated (PDCa) thyroid carcinomas has not been thoroughly investigated. Napsin A positivity in metastatic thyroid carcinoma, especially in conjunction with TTF-1, could potentially be misdiagnosed as a lung metastasis. The aim of this study is to investigate the prevalence of napsin A expression in ATC, PDCa and the recently described micropapillary pattern (MPP) thyroid carcinoma, which show histologic similarity to a subset of lung adenocarcinomas.
Design: Immunohistochemistry for Napsin A, TTF-1 and PAX-8 was performed. Staining strength (weak, moderate or strong) and extent (1+ = 1-25%, 2+ = >25-50%, 3+ = >50-75%, 4+ = >75%) were evaluated.
Results: Twenty-six ATCs (21 primary, 4 regional lymph node metastases, 1 metastasis to larynx), as well as 16 PDCa (14 primary and 2 metastases—1 regional lymph node, 1 lung) and 2 MPP carcinomas (1 primary and 1 regional lymph node metastasis) were identified. A focal MPP component was seen in 3 PDCa and 3 ATCs. The immunohistochemistry results are summarized below:

Table 1. Immunohistochemistry results
Histologic TypeNapsin ATTF-1PAX-8
Poorly differentiated2/16(13%)16/16(100%)15/16(94%)
Micropapillary component3*/6(50%)6/6(100%)5/6(83%)
*All but one Napsin A positive micropapillary component were also Napsin A positive in the poorly differentiated or anaplastic component

All Napsin A positive cases were positive for TTF-1 and all but the 1 primary MPP carcinoma were also PAX-8 positive. Napsin A staining strength was moderate but extent was variable (1+ in 4 cases, 3-4+ in 5 cases).
Conclusions: A minority of ATCs and PDCas are Napsin A positive. Napsin A positivity is more common in carcinomas with a MPP component, a pattern that can also be seen in lung adenocarcinomas. PAX-8 may be diagnostically useful to distinguish these Napsin A positive thyroid carcinomas from lung adenocarcinomas, since PAX8 is typically negative in lung adenocarcinomas but positive in thyroid carcinomas. In this study, only one Napsin A positive thyroid tumor was PAX-8 negative.
Category: Endocrine

Wednesday, March 6, 2013 1:00 PM

Poster Session VI # 33, Wednesday Afternoon


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