[2077] Lack of Reactivity with TdT in Small Cell Carcinoma

Bahram R Oliai, Cary J Buresh, Rodney T Miller. ProPath Laboratory, Dallas, TX

Background: The differential diagnosis between small cell carcinoma (SCC) and Merkel cell carcinoma (MCC) can be difficult as both are high grade neuroendocrine carcinomas which share many cytomorphologic and immunophenotypic features. Previous reports have shown that significant expression of TdT can be seen in up to 73% of MCC. Thus far we are aware of only one published report examining the expression of TdT in a series of 30 SCC. Herein we report our experience with TdT expression in a series of 108 cases of SCC.
Design: We reviewed 108 cases of SCC sent to our group in consultation for confirmatory immunophenotyping over a 5 year period. All tumors demonstrated the typical perinuclear dotlike staining pattern with low molecular weight cytokeratin. All but 2 cases expressed a combination of the neuroendocrine markers CD56, synaptophysin, and chromogranin (1 case expressed only CD56 and 1 case only synaptophysin). All but 4 cases expressed strong diffuse nuclear TTF-1 immunoreactivity (with 2 negative cases and 2 cases showing only focal staining of 5% or less cells). All cases demonstrated very high proliferation rates, as assessed by Ki-67 immunostaining. Cytokeratin 20 immunostaining was performed in 83 cases, with only 6 cases demonstrating reactivity (5 cases showing focal, dim staining and 1 case showing diffuse perinuclear dotlike staining), all positive cases demonstrated strong, diffuse nuclear TTF-1 reactivity.
Results: Of 108 cases of SCC, only 3 (2.7%) demonstrated reactivity with TdT, with only 5-15% of cells staining.
Conclusions: Since previous work has shown that TdT is positive in up to 73% of MCC, TdT may well be a useful immunostain in the differential diagnosis between SCC and MCC, with negative staining arguing in favor of SCC.
Category: Techniques

Wednesday, March 6, 2013 9:30 AM

Poster Session V # 297, Wednesday Morning


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