ABSTRACT
<p><b>OBJECTIVE</b>To study the expression of squamous cell markers p63, p40 and CK5/6 in small cell carcinoma of lung (SCLC).</p><p><b>METHODS</b>Immunohistochemical study for squamous cell markers (p63, p40 and CK5/6), neuroendocrine markers (chromogranin A, synaptophysin and CD56) and TTF1 was carried out in 283 cases of SCLC. The diagnostic value of these markers was evaluated.</p><p><b>RESULTS</b>The expression rate of p63, p40 and CK5/6 were 20.7% (54/261), 7.9% (5/63) and 0.5% (1/221), respectively in the cases of SCLC studied. Amongst the squamous cell markers, CK5/6 had the lowest rate of positivity (P < 0.01). On the other hand, chromogranin A, synaptophysin and CD56 were positive in 61.8% (170/275), 85.5% (242/283) and 89.2% (248/278), respectively. The positivity rate for chromogranin A was lower than that for synaptophysin and CD56 (P < 0.01). TTF1 was expressed in 77.2% (217/281).</p><p><b>CONCLUSIONS</b>p63 and p40 are expressed in a subset of SCLC. In contrast, CK5/6 is rarely positive in SCLC. An immunohistochemical panel of CK5/6, synaptophysin and CD56 is recommended for differential diagnosis of SCLC.</p>