Can p40 (Polyclonal) Replace p63 (Clone 4A4) in the Cytologic Diagnosis of Pulmonary Non-Small Cell Carcinoma?

Abstract

Objectives: Differentiating squamous cell carcinoma from adenocarcinoma (ACA) in cytology specimens can be challenging. Recent literature showed p40 had higher specificity than p63 for this purpose. Methods: We identified 190 cytology cases with p40 (polyclonal) and p63 (monoclonal clone 4A4) immunohistochemistry, including specimens from fine-needle aspirations (FNAs) and effusions. Results: ACAs of lung origin stained for p40 and p63 in 21% and 20% of cases, respectively, regardless of specimen site. Among lung FNAs of primary pulmonary ACAs (n = 42), 14% were positive for p40 and 24% were positive for p63. Of the 20 pulmonary ACAs in effusions, more cases showed p40 positivity (40%) compared with FNAs, whereas p63 were positive in 15%. Among metastatic ACAs from other sites (n = 14), more cases were positive for p40 than p63. Conclusions: Polyclonal p40 yields a level of false positivity in ACAs similar to p63, which is highest in effusions and is not limited to lung With the evolution of targeted therapies, accurate identification of squamous differentiation in cytology specimens has become critical. This issue is best exemplified in lung specimens, in which the distinction between squamous cell carcinoma (SCC) and adenocarcinoma (ACA) is crucial in guiding therapy. Cytology specimens are not uncommonly the only diagnostic material on which treatment is based. Often this biopsy specimen is from fineneedle aspiration (FNA), which has the advantage of being a minimally invasive tool that can sample sites not amenable to core biopsy or surgical excision. These samples are often of modest cellularity and must be used judiciously for special stains to conserve material for possible subsequent molecular studies. Other cytology specimens such as effusions are also common and sometimes the only material available for diagnostic testing in metastatic cancer. To conserve tissue for molecular studies, a minimalist approach for immunohistochemistry (IHC) panels performed on cytology specimens is of the utmost importance. Antibodies against p63 have been the most common tool used to identify squamous differentiation in both cytology and histology specimens. p63 is a p53 homologue that is involved in stem cell commitment toward squamous differentiation. It is often used for lung specimens in a panel with markers of ACA differentiation, TTF-1 and napsin A. An N-terminally truncated isoform of p63 (p40) has recently been described. Antibodies to both p40 and p63 have consistently been shown to have high sensitivity for SCC in histology specimens. The superiority of p40 IHC has been attributed to its better specificity for lung SCC vs ACA, with p63 showing a higher rate of false positivity (range, 18%-31%) compared with p40 (0%-3%). Thus, recommendations have been made that p40 should replace p63 for routine use in differentiating SCC from ACA. We have been using p63 (clone 4A4), and for the past 2 years, we added p40 (polyclonal) to a panel we implemented to distinguish SCC from ACA. Few studies have evaluated the clinical performance of p40 IHC in cytology specimens. The existing studies are focused on imageguided FNA specimens of primary lung masses, have variable sample sizes (ranging from 30-144), and use different antibody clones. These studies report high sensitivities for detecting lung SCC using both p63 and p40 on IHC-stained cell block sections similar to those seen in histologic studies. They also report greater specificity using p40 than p63, with false-positive rates for p63 of up to 63% in adenocarcinomas compared with 0% to 6% for p40. Other important and common cytology specimens that have not been well studied with regard to p40 staining include effusions and metastases as well as specimens from sites other than lung. Our study is the largest to date to evaluate p40 and p63 staining in cytology specimens and includes a variety of types of specimens from multiple primary sites.