Greater specificity of p40 compared with p63 in distinguishing squamous cell carcinoma from adenocarcinoma in effusion cellblocks

OBJECTIVE: Squamous cell carcinoma (SCC) rarely causes malignant effusions. Distinguishing between SCC and adenocarcinoma in effusion cytology can be a challenge. p63 and p40 have been frequently used to support squamous cell differentiation in both histological and cytological specimens. However, similar results in cytological preparations of effusion fluids have been rarely reported. This study was designed to assess the diagnostic value of p63 and p40 immunoreactivity for the differentiation of SCC from adenocarcinoma in malignant effusions. MATERIALS AND METHODS: Immunocytochemical staining of p63 and p40 was performed on thirty cellblock specimens, including ten malignant effusions carrying SCC and twenty malignant effusions showing adenocarcinoma. Any degree of nuclear staining was considered positive. RESULTS: Of the ten SCC cases, 100% tested positive for both p63 and p40, and most cases showed diffuse staining (>25% of tumor cells). The expression of p63 and p40 was detected in 4 (20%) and 2 (10%) of twenty adenocarcinoma cases, and the extent of staining was all focal (≤25% of tumor cells). The p63 reactivity showed 100% sensitivity, 80% specificity, 71% positive predictive value, and 100% negative predictive value for the differentiation of SCC from adenocarcinoma in malignant effusions. The sensitivity of p40 for SCC was 100%, the specificity was 90%, the positive predictive value was 83%, and the negative predictive value was 100%. CONCLUSION: Although p63 and p40 are both useful markers for the diagnosis of SCC in malignant effusions, p40 is more specific than p63 in distinguishing SCC from adenocarcinoma.