p63 immunostaining in destained bronchoscopic cytological specimens

The p53 homologous squamous stem-cell regulatory protein p63 is expressed in squamous carcinomas but is not characteristically detected in small-cell carcinomas (SCCs). A panel of thyroid transcription factor (TTF) 1 and p63 has been shown to be useful in distinguishing SCCs from poorly differentiated squamous carcinoma of the lung (PDSLC) in small biopsies and cytological cell blocks. Because tumor samples frequently are limited to cytological smears, we attempted to detect p63 in destained slides from a spectrum of pulmonary malignancies. Archival alcohol-fixed smears from 60 cases of cytologically diagnosed malignancies in bronchoscopically (n = 59) or fine-needle aspiration-obtained specimens (n = 1) were destained in acid alcohol, postfixed in 10% formalin, subjected to citrate-based antigen retrieval, and immunostained by exposure to anti-p63 monoclonal antibody 4A4, followed by reagents from a streptavidin-biotin immunoperoxidase kit, and diaminobenzidine as the chromogen. Postfixation in 10% formalin was found to be necessary for immunostaining. Normal ciliated and goblet cells were p63 negative, but reserve cells were p63 positive. All cases of squamous-cell carcinoma were positive for p63. Of 10 tumor samples originally diagnosed as SCC, only 6 samples were p63 negative and 4 samples exhibited positive staining. However, proper interpretation of the immunohistochemical (IHC) staining pattern and careful scrutiny of the cytological features and biopsy specimens in three of four cases led us to reclassify three cases into PDSLC. All adenocarcinomas (ACAs; n = 12), large-cell carcinomas (n = 4), and metastatic ACAs (n = 5) were p63 negative. Positive staining was seen in 9/16 tumors designated as non-SCCs; these tumors were not classified further into distinct histological categories. p63 staining in destained slides may be of value in facilitating the differential diagnosis between PDSLC and SCC. Criteria for conservative interpretation of results are discussed and include examination of reserve cells and ciliated cells on the same slide as internal positive and negative controls.