Comparison of ProEx C with p16INK4a and Ki-67 immunohistochemical staining of cell blocks prepared from residual liquid-based cervicovaginal material: A pilot study

BACKGROUND. Although liquid-based cervicovaginal cytology has high sensitivity for detecting dysplastic/malignant lesions, many pitfalls exist. Cell blocks can be prepared from residual liquid-based cervicovaginal material and used for immunohistochemistry. The aim of this study was to evaluate a new marker, ProEx C, on cell blocks and its ability to distinguish dysplastic/malignant lesions from morphologically abnormal but benign cells. The results of this study were compared with previously reported results for p16 and Ki-67 on the same material. METHODS. ProEx C is a cocktail of monoclonal antibodies against proteins associated with aberrant S phase cell cycle induction (topoisomerase IIA, minichromosome maintenance protein 2). ThinPrep (CytycCorp., Boxborough, Mass) cervicovaginal specimens from 79 patients were selected. Four cases had no residual abnormal cells in the cell block. On the basis of the cell block diagnosis, 29 cases were negative for intraepithelial lesion or malignancy (NILM), 27 had low-grade squamous intraepithelial lesions (LSIL), 16 had high-grade squamous intraepithelial lesions (HSIL), and 3 had squamous cell carcinomas (SCC). Cell block sections were immunostained with ProEx C. RESULTS. Thirteen of 16 (81%) cases of HSIL stained positively with ProEx C. Two of 27 (7%) LSIL stained positively, and 2 (7%) cases of NILM stained positively. All 3 cases of SCC were strongly positive (100%). Staining for ProEx C showed a higher positive predictive value compared with p16. CONCLUSIONS. ProEx C can be used on cell blocks prepared from residual liquidbased cervicovaginal cytologic specimens. Being a nuclear only stain, it is cleaner and easier to interpret even in scant specimens.