CD117 ( KIT ) is a useful immunohistochemical marker for differentiating porocarcinoma from squamous cell carcinoma

Background Distinguishing porocarcinoma from squamous cell carcinoma ( SCC ) is clinically significant but can pose a diagnostic dilemma. The present study sought to confirm the diagnostic utility of CD117 immunohistochemistry in distinguishing porocarcinoma from SCC and to examine histologic, carcinoembryonic antigen ( CEA ) immunohistochemical and CA19 ‐9 immunohistochemical differences between these tumors. Methods Immunostaining with anti‐ CD117 , anti‐ CEA and anti‐ CA19 ‐9 antibodies was performed for 22 porocarcinomas and 31 SCCs . The extent of CD117 , CEA and CA19 ‐9 staining was classified as negative (<1%), rarely positive (1–4%), focally positive (5–29%) or diffusely positive (30–100%). CD117 staining intensity was semi‐quantitatively graded as weak, moderate or strong. Results All (100%) porocarcinomas were positive for CD117 , with mainly focal (8/22) or diffuse (11/22) and moderate (9/22) to strong (8/22) staining. In contrast, only 6 of 31 SCCs (19.4%) expressed CD117 focally, and this expression was limited to the basal layer of the tumor in four cases. CEA immunostaining highlighted the lumina of all 22 porocarcinomas; however, CEA expression was not significantly different between porocarcinomas and SCCs (100 vs. 71.0%, respectively). CA19 ‐9 was not expressed in the lumina of 5 of 22 porocarcinomas. Conclusions Along with CEA , CD117 immunohistochemistry could be helpful in distinguishing porocarcinomas from SCCs .