CD24 is highly useful in differentiating high-grade serous carcinoma from benign and malignant mesothelial cells.

CD24 was previously shown to be overexpressed in high-grade serous carcinoma (HGSC) effusions compared to malignant mesothelioma (MM) in gene expression array analysis. The present study validated this observation in a large series consisting of both effusions and surgical specimens. Effusions (n = 206; 100 HGSC, 16 ovarian carcinomas of other histological types, 54 breast carcinomas, 36 MM) and surgical specimens (n = 182; 117 ovarian carcinomas, 65 MM) were analyzed for CD24 expression using immunohistochemistry. CD24 was expressed in 105/116 (91%) ovarian carcinoma and 16/54 (30%) breast carcinoma effusions, while it was uniformly absent in MM (0/36; 0%; P < .001). Reactive mesothelial cells were CD24-negative in all carcinoma specimens. Comparative analysis of 117 solid primary (n = 43) and metastatic (n = 74) ovarian carcinomas and 65 solid MM specimens showed CD24 expression in 46% (54/117) of the former compared to 3% (2/65) of the latter (P < .001). Comparative analysis of ovarian carcinomas at different anatomic sites showed significantly higher CD24 expression in effusions compared to solid ovarian and metastatic lesions (P < .001), with similar results when analysis was limited to HGSC (P < .001). In conclusion, CD24 is a highly sensitive and specific marker of ovarian carcinoma in the differential diagnosis from MM and reactive mesothelium in effusions. CD24 is similarly a specific marker in surgical specimens, though with lower sensitivity. The overexpression of CD24 in ovarian carcinoma effusions compared to solid lesions may be due to the acquisition of cancer stem cell characteristics.