CD34 immunohistochemistry in female genital tract carcinosarcoma (malignant mixed müllerian tumors) supports a dominant role of the carcinomatous component.

: Female genital carcinosarcomas (FGTCSs) are aggressive biphasic malignant neoplasms with histologic features of both carcinomas and sarcomas. However, their behavior is dominated by the carcinomatous component. CD34 is useful in distinguishing carcinomas with sarcomatoid features from epithelioid sarcomas, as it is never expressed in carcinomas although often expressed in epithelioid sarcomas. This investigation hypothesizes that CD34 expression in FGTCSs will contribute useful histogenetic and clinical information. Paraffin sections from a surgical series of 43 FGTCSs were stained using anti-CD34 with an automated immunohistochemical stainer, antigen retrieval, and appropriate external/internal controls. Reactions were graded as follows: negative (N) or S1-3, strong reactivity in 50% = S3 of cells, respectively. The series included 31 endometrial, 6 ovarian, 5 cervical, and 1 fallopian tubal FGTCSs, 70% exhibiting heterologous elements (22 rhabdomyosarcomas, 11 chondrosarcomas, 1 osteosarcoma, and 1 liposarcoma). The carcinomatous component included 19 endometrioid, 12 serous, 6 adenosquamous, 4 clear cell, and 2 poorly differentiated carcinomas: modified International Federation of Gynecologic Oncology grade I, 9%; II, 21%; and III, 70%. CD34 reactivity was 100% N in the carcinomatous components. CD 34 stained 21 % of sarcomatous components as follows: 2S1, 6S2, and 1S3 (three homologous and six heterologous). Of the heterologous components, only rhabdomyosarcoma stained with the following patterns: 1S1, 2S2, 1S3. Two of six heterologous FGTCSs exhibited staining of only the homologous sarcomatous component with the following pattern: 2S2, as the heterologous sarcomatous component (pure rhabdomyosarcoma in both cases) showed no staining. The staining pattern was unrelated to primary site of the FGTCSs. The pattern of CD34 expression (N in the carcinomatous component like carcinomas) supports the generally accepted opinion of dominance of FGTCS by the carcinomatous component. The staining of the sarcomatous component was rare (21%) and focal (only one of nine was S3). CD34 staining may help distinguish FGTCSs from epithelioid sarcomas, which strongly express CD34.