Alpha-smooth muscle actin-positive myofibroblasts in endometrial stroma are not a reliable criterion for the diagnosis of well differentiated endometrioid adenocarcinoma in small tissue samples.

Although a desmoplastic stromal reaction in well-differentiated endometrioid adenocarcinoma is considered a major criterion in the differential diagnosis with atypical hyperplasia, this histologic feature has not met with universal approval. Since alpha-smooth muscle (alpha-SM) actin positive myofibroblasts characterize the desmoplastic stromal response in a variety of neoplasms, the present study was undertaken in order to establish whether these cells are also prominent in the stroma of endometrioid carcinoma and if present could be used as a valid criterion in the differential diagnosis between benign and malignant lesions. The present study of 100 endometrial samples showed focal desmoplastic stromal reaction with alpha-SM actin positive myofibroblasts in 30% of small samples and in 50% of hysterectomy specimens with endometrioid carcinoma. In normal endometrium and in benign lesions lacking a desmoplastic reaction, focal stromal alpha-SM actin positivity was a very common finding. Stromal alpha-SM actin-positive cells were also frequently seen in nondesmoplastic stroma of endometrioid carcinoma. Thus the common presence of alpha-SM actin-positive myofibroblasts in normal endometrial stroma and in benign and malignant lesions precludes its usefulness in the diagnosis of well differentiated endometrioid adenocarcinoma, especially in small tissue samples.