Fine-needle aspiration biopsy of endometriosis of the abdominal wall: a potential diagnostic pitfall

Endometriosis is a relatively common disease in women, usually of reproductive age. In rare occasions, endometriosis may occur in unusual body sites, including extragonadal, extra-pelvic sites, or in the skin. We report a case of endometriosis of the abdominal wall diagnosed by fine-needle aspiration biopsy (FNAB). The patient was a 41-year-old woman with a clinical impression of "desmoid" tumor of the abdominal wall. FNAB showed groups of epithelial cells with mild nuclear atypia and rare mitoses. A few small clusters of slightly atypical spindle cells were also seen. The initial screening impression was adenocarcinoma due to the atypical cytological features. However, after review with the pathologist, the FNAB was signed out as "endometriosis." Subsequent local excisional biopsy confirmed the diagnosis of endometriosis. In conclusion, endometriosis may present diagnostic challenges in FNAB and this possibility should be considered in differential diagnosis. A clinical cytopathological correlation is necessary to avoid a misdiagnosed as an adenocarcinoma.