Endometrioid adenocarcinoma arising from inguinal endometriosis during follow-up for gastric cancer surgery: a case report and literature review.

Malignant transformation of inguinal endometriosis is rare. A 56-year-old woman underwent surgery for advanced gastric cancer 5 years ago and received postoperative adjuvant chemotherapy. She had no recurrence since then. However, 5 years after surgery, contrast-enhanced computed tomography (CT) showed a mass in the right inguinal region suspected to be a hydrocele of the canal of Nuck, with a blood test showing a slightly elevated CA19-9 level (63.0 U/mL). Six months later, CT showed an enlarged mass in the right inguinal region and inflammation in the surrounding area. In addition, both inguinal lymph nodes and those in the right iliac artery area were enlarged, suggesting the possibility of malignancy. For diagnostic purposes, a right inguinal mass was excised. Histopathological examination revealed that it was endometrioid adenocarcinoma with ectopic endometriois as the origin. The differential diagnoses for inguinal masses in women include an inguinal hernia, hydrocele of the canal of Nuck, ectopic endometriosis, lymphoma, and metastatic malignancy. The presence of a primary malignancy in the inguinal region is sporadic but must be differentiated. This is the first case of malignant transformation of inguinal endometriosis developed during postoperative follow-up of another cancer.