Endometriosis is commonly seen in the pelvis, including the peritoneum, with related lesions rare in extrapelvic sites. 1 We report a case of a 56-year-old female with a benign endometrioid adenofibroma/ polypoid endometriosis in an uncommon site: the left paracolic gutter inferior to the spleen, found incidentally in 2020 (measuring 3.3 cm) and was slowly growing. She was known to have endometriosis. The nodule was biopsied in 2025, which suggested endosalpingiosis. She subsequently underwent excision for the nodule, which measured 8 cm grossly. Cut sections showed multiple small gelatinous cystic areas with surrounding grey-whitish, firm and haemorrhagic areas. Microscopy showed a variably cystic lesion within mildly cellular fibrous stroma. The cystic areas were lined by cuboidal epithelium with areas of mucinous metaplasia. Small endometrioid-like and tubal-like glands were seen within the fibrous stroma. The glands were positive for BerEP4, PAX8, BCL2, WT1 (focal) and CK5/6 (focal). Calretinin was negative, and Ki-67 proliferative index was low. A tiny focus of endometriosis was seen in the surrounding fibroadipose tissue. The overall features were overlapping of polypoid endometriosis and a benign endometrioid adenofibroma.
Chia et al. (Sun,) studied this question.