Case Report: A patient with lynch syndrome with vaginal endometriosis-associated malignancy and synchronous colonic tubulovillous adenoma

Endometriosis is a common gynecological condition. However, endometriosis-associated malignancies occur in up to 1% of women with endometriosis. Most cases of endometriosis-associated malignancy occur in the ovary, whereas 20% of cases occur at extragonadal sites. Herein, we report the case of a patient with an incidental finding of vaginal endometriosis-associated malignancy who was later diagnosed with Lynch syndrome due to MSH6 deletion with loss of protein expression and was subsequently found to have a high-grade colonic tubulovillous adenoma. The patient was a 50-year-old (Para 2 + 2) woman without any previous history suggestive of adenomyosis or endometriosis, who was examined at a local hospital and was found to have swelling in the posterior vaginal fornix. Colposcopy was performed, and the mass was biopsied, revealing endometrial adenocarcinoma. She was then transferred to our hospital, where, after a series of assessments, she underwent surgery (including total hysterectomy, double adnexectomy, partial vaginal hysterectomy, and lymph node dissection). A postoperative pathological examination indicated a diagnosis of vaginal endometriosis-associated malignancy. A paclitaxel/carboplatin (TC) regimen (paclitaxel 175 mg/m² + carboplatin AUC 5) was initiated 9 days postoperatively. Loss of MSH6 protein expression in the mass was observed using postoperative immunohistochemistry. Genetic sequencing revealed pathogenic MSH6 variants, including p. F1104Lfs*11 (c.3312delT) and c.3556 + 1G > A, indicating germline mutations. These findings suggest the presence of Lynch syndrome. Before the second postoperative chemotherapy cycle, the patient underwent a colonoscopy, and a mass measuring approximately 6 cm in diameter was identified in the right half of the transverse colon. One month after the second cycle of chemotherapy, the patient underwent laparoscopic radical right hemicolectomy. Histopathological examination revealed a tubulovillous adenoma with high-grade intraepithelial neoplasia of partial glands. One month after the second surgery, the patient was referred to our department, completed four cycles (a total of six cycles) of combination chemotherapy (carboplatin and paclitaxel), and was recurrence-free at the last follow-up (July 2025). Lynch syndrome with both extra-ovarian endometriosis-associated cancer and intestinal lesions is rare. We report a case of incidentally identified vaginal endometriosis-associated malignancy in a patient with Lynch syndrome due to MSH6 protein deficiency and MSH6 germline mutations and the discovery of a high-grade tubular choriocapillaris adenoma of the colon. This case highlights the critical importance of MMR protein testing in the screening for Lynch syndrome in patients with extra-ovarian endometriosis-associated cancer and preoperative colonoscopy. Although there are no established guidelines for the treatment of extra-ovarian endometriosis-associated cancer, its management is currently based on protocols for the treatment of primary ovarian cancer.