Müllerian clear cell carcinoma arising from rectal endometriosis with pericolonic lymph node metastasis

We present a case of clear cell carcinoma arising in endometriosis of the rectum that provides insights to the biological behavior of this extremely rare tumor. The patient presented with a large rectal mass, and the biopsy was suggestive of clear cell carcinoma of Müllerian origin based on the morphology and immunoprofile. The patient underwent a proctosigmoidectomy and hysterectomy with bilateral salpingo-oophorectomy with clear cell carcinoma involving the colon. Five pericolonic lymph nodes were positive for carcinoma. Endometriosis was identified in the colonic wall adjacent to the carcinoma. No tumor was identified in the endometrium, fallopian tubes, or ovaries. ARID1A mutations were identified by next-generation sequencing. The patient underwent adjuvant chemotherapy and is disease-free 4 years after the diagnosis. To our knowledge, this is the first case of clear cell carcinoma arising in colon endometriosis with pericolonic lymph node metastasis.