Question: A 37-year-old woman presented at our hospital with hematochezia (approximately 200 cc). She had experienced constipation and abdominal pain for one month, and four days previously had been admitted to the Department of Gynecology for a cervical polyp with vaginal bleeding. She had undergone bilateral salpingectomy for blocked fallopian tubes at another hospital 3 years prior to the current admission.

On admission, initial laboratory findings revealed anemia(hemoglobin concentration, 7.4 g/dL), and elevated CA-125 levels (75.10 U/mL). Sigmoidoscopy was performed, showing a polypoid, fungating mass with colonic obstruction in the rectum, located 11 cm from the anal verge (Fig. A). Chronic inflammation with ulceration and epithelial hyperplasia were observed in the mucosal layer of endoscopic biopsies. A CT scan of the abdomen and pelvis revealed an ill-defined enhancing lesion in the rectum, abutting the uterus (Fig. B). Strong fluorodeoxyglucose uptake in the rectum was noted on PET-CT scanning (Fig. C). Further analysis by T2W sagittal imaging and MRI confirmed the presence of a fungating rectal mass that was infiltrating the uterus (Fig. D).

Although the presence of cancer was not confirmed in colonoscopic biopsies, we decided to operate based on clinical suspicions of invasive rectal malignancy and colonic obstruction. The patient underwent low anterior resection and total abdominal hysterectomy. Numerous polypoid rectal masses were subsequently detected in the gross specimen. Severe fibrosis and adhesions between the rectum and the lateral wall of uterus were also observed (Fig. E). A pathologic analysis of H&E staining (×100) in the surgical specimen revealed an endometrial-like gland with multifocal stromal cells in the rectum and regional lymph node (Fig. F). Cytokeratin 7-positive cells were detected by immunohistochemical staining for cytokeratin 7 (Fig. G, ×100).

What is the most likely diagnosis?