P4704 - Colorectal Endometriosis Masquerading as Metastatic Colorectal Cancer: Rethinking Diagnostic Certainty

A 48-year-old female with no medical history had a screening colonoscopy, revealing a 5cm rectosigmoid mass, with unrevealing endoscopic mucosal biopsies. A CT scan of the abdomen, and pelvis demonstrated short segmental thickening in the sigmoid colon, enlarged inferior mesenteric artery lymph nodes, and a large uterine fibroid. MRI results raised concern for a T2N2 stage rectal cancer, identifying 4 suspicious mesorectal lymph nodes.  A rectal endoscopic ultrasound (EUS) with biopsy showed no distinct mucosal lesion and non-diagnostic biopsies, revealing mildly inflamed anorectal mucosa with underlying fibromuscular tissue. A subsequent diagnostic laparoscopy demonstrated a large uterine fibroid and normal-appearing bilateral adnexa. However, the rectosigmoid mass could not be assessed due to the size and shape of the uterus, and there was no evidence of an intraperitoneal process. The patient was seen by Gynecology and Oncology with a Papanicolaou (PAP) test and cervical biopsy, which were negative and non-diagnostic for malignancy. She underwent a pelvic examination under anesthesia, and a true cut transvaginal biopsy from the posterior cervical area was unremarkable.   

PET-CT later demonstrated hypermetabolic activity in the rectal mass and other pelvic areas, highly suggestive of malignancy. Given non-diagnostic surgical and gynecologic workup, and repeat imaging with ongoing concern for malignancy, an EUS with fine needle biopsy was repeated. Biopsies ultimately revealed the mass-like lesion as endometriosis. The patient was ultimately diagnosed with benign endometriosis.


Discussion: Colorectal endometriosis results from deep endometrial infiltration, typically in the rectum and sigmoid colon. CEM can appear as a mass with heterogeneous density, irregular contours and associated lymphadenopathy, often mimicking colorectal malignancy on cross-sectional imaging and colonoscopy. Clinicians should be cautious in interpreting imaging suggestive of malignancy in the absence of confirmatory biopsy findings, as endometriosis may present similarly but require different management.

Disclosures:
Nargiz Gasimova indicated no relevant financial relationships.
Devon Thorpe indicated no relevant financial relationships.
Shahtaj Shah indicated no relevant financial relationships.
Bessie Roca Loor indicated no relevant financial relationships.
Peter Bhandari indicated no relevant financial relationships.
Sanjay Salgado indicated no relevant financial relationships.
Irene Rossmer indicated no relevant financial relationships.

Nargiz Gasimova, MD, Devon Thorpe, MD, Shahtaj Shah, MD, Bessie Roca Loor, MD, Peter Bhandari, MD, Sanjay Salgado, MD, Irene Rossmer, MD. P4704 - Colorectal Endometriosis Masquerading as Metastatic Colorectal Cancer: Rethinking Diagnostic Certainty, ACG 2025 Annual Scientific Meeting Abstracts. Phoenix, AZ: American College of Gastroenterology.