S4054 Colorectal Endometriosis Masquerading as Metastatic Colorectal Cancer: Rethinking Diagnostic Certainty

Introduction: Magnetic resonance imaging (MRI) is a key diagnostic modality in assessing and staging rectal cancers with a 75% specificity and 87% accuracy. However, despite its effectiveness, MRIs can sometimes lead to misdiagnosis due to the overlap in radiographic features. We present a case of colorectal endometriosis (CEM) that was misinterpreted on cross-sectional imaging as metastatic malignancy. Case Description/Methods: A 48-year-old woman with no medical history had a screening colonoscopy, revealing a 5 cm rectosigmoid mass, with unrevealing endoscopic mucosal biopsies. A computed tomography (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. Positron emission tomography-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 endoscopic ultrasound 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.