What is the recommended diagnostic workup for an adult patient with circumferential mucosal thickening worrisome for a constricting type mass lesion at the rectosigmoid junction, possibly with a history of gastrointestinal symptoms?

Diagnostic Workup for Circumferential Mucosal Thickening at the Rectosigmoid Junction

Immediate colonoscopy with multiple biopsies (minimum 6 specimens from the lesion and surrounding mucosa) is the essential first step to establish histological diagnosis and differentiate between malignancy, inflammatory bowel disease, endometriosis, and other etiologies. 1, 2

Initial Endoscopic Evaluation

Perform complete ileocolonoscopy to the cecum to exclude synchronous lesions (present in 2.5% of cases) and assess disease distribution, as approximately 35% of colorectal tumors occur proximal to the sigmoid. 3, 4

Critical Documentation During Endoscopy

• Exact location relative to the gastro-oesophageal junction, distance from anal verge, and extension into adjacent segments 1
• Circumferential involvement and degree of luminal narrowing 1
• Lesion characteristics: length, surface appearance (ulcerated, nodular, stricturing), and presence of obstruction 1, 4
• Pattern of inflammation: continuous vs. skip lesions, rectal involvement, and mucosal appearance in non-affected areas 1, 2

Biopsy Protocol

Obtain at least 6 biopsies from the lesion itself, plus additional biopsies from both inflamed and normal-appearing mucosa in all colonic segments including terminal ileum. 1, 2 This distinguishes:

• Malignancy: Look for adenocarcinoma (95-98% of rectal masses), neuroendocrine tumors, lymphoma, or rare collision tumors 4, 5
• Inflammatory bowel disease: Basal plasmacytosis, crypt architectural distortion, and granulomas suggest IBD; preserved crypt architecture with acute inflammation only suggests infection 2
• Endometriosis: Can mimic malignancy in women of reproductive age, presenting with obstruction and rectal bleeding 6

Laboratory Assessment

Obtain comprehensive baseline laboratory studies including:

• Complete blood count, liver function, renal function, albumin, iron studies 1, 2
• C-reactive protein and fecal calprotectin: Calprotectin >100 μg/g has 93% sensitivity and 96% specificity for IBD diagnosis 2
• Carcinoembryonic antigen (CEA) if malignancy suspected 4
• Stool cultures, C. difficile toxin, and ova/parasites before diagnosing IBD, as infectious colitis (Salmonella, Shigella, Campylobacter, CMV) can mimic inflammatory or neoplastic disease 2

Cross-Sectional Imaging

Contrast-enhanced CT or MRI of abdomen/pelvis is mandatory to evaluate:

• Extent of wall thickening and transmural involvement 1
• Extramural complications: abscesses, fistulas, or lymphadenopathy 1
• Distant metastases: liver lesions and peritoneal carcinomatosis 4
• Adjacent organ involvement: particularly ovarian masses in women (ovarian cancer commonly seeds to rectosigmoid) 7

Imaging Modality Selection

• MRI pelvis without and with IV contrast is superior for rectal/rectosigmoid lesions, providing detailed assessment of T-stage, circumferential resection margin status, and sphincter involvement if malignancy confirmed 1, 4
• CT colonography can identify proximal lesions beyond endoscopic reach if colonoscopy incomplete due to obstruction 1

Differential Diagnosis Considerations

High-Priority Diagnoses to Exclude

Colorectal adenocarcinoma (most common): Sigmoid colon is the most frequent site (52.6% of malignant polyps), and rectosigmoid junction tumors have 37% obstruction rate and lower resectability (44.4%) compared to other sites 3, 8

Inflammatory bowel disease:

• Ulcerative colitis shows continuous inflammation from rectum proximally (>97% have rectal involvement) 2
• Crohn's disease demonstrates skip lesions, transmural inflammation, and potential perianal disease 2

Endometriosis: Consider in women aged 20-50 with cyclic symptoms, weight loss, and rectal bleeding; can create mass lesions mimicking malignancy 6

Ischemic colitis: Particularly in elderly patients with vascular risk factors; typically affects watershed areas 2

Medication-induced colitis: NSAIDs, immunosuppressants, and chemotherapy agents can cause erosive changes 2

Multidisciplinary Discussion

All cases with confirmed or suspected malignancy require multidisciplinary team review including gastroenterology, surgery, radiology, pathology, and oncology before definitive treatment planning. 1, 4 For patients >70 years, formal geriatric assessment is recommended. 4

Common Pitfalls to Avoid

• Inadequate biopsy sampling: Friable mucosa may yield non-diagnostic specimens; obtain minimum 6 biopsies from multiple sites 1
• Assuming single pathology: Collision tumors (e.g., adenocarcinoma with neuroendocrine component) occur rarely but require different management 5
• Overlooking infectious causes: Always exclude C. difficile and other pathogens before diagnosing IBD 2
• Incomplete colonoscopy: Synchronous lesions occur in 2.5% of cases; if obstruction prevents complete examination, perform CT colonography or interval colonoscopy after treatment 3
• Missing extracolic pathology: Rectosigmoid can be secondarily involved by ovarian, uterine, or bladder malignancies 7