CT Colonography (Virtual Colonoscopy) Explained: When It Replaces Colonoscopy

CT colonography (CTC, or CT pneumocolon) offers a minimally invasive alternative to optical colonoscopy for the assessment of colorectal cancer and polyps. NICE recommends CTC when: colonoscopy is incomplete (~5% of cases), colonoscopy is contraindicated (e.g., frailty, severe cardiorespiratory disease, anticoagulation), or patient preference. Preparation is similar to colonoscopy (bowel cleansing) with the addition of faecal tagging (dilute barium or iodine) to distinguish stool from polyps. Rectal air or CO₂ insufflation distends the colon; low-dose CT is acquired in prone and supine positions. C-RADS (2005, revised) classifies findings: C0 (inadequate), C1 (normal or benign), C2 (indeterminate 6–9 mm polyp), C3 (polyp ≥ 10 mm or > 3 polyps 6–9 mm), C4 (mass suspicious for cancer), plus E-classification for extra-colonic findings (E1 normal to E4 requiring urgent workup). CT-C changes management in ~15% of cases through extra-colonic findings (AAAs, renal masses, adnexal lesions). Sensitivity for polyps ≥ 10 mm > 90%; < 6 mm poor and generally not reported. Any polyp ≥ 6 mm triggers colonoscopy for removal in most protocols. Does not treat — needs colonoscopy for polypectomy if positive.