CT Scan (Computed Tomography)

What It Shows

• Tumour size, shape, density, and relationship to surrounding structures
• Lymph node enlargement and regional spread
• Liver, lung, adrenal, and bone metastases
• Vascular involvement — tumour encasing or invading blood vessels
• Response to treatment — tumour shrinkage measured by RECIST criteria
• Post-operative anatomy and surgical complications
• Pleural effusions, ascites, and other cancer-related complications
• CT-guided biopsy — real-time needle guidance for tissue sampling

How It Works

The patient lies on a motorised table that moves through a large ring (gantry). X-ray beams rotate around the body at multiple angles. Detectors measure the attenuation of X-rays through different tissues. Computer algorithms reconstruct the attenuation data into axial, coronal, and sagittal cross-sectional images. For contrast CT, iodinated contrast is injected IV and images are acquired at timed phases (arterial, portal venous, delayed) to characterise lesion vascularity. A radiologist reviews the images and reports findings.

• Initial staging after cancer diagnosis
• Response assessment during or after treatment (RECIST criteria)
• Surveillance scans in remission
• Emergency assessment for suspected complications (pulmonary embolism, bowel obstruction)
• Radiation therapy planning (4D CT for lung, CT simulation)
• CT-guided biopsy or drainage procedures

CT scanning is the backbone of oncology imaging with decades of validated use. RECIST (Response Evaluation Criteria in Solid Tumours) — the universal standard for measuring chemotherapy response in clinical trials — is CT-based. Low-dose CT lung cancer screening (LDCT) has Level 1 evidence: the NLST trial (N=53,454) showed 20% reduction in lung cancer mortality with annual LDCT screening in high-risk smokers. The NELSON trial confirmed these findings. CT-guided biopsy has >90% accuracy for accessible lesions. Contrast-enhanced CT has sensitivity >90% for liver metastases ≥1 cm.

Preparation & What to Expect

For contrast CT: fast for 4 hours; declare allergy history and kidney function (creatinine/eGFR). Wear comfortable clothing without metal. For abdominal CT: oral contrast may be required 1–2 hours before. The scan itself takes 5–20 minutes. Results typically available within 24–72 hours.

• Soft-tissue contrast is inferior to MRI — poor for brain, spinal cord, liver lesions in some contexts
• Radiation exposure accumulates with repeated scans — relevant for young patients requiring long-term surveillance
• Contrast allergy risk — iodinated contrast can cause reactions from mild (urticaria) to severe (anaphylaxis)
• Contrast nephrotoxicity — caution in pre-existing kidney disease (eGFR < 30)
• Small tumours (< 5 mm) may be below detection threshold
• Cannot differentiate benign from malignant lesions with certainty — biopsy still required

• Radiation dose 2–20 mSv per scan — avoid repeated unnecessary CT in young patients
• Iodinated contrast contraindicated in severe contrast allergy or acute kidney injury
• Metformin should be withheld 48 hours after contrast in diabetic patients
• Inform the radiographer if pregnant — radiation risk to foetus requires dose optimisation