Four-Dimensional CT Simulation (4D-CT)

Overview

Four-dimensional CT (4D-CT) simulation is a radiation therapy planning technique that acquires CT images binned to phases of the respiratory cycle, enabling visualisation of tumour motion during breathing. For pancreatic SBRT, 4D-CT is used to define the internal target volume (ITV) — the envelope of tumour motion across all breathing phases — so that tight PTV margins can be applied without geographic miss.

Used by

• 4D-CT simulation is described as a standard component of the pancreatic SBRT technical workflow reviewed across nine prospective trials in locally advanced pancreatic cancer; enables motion-adaptive planning that allows tighter PTV margins (2–5 mm) and contributes to better OAR sparing and reduced toxicity compared to larger margin approaches. PMID:27826200

Notes

• Pancreatic tumours can move 1–2 cm craniocaudally with respiration; 4D-CT quantifies this motion to define the ITV.
• Used in combination with abdominal compression, respiratory gating, or real-time fiducial tracking to further reduce motion during delivery.
• Paired with cone-beam CT IGRT at each treatment fraction to verify tumour position relative to planning CT.
• Inadequate motion management (e.g., large craniocaudal PTV expansion of 1 cm without 4D-CT) was associated with lower local control and higher toxicity in the Danish SBRT series.

Sources

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