KEY POINTS
- Technical note using a CIRS Model 603A distortion phantom to quantify geometric distortion in computed tomography scans used for radiotherapy planning.
- The workflow was tested across 8 computed tomography scanners and 4 linear accelerator-based cone-beam computed tomography systems.
- All 8 computed tomography scanners showed measurable distortion. Compared with cone-beam computed tomography, mean distortion ranged from 0.13 to 0.52 mm, and maximum distortion ranged from 0.45 to 1.09 mm.
- Compared with the computer-aided design model, mean distortion ranged from 0.25 to 0.82 mm, and maximum distortion ranged from 0.58 to 1.88 mm.
- The authors identified longitudinal couch inaccuracy or non-linear couch sag as likely major contributors, and noted that current computed tomography quality assurance methods may miss submillimetre distortion relevant to stereotactic radiotherapy.
CLINICAL TAKEAWAY
This study challenges the routine assumption that computed tomography is geometrically distortion-free for high-precision radiotherapy planning. For stereotactic treatments, especially when using small margins or computed tomography as the reference for magnetic resonance imaging distortion assessment, submillimetre computed tomography distortion should be treated as a measurable uncertainty. The evidence is technical and phantom-based, but operationally relevant for quality assurance and margin-risk assessment.