Assessment and improvement of the quality of radiotherapy treatment planning CT images using a clinically validated phantom based method and a multicentre intercomparison.

Purpose: To develop a phantom method of image quality assessment for radiotherapy planning CT protocols (head and neck (H&N) and prostate) and validate results against clinical image quality. Test with data from different scanners and suggest protocol adjustments.

Methods: Macros measured patient water-equivalent diameter and noise from clinical CT images. Target transfer function (TTF), contrast, noise-power spectrum (NPS), detectability index and the edge visibility of a low contrast target were measured using Catphan 604 and bespoke phantoms. Ten centres scanned the phantoms with modified clinical protocols and collected data from patient images using the macros. Clinical experts, ranked the quality of images for contouring and correlated results against phantom metrics.

Results: Clinical image review showed a large range of results from different scanners for H&N scans and fewer differences for prostate. The phantom metrics best correlated with high clinical image scores were, for H&N: high TTF50 (r = 0.73, p = 0.003), contrast (r = 0.58, p = 0.003) and target edge visibility (r = 0.70, p = 0.004); for prostate: high TTF50 (r = 0.83, p = 0.002), low noise (r = 0.37, p = 0.26) and target edge visibility (r = 0.59, p = 0.05). Hence, optimal contrast, resolution and noise are important for good contouring image quality. Reconstruction kernel, field of view and noise, or X-ray tube current and rotation time, are possible parameters for adjustment.

Conclusions: This phantom method (using Catphan 604) was a good surrogate for clinical quality assessment of CT images for radiotherapy contouring. Results identified the poorest performing scanners, allowing recommendations for image quality improvement and confirming scan protocol optimisation is necessary in some centres.