HU Coefficient: A Clinically Oriented Metric to Evaluate Contour Accuracy in Radiation Therapy

Purpose To propose a clinically oriented quantitative metric, the HU coefficient, to evaluate contour quality, gauge the performance of auto contouring methods, and aid effective allocation of clinical resources. Materials and Methods Publicly available pelvic CT data from the Cancer Imaging Archive was used to demonstrate the clinical utility of the HU coefficient in contour evaluation. The bladder was selected as the organ of interest. It was contoured by a certified medical dosimetrist and reviewed by a certified medical physicist. This contour served as the ground truth contour. From this contour, we simulated two contour sets. The first set had the same Dice similarity coefficient (DSC) but different HU coefficients, whereas the second set kept a constant HU coefficient while exhibiting different DSCs. Four individuals were asked to adjust the simulated contours until they met clinical standards. The corresponding contour modification times were recorded and normalized by individual’s manual contouring times from scratch. The normalized contour modification time was correlated to the HU and DSC to evaluate their suitability as quantitative metrics assessing contour quality. Results The HU coefficient maintained a strong correlation with the normalized contour modification time when both sets of simulated contours were included in analysis. The correlation between the DSC and normalized contour modification time, however, was weak. Compared to DSC, HU is more suitable for evaluating contour quality. Conclusions We demonstrated that the HU coefficient correlated well with the average normalized contour modification time. Clinically, contour modification time is the most relevant factor in allocating clinical resources. Therefore, the HU coefficient is better suited than DSC to assess contour quality from a clinical perspective.