Parametric delineation of the clinical target volume for glioma: Model-based approach that tailors to physician contouring practices.

BACKGROUND AND PURPOSE: Delineating the clinical target volume (CTV) is an essential part of a radiation oncologist's practice. However, this process is subject to variability due to factors such as clinical experience, institutional standards, and contouring guidelines being followed. We propose a parameterized CTV delineation model for glioma tumors that can tailor to CTV delineation practices. The model is validated based on its ability to replicate different physician-delineated CTVs within our institution. MATERIAL AND METHODS: Five delineation parameters define the model-based CTV for glioma: CTV margin expansion, reduction of CTV margins in the brainstem and optic structures for reduced likelihood of tumor infiltration or preferential sparing, and increase of CTV margins at the ventricles and falx cerebri to account for positional uncertainties. Parameters are fitted by matching the model CTV to the manually-delineated CTVs for patients, maximizing the Dice score. The model is developed using 93 retrospectively collected glioma cases treated by three physicians in our institution. The parameter values for different physicians provide CTV delineation starting points, which was tested by training a classification model using fitted CTV parameters. RESULTS: Physician quality assessments showed no statistically significant difference in quality ratings between model-generated and physician-delineated CTVs (p > 0.05), with both groups achieving an average quality rating of 2.3 (0: unusable, 1: major edits, 2: minor edits, 3: acceptable). The distribution of the delineation parameters varied systematically across physicians, especially in the CTV contours near the brainstem and ventricles. The distribution of the fitted delineation parameters varied systematically across physicians, especially in the CTV contours near the brainstem and ventricles. The best-performing classification model identified physician-specific CTV delineation starting points with 80% accuracy and Cohen's Kappa score of 0.68. CONCLUSION: Our parametric CTV delineation system effectively captures and quantifies physician contouring practices. The model can be used in a contour quality assurance program to promote consensus-building and CTV standardization across physician practices.