3727 - Global Interobserver Variations of Prescription and Plan Evaluation within Heterogeneous Clinical Target Volume (CTV) Delineations among International Experts in Head and Neck Cancers

U. Selek¹, K. S. Hu², H. Tatli³, J. R. Palta⁴, and A. Jhingran⁵; ¹Koc University, School of Medicine, Department of Radiation Oncology, Istanbul, Turkey, ²Department of Radiation Oncology, NYU Langone Health, New York, NY, ³RTIMEA Education and Training Department, Elekta Instrument AB, Istanbul, Turkey, ⁴Department of Radiation Oncology, Virginia Commonwealth University, Richmond, VA, ⁵Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX

Purpose/Objective(s): To define the extent and pattern of global interobserver variations of prescription and plan evaluation in predefined gross tumor volume (GTV) and organs at risk (OAR) volumes among international experts in head and neck cancers Materials/

Methods: Experts were invited by email globally from ASTRO, ESTRO, IAEA, FARO, and TROD for this study, and DICOM images of two anonymized cases were delivered online via cloud large file share to each colleague. The data set included delineated gross tumor volumes (GTV) of primary tumor & nodal disease and organs at risk (OAR) to decrease the discrepancy and interobserver variability for this step. Experts were asked to delineate the clinical tumor volumes (CTV) and planning tumor volumes (PTV) based on their institutional guidelines; to generate a treatment plan in their planning software, as if they were to be treated in their center, based on their institutional / departmental acceptance criteria. We used a systematic process to create and analyze a consensus atlas among multiple observers facilitated by ProKnow DS Version 1.35.2 (Precision radiation medicine company, Golden Rule Version 1.2.8531.2131) The plans were submitted to a precision radiation medicine company ProKnow (Elekta AB) in the standard DICOM format, encompassing DICOM images, RT structure sets, RT plans, and RT doses. Each plans dose-volume histogram (DVH) was automatically recalculated to ensure consistency across different planning systems. A consensus plan, for consensus CTV and PTV atlas, generated using treatment planning software 6.1.2 treatment planning software with VMAT technique was uploaded to a precision radiation medicine company ProKnow DS for statistical analysis to create scorecards adhering to clinical dose criteria, with subsequent comparison of target and organ-at-risk (OAR) doses against the consensus plan.
Results: 25 experts participated the study. NPX: The dose coverages within the target volumes are compatible. OAR dose reductions were significant in doses to the chiasm, brainstem, optic nerves, lenses, and eyes, but not for parotids and larynx via the plan generated based on consensus CTV and PTV. Though in limits, there was wide variances among expert plans for OAR doses of D50% of cochlea, brainstem, and optic chiasm. OPX: The dose coverages within the target volumes are compatible. OAR dose reductions were significant in doses to the optic nerves, lenses, and eyes, but not for chiasm, brainstem, parotids, and larynx via the plan generated based on consensus CTV and PTV. Though in limits, there was wide variances among expert plans for OAR doses of D50% of constrictors, larynx, and brachial plexus.
Conclusion: Though in normal range of dose limits, OAR doses were significantly heterogeneous. Even in an environment of predefined primary and nodal GTV to decrease the discrepancy, interobserver variability is evident especially starting from CTV2, accordingly which directly effected the PTVs, which generated different OAR scenarios; appealing the need for standardized peer review processes and more structured guidelines for CTV.