2134 - Global Interobserver Variations of Prescription and Plan Evaluation within Heterogeneous Clinical Target Volume (CTV) Delineations among International Experts in Lung Cancer

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 interobserver variations of clinical and planning target volume (CTV and PTV) delineation among international experts in predefined primary and nodal gross tumor volumes (iGTV) and organs at risk (OAR) volumes in lung cancer. Materials/

Methods: Experts were invited by email globally from ASTRO, ESTRO, IAEA, FARO, and TROD. DICOM images of two anonymized cases (T1N0M0 adenocancer: Stereotactic ablative body radiotherapy, SABR & T2aN2M0 adenocancer: LA-NSCLC) were delivered online via cloud large file share to each colleague. The average data set included delineated iGTV based on 4 D-CT scan including 10 respiratory phases of primary tumor for both cases & nodal disease for LA-NSCLC, and OAR to decrease the discrepancy and interobserver variability for this step. Experts were asked to delineate the planning tumor volumes (PTV) for SABR and CTV & PTV for LA-NSCLC based on their institutional guidelines. We used a systematic process to create and analyze a consensus atlas among multiple observers facilitated by a precision radiation medicine company; forming 50% ground truth area common in all participants contours. The plans were submitted to a precision radiation medicine company in the standard DICOM format, encompassing DICOM images, RT structure sets, RT plans, and RT doses. A consensus plan, for consensus CTV and PTV atlas, generated using treatment planning software with VMAT technique was uploaded to a precision radiation medicine company 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: 21 experts participated the study. SABR: The target coverage in both the consensus plan and the overall population exhibits similarity. Though in limits, there was significant dissociation at each expert’s discretion in doses of heart, contralateral lung, chestwall, esophagus and spinal cord. OAR dose reductions were significant in doses to the total lung and chest wall via the plan generated based on consensus PTV. LA-NSCLC: The target coverage in both the consensus plan and the overall population exhibits similarity. Though in limits, there was significant dissociation at each expert’s discretion in doses of total lung including volume receiving 5 Gy, 10Gy and 20Gy, heart, ipsilateral & contralateral lung, chestwall, esophagus and spinal cord. There was a wide dose range of OAR among expert plans due to prioritization of OAR discretion. OAR dose reductions were significant in doses to the spinal cord and total lung receiving 5 Gy via the plan generated based on consensus PTV.
Conclusion: OAR doses being in normal range were significantly heterogeneous due to prioritization of OAR discretion, even in a single target SABR treatment, in this cohort of expert plans. Standardized peer review process is appealing for a more structured OAR dose criteria, along with a more structured CTV & PTV guideline for more than one target.