Inter-planner variation in treatment-plan quality of plans created with a knowledge-based treatment planning system

•Clarify the inter-planner variation of knowledge-based plan (KBP) quality.•Ninety VMAT plans were created using the RapidPlan by nine planners.•Single optimization with only priority modification was performed.•Large variation of plan quality was observed in beginner planners.•Inter-planner variation remains for complicated plans even though using KBP. This study aimed to clarify the inter-planner variation of plan quality in knowledge-based plans created by nine planners. Five hypofractionated prostate-only (HPO) volumetric modulated arc therapy (VMAT) plans and five whole-pelvis (WP) VMAT plans were created by each planner using a knowledge-based planning (KBP) system. Nine planners were divided into three groups of three planners each: Senior, Junior, and Beginner. Single optimization with only priority modification for all objectives was performed to stay within the dose constraints. The coefficients of variation (CVs) for dosimetric parameters were evaluated, and a plan quality metric (PQM) was used to evaluate comprehensive plan quality. Lower CVs (