Defining a Quantitative Threshold for Adaptive Re-planning using a Percentile-based Scoring System.

Abstract

Background: Despite the frequent need for adaptive planning in head and neck proton therapy, no quantitative indicators have been identified to determine when a plan should be adapted. Here, we aimed to determine a quantitative threshold for the change in plan quality between the primary treatment plan and verification plans that correlates with the physician's decision to adapt.

Methods: Primary treatment and verification plans were extracted from the clinical treatment planning system for 167 patients who underwent intensity-modulated proton therapy for base of tongue, tonsillar, or oropharyngeal cancers, with and without nodal involvement, between 2016 and 2021. All plans were graded using a previously validated percentile-based Daisy scoring system, and differences between the primary plan and its corresponding verification plans were calculated. Two generalized linear mixed models (GLMM) were created using the difference in plan scores and the primary plan score to predict the probability of a plan being adapted. Additionally, five radiation oncologists evaluated 65 verification plans using a 3-point Likert scale (1="Adaptive Planning Necessary", 2="Consider Adapting", and 3="Acceptable"). Wilcoxon rank sum analysis was used to determine statistical significance between the score differences in each category.

Results: The GLMM results demonstrated a significant relationship between score differences and the need for adaptive planning (P=0.034), and the combined score difference plus primary plan score with the need for adaptive planning (P=0.004, P=0.027). Physician review indicated that plans requiring adaptation had a mean score drop of 37.7%±16.4, whereas acceptable plans had a mean drop of 19.3%±15.0. Structurally, the high-dose clinical tumor volume D_95%[%] and hotspots (Body_D2cc[Gy]) were most indicative of whether a plan should be adapted.

Conclusions: A percentile-based Daisy scoring system can be used to identify a threshold at which a change in plan score between primary and verification plans indicates that an adaptive plan is necessary.