Prediction and Monitoring of Adaptive Radiation Therapy Timing Using Two-dimensional X-ray Image-based Water Equivalent Thickness

Abstract

Background/Aim: This study aimed to predict the optimal timing for adaptive radiation therapy (ART) using two-dimensional X-ray image-based water equivalent thickness (2DWET). Patients and Methods: Forty patients with oropharyngeal and hypopharyngeal cancer underwent Computed Tomography (CT) rescanning during treatment. An adaptive score (AS) was proposed to guide ART decisions based on changes in four dose indices: target coverage, spinal cord dose, parotid gland dose, and over-dose volume. Delivered dose distributions were reviewed by two head and neck radiation oncologists, with ART requirement evaluated using a four-point scale. Logistic regression determined the AS cutoff, while receiver operating characteristic analysis assessed 2DWET as a predictor of ART timing. Results: The AS strongly correlated with the oncologists’ ART decisions (Pearson coefficients of 0.74 and 0.64). An AS cutoff of 7.5 optimally indicated ART requirement, matching oncologist decisions with sensitivities of 79.2% and 89.5%, and specificities of 87.5% and 81.0%, respectively. The 2DWET method identified the AS threshold of 7.5 with sensitivity and specificity of 63.2% and 81.0%. Conclusion: An AS of 7.5 was highly indicative of ART timing, aligning well with oncologists’ decisions, and 2DWET demonstrated potential as a low-exposure, efficient tool for predicting ART timing in patients with oropharyngeal and hypopharyngeal cancers.