3760 - Anatomical and Dosimetric Variations during VMAT in Patients with Locally Advanced Nasopharyngeal Carcinoma after Induction Therapy: Implications for Adaptive Radiation Therapy

S. Zhao¹, J. Han², Z. Yang¹, X. Chen³, X. Liu¹, F. Zhou¹, L. Wen¹, Z. Zhang¹, Y. Wang¹, G. Liu¹, B. Wu¹, S. Zhang¹, J. Huang¹, and K. Yang¹; ¹Cancer Center, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China, ²Cancer Center, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China, ³Department of Epidemiology and Statistics, School of Public Health, Medical College, Zhejiang University, HangZhou, China

Purpose/Objective(s): Induction therapy (IT) results in significant tumor shrinkage in patients with locally advanced nasopharyngeal carcinoma (LANPC), potentially leading to a unique pattern of variations in the target volumes (TVs) and organs at risk (OARs) during subsequent radiotherapy. This study aimed to fill the knowledge gap regarding anatomical and dosimetric changes in patients with LANPC during volumetric modulated arc therapy (VMAT) after IT and to explore the characteristics of patients with significant variations, providing insights for adaptive radiation therapy (ART). Materials/

Methods: Patients with LANPC treated with VMAT after IT between July 2021 and June 2023 were retrospectively included. Adaptive computed tomography (aCT), reconstructed from weekly cone-beam computed tomography, facilitates recontouring and planning transplantation. Variations in volume, dice similarity coefficients (DSCs), and delivered dose to TVs and OARs on planning CT and aCT were compared to identify changing patterns. Logistic regression was used to investigate potential risk factors.
Results: This study included 60 patients with LANPC who underwent IT combined with VMAT at our center. Throughout the course of VMAT, there was a gradual volumetric decrease in the planning gross tumor volumes of the primary tumor (PGTVp), the lymph nodes (PGTVn), and the parotid glands, while no significant dosimetric changes were observed in the PGTVp or brainstem. Notably, the D99 and D95 of PGTVn declined the most rapidly at 25 fractions (F), decreasing by 7.94 (3.28%, 14.90%) and 4.18 (1.73%, 8.40%) from baseline, respectively (P <0.001). Peak dosimetric variations in both the ipsilateral and contralateral parotid glands were also noted at 25F, with Dmean increasing by 7.80±11.33% and 6.50±8.55%, respectively, compared to baseline (P <0.001). Despite an increasing trend in the dose delivered to the spinal cord before 10F, particularly the upper spinal cord, the dose consistently remained within the dose limits. Furthermore, multivariate logistic regression revealed that initial body mass index (BMI) =23.5 kg/m2 (P=0.034) and non-objective response (OR) during IT (P=0.045) were identified as independent risk factors for a substantial reduction in PGTVn dose in the later stages of VMAT.
Conclusion: Replanning for post-IT LANPC patients appears reasonable at approximately 25F during VMAT. Patients with an initial BMI =23.5 kg/m² and non-OR during IT should be actively considered for ART to stabilize the delivered dose.