270 - Quantitative DW-MRI Monitoring of Metastatic Lymph Nodes in HPV+ Head and Neck Cancer Using MR-Linac

D. H. El-Habashy¹, R. He¹, B. McDonald², K. A. Wahid², Y. Ding³, S. Mulder¹, J. Christodouleas⁴, M. Naser¹, A. Dresner⁵, A. S. Mohamed¹, and C. D. Fuller¹; ¹Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, ²MD Anderson Cancer Center, Houston, TX, ³Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, ⁴Elekta Inc., St. Charles, MO, ⁵Philips Healthcare, Fitchburg, WI, United States

Purpose/Objective(s): This study aims to utilize the MR-Linac device to obtain serial quantitative DW-MRI during RT. The objective is to monitor and compare weekly changes in both cystic and solid portions of metastatic lymph nodes. Subsequently, these changes will be correlated with the response to radiation therapy (RT) and oncologic outcomes. This study is categorized as Stage 2a, according to the R-IDEAL framework. Materials/

Methods: Thirty-one patients with Human Papillomavirus (HPV)-related head and neck squamous cell carcinoma (HNSCC) who underwent curative-intent RT at MD Anderson Cancer Center were enrolled. Segmentation of the entire gross nodal disease volumes (GTVN), as well as the cystic (GTVNC) and solid (GTVNS) subvolumes, was conducted. Baseline and weekly MRI scans were obtained, and various ADC parameters were extracted from the regions of interest (ROIs). Baseline and weekly ADC parameters were correlated with response to RT and the recurrence using the Mann-Whitney U test. The Wilcoxon signed-rank test was utilized to compare the weekly ADC versus baseline values. Weekly volumetric changes (?volume) for each ROI were correlated with ?ADC using Spearman’s Rho test.
Results: There was an incremental increase in ADC parameters from baseline throughout the course of RT for GTVN. A similar trend of increased ADC parameters across RT was observed for GTVNS, while GTVNC exhibited a peculiar change in the pattern of ADC across RT. Baseline ADC parameters at the 5^th, 10^th, and 20^th percentiles for GTVNC significantly correlated with the development of recurrence after the end of RT (p = 0.004, 0.012, and 0.019, respectively). However, no significant correlation was found between any baseline ADC parameters for GTVNS and the development of recurrence. A significant positive correlation was found between mean, median, 5th, 10th, 20th, 30th, 40th, 60th, and 70th percentiles ?ADC and ?volume for GTVNC, but not for GTVNS at the 6th week of RT. No significant correlation was detected between changes in ADC parameters across RT and either response to RT or the development of recurrence.
Conclusion: Through the analysis of weekly changes in both cystic and solid portions of metastatic lymph nodes, we identified distinct patterns of ADC alterations. These findings may underscore the importance of considering both solid and cystic components when assessing treatment response in metastatic lymph nodes. Further prospective multi-institutional studies to elucidate the correlation between these ADC kinetics for the solid and cystic subvolumes of metastatic lymph nodes and oncologic outcomes and subsequently how to modify RT dose accordingly, are warranted.