Human papillomavirus-negative (HPV-) head and neck cancers have poor outcome compared with HPV positive cancers¹. Thus, in an effort to perform biologically guided radiotherapy it is critical to understand how different functional imaging techniques² interact and potentially complement each other. Multimodality imaging can provide useful anatomical and functional data about tumors, including tumor cellularity measured by diffusion weighted (DW)-MRI and glucose metabolic status measured by 18F-fluorodeoxyglucose (18F-FDG) PET. In order to characterize the tumor and to implement new predictive models based on functional imaging data, we must ensure we can extract as much information as possible from the available data. Some of the main parameters to characterize tumor behavior, along with radiation therapy treatment, must be initial tumor density, hypoxia, malignancy-proliferation, dose to each voxel, and timing of the dose. In this work we focus on DW-MRI and 18F FDG-PET derived parameters. The purpose of our study was to non-invasively assess the tumor biology and metabolism of neck nodal metastases in HPV- head and neck squamous cell carcinoma (HNSCC) by investigating the relationship between tumor cellularity measured using DW-MRI and glucose metabolism measured by 18F-FDG PET. This study was approved by local institutional review board and we obtained informed consent from all patients. Five newly diagnosed HPV- HNSCC patients with metastatic neck nodes underwent 20 DW-MRI imaging studies i.e. pre-, intra- [2 and 3 weeks] and post- chemo- radiation (dose of 70Gy) therapy. These patients also had pre- and post-and 10 18F-FDG PET/CT scans. All MRI examinations were performed on a 1.5-T scanner (Achieva; Philips Healthcare) with a Philips Sense Flex Medium coil. Standard MR images for localization and T2w Turbo Spin Echo images were obtained followed by DW-MRI acquisition using a single-shot echo planar imaging (SS-EPI) (TE/TR (ms) = 77/5270; NEX=4, FOV (cm): 23-25, slice thickness (mm)= 6 with 3 b values of b=0, 600 and 1000 s/mm², respectively. Whole-body PET/CT scan (Discovery, GE Healthcare Bio-Sciences Corp.) was performed in 3D mode from head to thigh, 60min after intravenous administration of approximately 370MBq (±10%) of 18F-FDG with a FOV(cm)= 70, matrix 218 × 218. The pixel spacing was 5.47mm with a slice thickness =3.27 mm. PET images were corrected for attenuation, scatter, decay, dead time, random coincidences and slice sensitivity. Regions of interest (ROIs) were delineated for each patient by the neuroradiologist on the primary and nodes using in-house software. The matched regions of interests from both modalities were analyzed. We registered all datasets (MRI studies, radiation dose distributions and PET/CT) in order to extract each voxel’s information (signal intensity, radiation dose and Uptake) using the software: ARTFIBio 0.6.23. To examine the correlations between DW-MRI derived apparent diffusion coefficient (ADC [mm2/s]) metric using mono-exponential fitting4 and mean of standard uptake value (SUV) measurements using established method5 from 18F-FDG PET, the non-parametric Spearman correlation coefficient was calculated. All patients had a clinical follow-up as standard of care and survival status was documented into groups at 1year: no evidence of disease [NED], alive with disease [AWD], and dead of disease [DOD].For the 5 patients, a total of 4 primary tumors and 9 nodes were analyzed. One patient had unknown primary tumor. There was a strong negative correlation between the mean of the pretreatment ADC (ρ=-0.6, p=0.008) and the 18F-FDG SUV (Figure 1). On last 1 year clinical follow up the survival status of patients was as follows: 1 NED, 2 AWD and 2 DOD. No primary was visible by MRI for delineation on the post-treatment MRI study for all patients. No node was visible by MRI for delineation on the post-treatment MRI except in the DOD patients. ADC values for pre-, 2 intra- and 1 post -treatment MRIs from primary and node of representative DOD, AWD and NED patients are shown in Table 1 (Figure 2). Figure 3 shows different trends depending on the outcome when ADC is plotted versus radiation dose. Multi modality imaging offers much more information about tumor biology and metabolism than the individual datasets on their own. The initial evaluation of the preliminary results suggests that in these solid tumors cellularity is inversely proportional to the glucose metabolic uptake. The survival status and functional metrics show different trends for NED, AWD and DOD. These initial findings need to be validated in larger populations so that these patients can receive individualized radiation therapy.