Correlation of 18F-FDG Uptake with Apparent Diffusion Coefficient Ratio Measured on Standard and High b Value Diffusion MRI in Head and Neck Cancer

Although the clinical applications of F-18-FDG PET/CT and diffusion-weighted MRI (DWI) are similar to each other in head and neck cancer, the image acquisition methods in the 2 modalities are significantly different. F-18-FDG PET/CT traces glucose metabolism, a nonspecific process essential for tumor growth. On the other hand, DWI provides information on Brownian motion of water molecules in tissues, which represents cellularity. The aim of our study was to investigate whether apparent diffusion coefficient (ADC) values at b = 1,000 (ADC(1,000)) and 2,000 (ADC(2,000)) s/mm(2) or whether the change (ADC(ratio)) of ADC values from b = 1,000 to 2,000 s/mm2 has any significant correlation with the standardized uptake value (SUV) in patients with head and neck squamous cell carcinoma (HNSCC). Methods: Our hospital's institutional review board approved this retrospective study. We included 47 patients (32 men and 15 women) with histopathologically proven HNSCC, who underwent both DWI (at both b = 1,000 s/mm(2) and b = 2,000 s/mm(2)) and F-18-FDG PET/CT in the 2 wk before treatment. ADC(ratio) maps were generated using a pixel-by-pixel computation for which ADCratio is (ADC(2,000)/ADC(1,000)) x 100. The mean ADC(1,000), ADC(2,000), and ADCratio values were evaluated within a manually placed polygonal region of interest within the main tumor on every slice of the ADC(1,000), ADC(2,000), and ADCratio maps, respectively. In addition, the maximal SUV (SUVmax) and mean SUV (SUVmean) were measured for the entire tumor region of interest. Comparisons were made using Pearson correlation analysis, and partial correlation coefficients were derived. Results: No significant correlation was found between the mean ADC(1,000) and SUVmean (r = -0.222, P = 0.1325) or the mean ADC(2,000) and SUVmean ( r = -0.1214, P = 0.4163). However, the ADC(ratio) was significantly and positively correlated to both the SUVmean ( r = 0.667, P < 0.001) and SUVmax ( r = 0.5855, P < 0.001). Conclusion: The ADC(ratio) and SUV were significantly correlated with each other in primary HNSCC patients, possibly because of a higher-cellularity region as a result of relatively increased tumor proliferation. Further studies are warranted to investigate the possible complementary role of DWI and PET/CT in various clinical settings, including staging and treatment response.