Pharmacokinetic analysis of hypoxia ¹⁸F-fluoromisonidazole dynamic PET in head and neck cancer

This study used pharmacokinetic analysis of ¹⁸F-labeled fluoromisonidazole (¹⁸F-FMISO) dynamic PET to assist the identification of regional tumor hypoxia and to investigate the relationship among a potential tumor hypoxia index (K_i), tumor-to-blood ratio (T/B) in the late-time image, plasma-to-tissue transport rate (k₁), and local vascular volume fraction (β) for head and neck cancer patients. Methods: Newly diagnosed patients underwent a dynamic ¹⁸F-FMISO PET scan before chemotherapy or radiotherapy. The data were acquired in 3 consecutive PET/CT dynamic scan segments, registered with each other and analyzed using pharmacokinetics software. The (K_i, k₁, β) kinetic parameter images were derived for each patient. Results: Nine patients' data were analyzed. Representative images of ¹⁸F-FDG PET (of the tumor), CT (of the anatomy), and late-time ¹⁸F-FMISO PET (of the T/B) and parametric images of K_i (potentially representing tumor hypoxia) are shown. The patient image data could be classified into 3 types: with good concordance between the parametric hypoxia map K_i and high T/B, with concordant findings between the parametric hypoxia map and low T/B, and with ambiguity between parametric hypoxia map and T/B. Correlation coefficients are computed between each pair of T/B, K_i, k ₁, and β. Data are also presented for other potential hypoxia surrogate measures, for example, k₃ and k₁/k₂. Conclusion: There is a positive correlation of 0.86 between the average T/B and average hypoxia index K_i of the region of interest. However, because of the statistical photon counting noise in PET and the amplification of noise in kinetic analysis, the direct correlation between the T/B and hypoxia of the individual pixel is not obvious. For a tumor region of interest, there is a slight negative correlation between k₁ and K_i, moderate positive correlation between β and K_i, but no correlation between β and k₁.