The optimal ${}^{18}$F-fluoromisonidazole PET threshold to define tumor hypoxia in preclinical squamous cell carcinomas using pO${}_2$ electron paramagnetic resonance imaging as reference truth

To identify the optimal threshold in ${}^{18}$F-fluoromisonidazole (FMISO) PET images to accurately locate tumor hypoxia by using electron paramagnetic resonance imaging (pO${}_2$ EPRI) as ground truth for hypoxia, defined by pO${}_2$ ≤ 10 mmHg. Tumor hypoxia images in mouse models of SCCVII squamous cell carcinoma (n = 16) were acquired in a hybrid PET/EPRI imaging system 2 h post-injection of FMISO. T2-weighted MRI was used to delineate tumor and muscle tissue. Dynamic contrast enhanced (DCE) MRI parametric images of K${}^{trans}$ and $v_e$ were generated to model tumor vascular properties. Images from PET/EPR/MRI were co-registered and resampled to isotropic 0.5 mm voxel resolution for analysis. PET images were converted to standardized uptake value (SUV) and tumor-to-muscle ratio (TMR) units. FMISO uptake thresholds were evaluated using receiver operating characteristic (ROC) curve analysis to find the optimal FMISO threshold and unit with maximum overall hypoxia similarity (OHS) with pO${}_2$ EPRI, where OHS = 1 shows perfect overlap and OHS = 0 shows no overlap. The means of dice similarity coefficient, normalized Hausdorff distance, and accuracy were used to define the OHS. Monotonic relationships between EPRI/PET/DCE-MRI were evaluated with the Spearman correlation coefficient (ρ) to quantify association of vasculature on hypoxia imaged with both FMISO PET and pO${}_2$ EPRI. FMISO PET thresholds to define hypoxia with maximum OHS (both OHS = 0.728 ± 0.2) were SUV ≥ 1.4 × SUV${}_{mean}$ and SUV ≥ 0.6 × SUV${}_{max}$. Weak-to-moderate correlations (|ρ|< 0.70) were observed between PET/EPRI hypoxia images with vascular permeability (K${}^{trans}$) or fractional extracellular-extravascular space ($v_e$) from DCE-MRI. This is the first in vivo comparison of FMISO uptake with pO${}_2$ EPRI to identify the optimal FMISO threshold to define tumor hypoxia, which may successfully direct hypoxic tumor boosts in patients, thereby enhancing tumor control.