Model Interpretation and Clinical Implications

Useful characterizing parameters have been derived from historical positron emission tomography (PET) standardized uptake values (SUV) and influx constants K. Meta-analysis was performed on 30 multipatient PET oncological studies providing same patient SUVs and K's. Averaged results for fluorine-18 fluorodeoxyglucose (FDG) and L-methionine respectively were: SUV vs. K correlation coefficients = 0.89 and 0.80; SUV/K ratios = 192 and 63 minutes as average tracer clearance times T in these populations. For cancers, coefficients of variation (CV) for K's were 0.61 and 0.46, notably larger than the CVs (0.50 and 0.40) for SUVs. A Monte Carlo simulation model, matching these results, represents 1/T as (an effective tracer clearance rate) x (its initial distribution volume). We conclude that T is a characteristic tracer clearance time that is independent of cancer type. A measurement model is introduced that might help improve protocols. The higher CVs of K's vs. SUVs is worth noting clinically when seeking an effective diagnostic marker. Also, SUV conversions to K can provide some quality assurance in K measurements.