PET quantification with a histogram derived total activity metric: Superior quantitative consistency compared to total lesion glycolysis with absolute or relative SUV thresholds in phantoms and lung cancer patients

IntroductionThe increasing use of molecular imaging probes as biomarkers in oncology emphasizes the need for robust and stable methods for quantifying tracer uptake in PET imaging. The primary motivation for this research was to find an accurate method to quantify the total tumor uptake. Therefore we developed a histogram-based method to calculate the background subtracted lesion (BSL) activity and validated BSL by comparing the quantitative consistency with the total lesion glycolysis (TLG) in phantom and patient studies.MethodsA thorax phantom and a PET-ACR quality assurance phantom were scanned with increasing FDG concentrations. Volumes of interest (VOIs) were placed over each chamber. TLG was calculated with a fixed threshold at SUV 2.5 (TLG2.5) and a relative threshold at 42% of SUVmax (TLG42%). The histogram for each VOI was built and BSL was calculated. Comparison with the total injected FDG activity (TIA) was performed using concordance correlation coefficients (CCC) and the slope (a). Fifty consecutive patients with FDG-avid lung tumors were selected under an IRB waiver. TLG42%, TLG2.5 and BSL were compared to the reference standard calculating CCC and the slope.ResultsIn both phantoms, the CCC for lesions with a TIA ≤ 50 ml*SUV between TIA and BSL was higher and the slope closer to 1 (CCC = 0.933, a = 1.189), than for TLG42% (CCC = 0.350, a = 0.731) or TLG2.5 (CCC = 0.761, a = 0.727). In 50 lung lesions BSL had a slope closer to 1 compared to the reference activity than TLG42% (a = 1.084 vs 0.618 – for high activity lesions) and also closer to 1 than TLG2.5 (a = 1.117 vs 0.548 – for low activity lesions).ConclusionThe histogram based BSL correlated better with TIA in both phantom studies than TLG2.5 or TLG42%. Also in lung tumors, the BSL activity is overall more accurate in quantifying the lesion activity compared to the two most commonly applied TLG quantification methods.