PT - JOURNAL ARTICLE AU - John A. Kennedy AU - Rachel Lugassi AU - Ronit Gill AU - Zohar Keidar TI - Digital Solid-State SPECT/CT Quantitation of Absolute <sup>177</sup>Lu Radiotracer Concentration: In Vivo and In Vitro Validation AID - 10.2967/jnumed.119.239277 DP - 2020 Sep 01 TA - Journal of Nuclear Medicine PG - 1381--1387 VI - 61 IP - 9 4099 - http://jnm.snmjournals.org/content/61/9/1381.short 4100 - http://jnm.snmjournals.org/content/61/9/1381.full SO - J Nucl Med2020 Sep 01; 61 AB - The accuracy of 177Lu radiotracer concentration measurements using quantitative clinical software was determined by comparing in vivo results for a digital solid-state cadmium-zinc-telluride SPECT/CT system with in vitro sampling. Methods: First, image acquisition parameters were assessed for an International Electrotechnical Commission body phantom emulating clinical count rates loaded with a lung insert and 6 hot spheres with a 12:1 target-to-background ratio of 177Lu solution. Then, the data of 28 whole-body SPECT/CT scans of 7 patients who underwent 177Lu prostate-specific membrane antigen radioligand therapy were retrospectively analyzed. Three users analyzed SPECT/CT images for in vivo urinary bladder radiotracer uptake using quantitative software. In vitro radiopharmaceutical concentrations were calculated using urine sampling obtained immediately after each scan, scaled to SUVs. Any in vivo or in vitro identity relations were determined by linear regression (ideally, slope = 1 and intercept = 0), within a 95% confidence interval. Results: Phantom results demonstrated lower quantitative error for acquisitions using the 113-keV 177Lu energy peak rather than including the 208-keV peak, given that only low-energy collimation was available in this camera configuration. In the clinical study, 24 in vivo–in vitro pairs were eligible for further analysis, with 4 having been rejected as outliers (via Cook distance calculations). All linear regressions (R2 ≥ 0.82, P &lt; 0.0001) provided identity in vivo–in vitro relations (95% confidence interval), with SUV averages from all users giving a slope of 0.96 ± 0.13, an intercept of −0.07 ± 0.46 g/mL, and an average residual difference of 19.5%. In acquisitions with the lower-energy 177Lu energy peak, solid-state SPECT/CT imaging provided an accuracy to within approximately 20% of in vivo urinary bladder radiotracer concentrations. Conclusion: This noninvasive in vivo quantitation method can potentially improve diagnosis, patient management, and treatment response assessment and provide data essential to 177Lu dosimetry.