Validation of simplified uptake measures against dynamic Patlak K for quantification of lesional Zr-Immuno-PET antibody uptake.

Purpose: Positron emission tomography imaging of zirconium-89-labelled monoclonal antibodies (Zr-Immuno-PET) allows for visualisation and quantification of antibody uptake in tumours in vivo. Patlak linearization provides distribution volume (V) and nett influx rate (K) values, representing reversible and irreversible uptake, respectively. Standardised uptake value (SUV) and tumour-to-plasma/tumour-to-blood ratio (TPR/TBR) are often used, but their validity depends on the comparability of plasma kinetics and clearances. This study assesses the validity of SUV, TPR and TBR against Patlak K for quantifying irreversible Zr-Immuno-PET uptake in tumours.

Methods: Ten patients received 37 MBq 10 mg Zr-anti-EGFR with 500 mg/m unlabelled mAbs. Five patients received two doses of 37 MBq Zr-anti-HER3: 8-24 mg for the first administration and 24 mg-30 mg/kg for the second. Seven tumours from four patients showed Zr-anti-EGFR uptake, and 18 tumours from five patients showed Zr-anti-HER3 uptake. SUV TPR and TBR values were obtained from one to six days p.i. Patlak linearization was applied to tumour time activity curves and plasma samples to obtain K.

Results: For Zr-anti-EGFR, there was a small variability along the linear regression line between SUV (- 0.51-0.57), TPR (- 0.06‒0.11) and TBR (- 0.13‒0.16) on day 6 versus K. Similar doses of Zr-anti-HER3 showed similar variability for SUV (- 1.3‒1.0), TPR (- 1.1‒0.53) and TBR (- 1.5‒0.72) on day 5 versus K. However, for the second administration of Zr-anti-HER3 with a large variability in administered mass doses, SUV showed a larger variability (- 1.4‒2.3) along the regression line with K, which improved when using TPR (- 0.38-0.32) or TBR (- 0.56‒0.46).

Conclusion: SUV, TPR and TBR at late time points were valid for quantifying irreversible lesional Zr-Immuno-PET uptake when constant mass doses were administered. However, for variable mass doses, only TPR and TBR provided reliable values for irreversible uptake, but not SUV, because SUV does not take patient and mass dose-specific plasma clearance into account.