Quantification in Lutetium-177-DOTATOC SPECT/CT using NaI detectors: Application to neuroendocrine tumor patients

Ziel/Aim In peptide receptor radionuclide therapy (PRRT) for neuroendocrine tumors (NET), feasibility of lesion uptake quantification in Lu-177-DOTATOC SPECT/CT for therapy monitoring remains unclear. This is to evaluate the association of lesion uptake changes with morphologic changes.

Methodik/Methods Retrospective analysis in 10 patients with NET (median age: 73 [range, 57-79] years) undergoing first 2 cycles of PRRT (median activity: 7.0 [5.5-7.1] GBq). Maximum/mean/peak counts and metabolic tumor volume (MTV) of primary tumor (n = 1) or metastases (liver: 16, extrahepatic: 8) were measured on Lu-177 SPECT/CT 24h post injection (GE Discovery DR Pro 670) at both cycles. SUV (max/mean/peak) were calculated based on volume sensitivity for a homogenous cylindrical phantom. Data was reconstructed iteratively with attenuation correction, resolution recovery and scatter correction. Relative change of the product MTV*SUV between first and second cycle was correlated for each lesion to its relative change in maximum diameter (CT/MRI). In patient-based analysis, relative change in sum of target lesion MTV*SUV between two cycles was used for response assessment following RECIST into PD (>+20%), PR (>-30%) or SD (others).

Ergebnisse/Results Lesion-based, Pearson’s correlation coefficient for changes in MTV*SUV and maximum diameter was 0.6 (MTV*SUV_peak/mean) or 0.3 (MTV*SUV_max). Interdisciplinary consensus with all outcome data was available as standard of reference for patient-based analysis (PD: 3, SD: 5, PR: 2). 2/3 patients with PD were correctly identified as PD by MTV*SUV changes (1/3 pts, SD). 2/5 patients with SD were correct while 2/5 were classified as PD and 1/5 as PR. 2/2 patients with PR were correctly classified.

Schlussfolgerungen/Conclusions Quantification in SPECT/CT for Lu-177-DOTATOC PRRT is feasible, and therapy-associated changes in lesion uptake correlated with MRI/CT diameter changes. In this small patient series, changes of target lesion uptake during PRRT showed resemblance with response assessment by interdisciplinary consensus.