Feasibility of iodine-123-mIBG SPECT/CT quantification in neuroblastoma using CZT and NaI detectors

Abstract

Background In [ 123 I]mIBG SPECT/CT for neuroblastoma, lesion uptake quantification could improve therapy monitoring. This study compared quantitative accuracy of a CZT (WEHR45) and NaI (LEHR) system (GE Discovery 670). Methods Volume sensitivity VS hom was estimated from a homogenous cylindrical phantom (811 ml) acquired with body contour or fixed 27 cm detector radius. Relative accuracy of VS hom to retrieve true background activity concentration (AC) in an IEC body phantom was calculated. Maximum/peak contrast recovery (CR) of the sphere inserts used VS IEC estimated from the IEC phantom. In 16 children with 37 [ 123 I]mIBG SPECT/CTs (range, 1-6 per patient; CZT, 12; NaI, 25), normal organ SUVmean (liver r/l, spleen, myocardium, blood pool, spine, muscles) were calculated using VS hom and VS IEC . Iterative reconstruction (Q.Metrix) used resolution recovery with/without scatter correction (SC) by dual energy window (DEW; 159 ± 10% and 130 ± 10% keV). In 20 exams, metabolic tumor volume (MTV)*SUVmax changes of primary tumors were correlated with changes of MRI tumor volume in serial scans (Pearson). Results VS hom (cts * MBq -1 * s -1 ) using SC was slightly lower for 27 cm radius vs. body contour with CZT (48 vs. 50, p<0.001) but comparable with NaI (59 vs. 61, p=0.18). Relative error in IEC phantom AC based on VS hom for CZT vs. NaI was -1.3% vs. +4.1% with SC (p=0.22). Acquisition time reduction by 50% (CZT) showed similar VS hom and relative error. CRmax/peak overestimated true AC in largest spheres (diameter, 22-37 mm) with SC and underestimated it without SC. Average SUVmean in liver and myocardium of CZT vs. NaI patients were similar. In the remaining organs (high estimated scatter proportions of >60% [CZT] or >40% [NaI]), average SUVmean differed between CZT and NaI, and coefficient of variation was significantly higher in SC vs. non-SC with CZT. MTV*SUVmax changes correlated with MRI volume changes with r=0.62 (non-SC) or r=0.59 (SC). Conclusions In principle, both CZT and NaI allow quantification in [ 123 I]mIBG SPECT/CT, but quantitative accuracy remains limited if using DEW for SC due to effects from source geometry (e.g. phantoms and organs) on scatter. Detector radius and acquisition time showed minor effects. MTV*SUVmax changes might be surrogate of response to therapy.