Stability of SPECT uptake value (“SPECT-UV”) in normal abdominal organs for 111In-pentetreotide imaging

Objectives Quantitative PET has become important in oncologic imaging. Some cancer types, such as pancreatic neuroendocrine tumors, are however generally followed with SPECT radiopharmaceuticals. It would therefore be desirable to quantitate treatment response with SPECT in a manner analogous to PET. In order to implement quantitative SPECT, however, the normal variability of SPECT uptake in solid organs must be known.

Methods 7 patients imaged with 111In-pentetreotide 24 hours after injection at multiple time points to monitor progression or assess therapy of neuroendocrine disease were used (total 17 scans). Images were reconstructed with quantitative SPECT (QSPECT) methods that compensated all image degrading effects (OSEM, 30 iterations, 8 subsets). Normal iver and kidney volumes-of-interest (VOIs) were manually drawn on a clinical workstation. The maximum, mean, and peak (in a 1 cm spherical region) SPECT-UVs were computed in the VOIs analogously to methods used to evaluate PET SUV.

Results Image noise led to larger variability in the maximum and peak SPECT-UV values than for the mean. The mean SPECT-UV values for left and right kidneys (means 8.28 and 8.32) were highly correlated (r=0.962), indicating spatial reliability within the images. The coefficients of variation (COVs) in the kidneys and liver were, respectively, 28 and 72% over all scans and 17 and 38% between scans for the same patient averaged over all patients. The intraclass correlation coefficients, indicating the fraction of variation due to patient differences, were 0.32 and 0.73 for the liver and kidneys. The intra-patient variability in kidney SPECT-UV was similar to that previously reported for the liver SUV for FDG-PET.

Conclusions Inter-patient organ SPECT-UV variability was greater than intra-patient variability, suggesting that patient-specific criteria are needed. The reliability of SPECT-UV in normal organs indicates it may find an important role in standardizing the assessment of treatment response in tumors imaged with SPECT radiopharmaceuticals.

Research Support U01CA140204