Abstract
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Objectives: Fractionated therapy with 177Lu-octreotate is an effective treatment option for patients with generalized neuroendocrine tumors. The most frequent protocol in therapy with 177Lu-octreotate is to give all patients 4 cycles with 7.4 GBq each time but more and more centers individualize treatments basted on several things including absorbed dose calculations. To do absorbed dose calculations quantitative imaging with a gamma camera is needed. The current aim was to study imaging capabilities and differences of 177Lu on a GE Discovery 670 CZT and compare the results to standard imaging on a GE Discovery 670 PRO both from a diagnostic point of view and for quantitative imaging.
Methods: Using an Image Quality phantom for PET that is described in NEMA NU 2-2001 the spheres were filled with a concentration that was 8 times higher than the background. Imaging was performed with 120 angles (60 for each detector), 128 matrix and 30 s per angle using the only available collimator for the CZT system and a MEGP for the PRO system. Low dose CT was done for attenuation correction for both systems. Percent contrast of the concentration in the spheres was calculated for the different measurements. The measurements for the 670 CZT system was performed with an energy setting of both 113 keV ± 6 % and 208 keV ± 6 % and for the 670 PRO only 208 keV ± 10 % was performed.
Results: The percent contrast was for the smallest to the largest sphere for 208 keV peak was for the PRO system 4, 9, 15, 20, 26 and 37 % while it was 7, 13, 17, 22, 30 and 38 % using the CZT system. The results were 7, 13, 20, 26, 36 and 48 % for the 113 keV peak in the CZT system and applying a triple energy window correction for the 113 keV peak in the CZT system gives percent contrasts of 7, 16, 23, 32, 47 and 63 %. This seems to be fairly good, but it really need to be noted that for the upper energy window there was a clear septal penetration that is not acceptable for quantitative imaging while for the lower energy window there is a clear problem with scatter that can really not be corrected in this way due to earlier shown problems with energy sharing between pixels in the CZT camera (1).1. Fan P, Hutton BF, Holstensson M, et al. Scatter and crosstalk corrections for (99m)Tc/(123)I dual-radionuclide imaging using a CZT SPECT system with pinhole collimators. Med Phys. 2015;42:6895-6911.
Conclusion: Diagnostic imaging with a 670 CZT on 177Lu gives a good result and even the quantitative imaging possibilities really seems to have great potential even if there is still some work that needs to be done to make it work with the full potential. Research Support: None