Feasibility of post-therapy quantitative Lu-177-DOTATATE SPECT/CT in patients with metastatic neuroendocrine tumor for evaluation of response to treatment: Comparison with Ga68-DOTATATE-PET/CT

1530 Introduction: Feasibility of post-therapy quantitative Lu-177-DOTATATE SPECT/CT in patients with metastatic neuroendocrine tumor for evaluation of response to treatment: Comparison with Ga68-DOTATATE-PET/CT Purpose: Lu-177-DOTATATE (Lutathera™) peptide receptor radionuclide therapy (PRRT) is gaining widespread adoption for the treatment of neuroendocrine tumors (NET) since its FDA approval in early 2018. The utility of a novel quantitative SPECT/CT methodology in evaluating Lu-177-DOTATATE-avid tumor burden and its therapy response using the post-therapy gamma camera images was explored. We then compared the trend of these quantitative measures on serial treatments with baseline and follow up Ga68-PET/CT-DOTATATE images in two major groups of improving and stable disease. Methods: 51 whole body SPECT/CT images were analyzed using a dedicated SPECT/CT quantitation software (MIM®) and workflow developed for quantitative Lu-177-DOTATATE SPECT/CT images. 15 patients with metastatic NET had enrolled for lutathera therapy, each for a total of 4 doses at 8 week intervals. 2 patients refused therapy halfway through the cycles. SPECT-CT was not obtained in 1 patient due to severe backpain. The remaining 12 had their Lutathera-avid lesions semi-automatically drawn. The tumor volume, total, max, and mean counts of the lesions were calculated. The baseline and 6-month follow up Ga68-DOTATATE-PET/CT images of each patient were reviewed and grouped as “improved” or “stable”. Friedman’s test with multiple comparisons was used to evaluate statistical significance of quantitative measures in each outcome group over course of treatment. Correction for multiple comparisons was performed using Dunn’s test. P values <0.05 were considered significant. Results: Out of 12 patients (48 scans), 5 showed improved on imaging, 6 were stable and 1 had progressed (13.1±5.1 months after last therapy). The progressed group was excluded due to small sample size. In the improving group, there was no statistically significant difference between 4 time-points of total count, max count and volume (P>0.08). Mean counts at dose 1 (836.8; 124.8 to 1549 95%CI) vs. dose 3 (483.6; -214.3 to 1182 95%CI) and dose 1 vs. dose 4 (464.8; -100.1 to 1030 95%CI) were significantly decreased (P 0.06). Conclusions: This pilot study shows a proof of concept method for quantification of post-therapy Lu-177-DOTATATE SPECT/CT in early evaluation of PRRT response compared to Ga68-DOTATATE-PET/CT. Statistically significant decline of tumoral uptake over the course of serial treatments was seen in both improving and stable disease groups. Unexpected similar behavior of both improving and stable groups could be due to small sample size of this study, qualitative analysis of the PET/CT images or possibly inherently different imaging characteristics of post-therapy SPECT/CT vs. Ga68-DOTATATE-PET/CT. A longer term clinical and imaging follow up using a larger sample size could help further understand the significance of such serial quantification methodology and likely pave way for a more individual patient-based tumor dosimetry.