SUVR for the Interpretation of Neuroendocrine Tumor PET Imaging with⁶⁸Ga-dotatate

Objectives: PET imaging with 68Ga-dotatate (NETSPOT) is a highly sensitive technique for the detection of neuroendocrine tumors. However, the interpretation of 68Ga-dotatate scans largely remains dependent on visual inspection, introducing ambiguity to the characterization of tissue(s) with high physiologic tracer retention (e.g. the head and uncinate process of the pancreas). While the use of reference tissue and SUVR has become commonplace in PET neuroimaging, their application in routine oncology studies remains limited. This work seeks to identify potential reference tissue(s) for the interpretation of 68Ga-dotatate scans and provide a preliminary assessment of the utility of the corresponding SUVR measurement.

Methods: Retrospective analysis was conducted on static, whole-body scans from 115 consecutive adults imaged with 68Ga-dotatate at the University of Michigan between July 2018 and December 2018. 2D regions were manually drawn on the liver, spleen, pancreatic head & uncinate process, and thyroid, pituitary, and adrenal glands, using MEDVIEW (MedImage, Ann Arbor, MI) to determine the SUV characteristics of the tissue section. For positive scans, the lesion with the highest avidity for 68Ga-dotatate was identified and analyzed in the same manner, with all regions confirmed anatomically by a low-dose CT scan acquired at the time of PET imaging. SUVR was calculated as the ratio SUVmax (lesion) to SUVmean (reference region).

Results: Images were acquired following intravenous administration of 158 ± 3.7 MBq (4.27 ± 0.1 mCi) of ⁶⁸Ga-dotatate. Mean SUV values were particularly variable in the thyroid (3.06 ± 1.63, n = 204), pituitary glands (6.74 ± 2.46, n = 115), and spleen (18.42 ± 7.34, n = 109), while hepatic (6.06 ± 2.04, n = 114) and adrenal (13.12 ± 4.21, n = 217) tracer retention showed higher inter-individual agreement. Analyzed lesions displayed highly variable uptake (SUV_max = 44.37 ± 30.09, n = 77), as did physiologic uptake in the pancreatic head & uncinate process (SUV_max = 9.96 ± 5.01, n = 86). Using the liver as a reference region, ⁶⁸Ga-dotatate positive tumors in the head and uncinate process showed a SUVR of 12.64 ± 8.99 (n = 13), in contrast to control tissue in the same region (1.72 ± 1.11, n = 86). The size of the 2D shape used to determine the SUV_{mean (reference region)} was large (mean = 64 cm² or 389 pixels).

Conclusions: The significant variability in ⁶⁸Ga-dotatate retention found in our study and others underlies a need for improved interpretation guidelines. Here, we identify the liver as a potential reference region for analyzing NETSPOT images and establish SUVR as a tool which facilitates a semi-quantitative interpretation of ⁶⁸Ga-dotatate scans without significant disruptions to established imaging and analysis workflows. References: Moradi et al. Clinical Nuclear Medicine, 2016, 41 (6): e281-e287.