Abstract
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Objectives: Highly sensitive, specific and accurate 68Gallium DOTATATE PET/CT (68Ga-Dotatate) is rapidly becoming a preferred imaging modality for evaluating neuroendocrine tumors [1,2,3]. Awareness of normal distribution pattern of the radiotracer is essential for diagnosing abnormal uptake. Visual analysis is aided by semi-quantitative measures such as maximum standardized uptake value (SUVmax) which helps in differentiating abnormal activity from physiological activity and comparing it to reference organs such as liver and spleen [4,5,6]. Several articles document the normal bio-distribution and SUVs using the standard non time-of-flight PET/CT (TOF)cameras [7,8,9]. With eventual retirement of older non TOF cameras and their replacement with newer, highly sensitive TOF cameras, where SUVmax measurements are generally higher due to better special resolution and reduced partial volume effect , updated knowledge of normal ranges of SUVmax is needed. To our knowledge, TOF SUVmax in normal structures on 68Ga-Dotatate scans has not been previously reported. Additionally, awareness of how much the TOF SUVmax differs from the non TOF value is required for adequate scan comparisons while the different technologies coexist. Our study objectives are:
To establish normal average TOF SUVmax values for common structures on 68Ga-DOTATATE PET/CT
To compare SUVmax values obtained using the TOF and non-TOF algorithms.
Objectives:
Methods: Retrospective analysis of 77 consecutive 68Ga-Dotatate scans was performed out of which 50 patients (20 men and 30 women; median age, 55 years; median weight, 88.5 kilograms) who underwent imaging on a PET-CT camera with both TOF and non TOF reconstruction capability were included in the study. All patients were greater than 18 years and were imaged for known or presumed neuroendocrine tumor using the standard imaging protocol. This included 2 hour fasting, imaging a minimum of 24 hours after short acting somatostatin (SST) analogues or just prior to the next dose of long acting SST analogues, and imaging with a prescribed dose of 185 megabecquerels (MBq) at 60 minutes post injection. Each image was processed using TOF and non TOF reconstruction algorithms. A one centimeter square region of interest (ROI) was drawn on an anatomically normal appearing area of 24 preferentially chosen structures to measure SUVmax while avoiding any confounding adjacent activity or areas concerning for any possible disease or abnormality on CT and when unavoidable, a manually contoured region was drawn. Same ROI was analyzed using both algorithms and the SUVmax were compared.
Results: A total of 1200 ROIs were evaluated. 68Ga-Dotatate uptake was seen in normal structures with SST receptor subtype 2 in the following decreasing order: spleen, pituitary, renal cortex, adrenals, uncinate pancreas, liver, pancreas except uncinate, salivary gland, thyroid, bone marrow, myocardium, lymph node, blood, breast, muscle, lung, fat, pineal gland and brain. TOF images had better target to background ratio visually. TOF SUVmax were higher for all structures with the exception of lung and brain. As compared to non TOF SUVmax, TOF SUVmax measured more than double in adrenals, and uncinated pancreas; approximately 1.8 times in concerning lesions, lymph nodes, pineal gland; and greater than 1.5 times in thyroid, breast, and pancreatic head.
Conclusions: Our study demonstrates the 68Ga-Dotatate pattern on newer, sensitive TOF PET/CT cameras, and documents the average TOF SUVmax and their ranges in normal structures. Overall, TOF SUVmax measure higher in the same ROIs, with the abnormal lesions measuring approximately 1.8 times higher than non TOF. These findings need to be taken in consideration when comparing patient scans between PET/CT technologies. Our study provides reference TOF SUVmax values, and hopes to assist in accurate evaluation and interpretation of 68Ga-Dotatate scans between different imaging technologies.