Injection method alters diagnostic confidence and quantitative metrics in [18F]fluorocholine PET/CT for parathyroid adenoma localization

Studies evaluating [18F]fluorocholine PET/CT for parathyroid adenoma localisation fail to report the injection method used. At our department, manual injection method (MIM) was initially used, before converting to an automated injection system (AIS). Afterwards, physicians observed increased retention of the radiotracer within the veins leading from the injection site, which had not been as present previously. The aim of this study was to evaluate whether the injection method had a clinical impact. The study compared two groups consisting of 60 patients, where the injection method was the only difference. Two physicians, blinded to injection method, evaluated if retention was present and interpretation was jeopardised. Quantitative measures were determined based on the original diagnostic confidence scores (DCS) for each suspected lesion, as these were compared between patient groups. Additionally, the maximum standardised uptake values (SUVmax) were measured for the suspected lesions along with the thyroid and salivary glands as a physiological reference point. Comparisons of the quantitative parameters were also performed between patients with or without retention. There was significant more occurrence of retention in AIS-group (83.3%) than MIM group (36.7%), (p < 0.00001). Retention did not influence the interpretation of any patient. A significant difference of DCS was observed, as 18% of lesions in the AIS-group had a low score, compared to only 1% in the MIM-group (p < 0.001). SUVmax was significantly lower in the AIS-group than the MIM-group for the reported adenomas (10.1 vs. 11.8, p = 0.008). In addition, SUVmax in the AIS-group was significantly lower in both the thyroid (4.6 vs. 5.2, p = 0.023) and salivary glands (10.8 vs. 12.4, p < 0.001). Comparison between patients with and without retention revealed similar differences in DCS and SUVmax for adenomas. The AIS is associated with a significant higher presence of retention, low diagnostic confidence and overall lower SUVmax, which indicates an aspect of clinical impact. It could potentially lead to a parathyroid adenoma being missed or classified as normal parathyroid tissue. Thus, manual injection method might be the method of choice, to ensure the most optimal clinical outcome.