The Influence of Early Measurements Onto the Estimated Kidney Dose in [177Lu][DOTA0,Tyr3]Octreotate Peptide Receptor Radiotherapy of Neuroendocrine Tumors

Multiple measurements have been required to estimate the radiation dose to the kidneys resulting from [Lu-177]DOTATATE therapy for neuroendocrine tumors. The aim of this study was to investigate the influence of early time-point measurement in the renal dose calculation. Anterior/posterior whole-body planar scintigraphy images were acquired at approx. 1, 24, 48, and 72 h after administration of [Lu-177]DOTATATE. Furthermore, we acquired planar 1-bed dynamic recordings in 12 frames (5 min each) during the first hour. We assessed kidney exposure with a three-phase model consisting of a linear increase to the maximum within the initial minutes p.i., followed a bi-exponential decline. This three-phase-model served as reference for evaluating accuracy of dose estimates in 105 kidneys calculated by conventional mono-exponential fitting of the final three and four whole-body images. Mean effective half-life times for the reference model were 25.8 +/- 12.0 min and 63.9 +/- 17.6 h, predicting a mean renal dose of 5.7 +/- 2.1 Gy. The effective half-life time was 46.3 +/- 15.4 h for the last four and 63.3 +/- 17.0 h for the last three data points. The mean start of the first whole-body measurement was 1.2 +/- 0.1 h p.i. The ratio of fast to slow phases was 28.1 +/- 23.9 % at this time point, which caused a mean absolute percentage dose deviation of 12.4 % for four data points, compared to 3.1 % for three data points. At a mean time of 2.4 h p.i. (max 5.1 h), the ratio of fast to slow phase declined below 5 %. Kinetic analysis of renal uptake using dynamic planar scans from the first hour after injection revealed a fast and a slow washout phase. Although the fast phase did not contribute substantially to the estimated renal dose, it could influence planar measurements performed within the first hours. We found that the presence of two clearance phases can hamper accurate dose estimation based on a single-phase model, resulting in approximately 12.4 % dose underestimation, thus potentially resulting in overtreatment. In the absence of dynamic initial recordings, the first dosimetry measurements should therefore be obtained later than 3-5 h after [Lu-177]DOTATATE injection. Omitting the early whole-body image reduced the dose estimation error to 3.1 %.