Absorbed doses to kidneys based two SPECT measurements versus seven SPECT measurements patients with neuroendocrine tumours receiving 177Lu-DOTATATE therapy

365 Aim: As in all therapies using ionizing radiation, a patient-specific optimization of the delivered radiation should be performed in therapy with 177Lu-DOTATATE. At our centre, the number of treatments is based on the absorbed dose (AD) to the kidneys. For logistical reasons, estimation of absorbed doses should be performed with as few measurements as possible. The aim of the present work was to study how well kidney AD estimations using a method with two measurement points agree with a method that is based on seven imaging points. Methods: Six patients (three female and three male) with neuroendocrine tumors with high somatostatin receptor expression were included. They were injected with 500MBq 177Lu-DOTATATE and no kidney protection was used. SPECT/CT over the abdomen were acquired at 0.5, 1, 1.5, 2.5, 4, 24 and 96 h after injection of 177Lu-DOTATATE. AD were calculated using single exponential fits to data from 2 measurements (24-96) or trapezoidal integration up to 24 h and then exponential fit after 24 h. Bias of AD values relative to those based on the seven-point measurement were calculated for both right and left kidney. To test statistical significance a Wilcoxon matched-pairs signed rank test with a P-value of 0.05 for significance was used. Median, Min and Max of the deviations were also calculated for the AD. For the early measurement points, estimations from the (24-96) for the time of measurement were compared with the measured values. Results: Bias versus AD(seven) was -0.4%(-1.1-3.8) (Median (Min-Max)) for the right kidney and -0.2%(-1.1-5.4) for the left kidney when the AD(24-96) method was used. The P-values were 0.69 for the right kidneys and 0.84 for the left kidneys and this show no significant difference between the two methods. With errors as small as these and using a fixed limit of the absorbed dose to the kidneys, this would mean that really few patients would be given either too many or too few treatments. The difference of the estimated activity concentrations for every point was in general small (less than 5%) but could vary up to about 30% for single values. A larger study including more patients and preferably also a later timepoint (maybe 7 days) is highly warranted. Conclusions: With a difference of less than about 5% for all the measurements the results from this study indicate that the influence of early measurements (earlier than 24 h) is fairly small and can be ignored. A larger study with more patients is needed to draw full conclusions.