Estimation of the absorbed dose to patients treated with 177Lu-Dotatate with regards to the long-term retention and radionuclide impurity in the form of 177mLu

The use of radio-labeled somatostatin analogs, such as 177Lu-labelled DOTATATE, in PRRT, has based on the studies performed until now, shown to be promising for RNT disseminated neuroendocrine tumors [1]. In this regard, the aim of treatment is to assure the delivery of the highest possible absorbed dose to the tumor cells, while saving the normal tissues [1], meaning, to achieve a higher activity uptake in tumors with neuroendocrine origin while assuring the lowest possible radio-toxicity in normal organs [2]. The key challenge associated with the absorbed dose distribution in the body upon administration of PRRT using 177Lu-DOTATATE is the absorbed dose to the kidneys, as being the dose limiting organs in PRRT [7]. The main excretion route of 177Lu-DOTATATE is through urine, where >65% of the total administered activity is excreted. This has lead us to investigate the long time retention of 177Lu in the total body, and also to estimate the absorbed dose given by possible radionuclide impurities. According to the declaration from the producer, a sample of 177Lu contains less than 0.05% of 177mLu. In order to investigate whether the 177mLu contribution to the absorbed dose is as low that it can be tolerated or even neglected, spectroscopy measurements have been performed on patients receiving therapy with 177Lu-DOTATATE at Skåne University Hospital [15]. Prior to performing these measurements, the spectroscopy equipment has been carefully calibrated. The data obtained from patient measurements have been analyzed using different methods, and have been combined with imaging based data, obtained as part of the routine dosimetry performed at Skåne University Hospital.