Improvement of the approach to definition of patient release criteria after radionuclide therapy

Current patient release criteria established in NRB-99/2009 relate to four radionuclides used in Russia that period of the document preparation. These criteria were calculated only considering the radionuclide decay. Thus, these criteria give conservative assessments which require the patient to stay in dedicated protected room (“hot” room) of the radiotherapy department for several days and do not allow to radionuclide therapy to be given in the outpatient hospital which certainly reduces the availability of this type of treatment. Consideration of the biological excretion of the radiopharmaceutical will be able to make the patient release criteria after radionuclide therapy significantly softer. Recently, new promising radionuclides and radiopharmaceuticals for therapy have appeared, the introduction of which into medical practice is obstructed by the lack of the patient release criteria. Current study is devoted to justification of improvement the approach to definition of patient release criteria after radionuclide therapy with promising and applied radiopharmaceuticals considering biological excretion of radiopharmaceutical. As examples, calculations of patient release criteria were performed for 177Lu-PSMA, 177Lu-DOTA-TATE, 131I-mIBG и Na131I. The dosimetric model used for calculation is based on the model which was used for definition of the current patient release criteria in NRB-99/2009. Additionally, the biological excretion of the radionuclide in radiopharmaceutical, which was assessed according to the published data, was considered. Two phases of biological excretion were evaluated for considered radiopharmaceuticals (fast and slow fractions). The main contribution of the radionuclide decrease in the patient’s body during the first hours after the injection is made by the fast-eliminated fraction. During 4-6 hours after the administration about 50% of these radiopharmaceuticals eliminated via urine. The calculation of patient release criteria were performed for slow-elimination fractions which characterize the decrease of radionuclide activity in the body after the patient release. The following effective half-lives were used: 177Lu-PSMA – 60 hours, 177Lu-DOTA-TATE – 100 hours, 131I-mIBG – 45 hours, Na131I – 7.5 days. The calculations demonstrate when planning a course of radionuclide therapy with four administrations of 177Lu-PSMA, the release of a patient is allowed at the dose rate of 20 μSv/h from the patient at the distance of 1 m; in case of radionuclide therapy with 177Lu-DOTA-TATE, release of a patient is allowed at the dose rate of 12 μSv/h. Considering the fast-eliminated fraction in the first hours after the administration, it can be assumed that more patients with normal renal function can undergo radionuclide therapy with 177Lu-PSMA in the outpatient hospital. Some patients after the 177Lu-PSMA administration and patients in the therapy with 177Lu-DOTA-TATE may be held in the department in the “hot” rooms for 1-2 days until the dose rate decrease to the acceptable levels. The consideration of biological excretion of radiopharmaceutical makes it possible to soften patient release criterion for 131I-mIBG by four times and for Na131I – by 10%. It will allow to release patients after radionuclide therapy earlier without reduce of radiation safety of people around the patient and will increase the capacity of radionuclide therapy department.

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