Abstract
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Aim: The aim of this study was to collect temporal radiation exposure rate data from subjects treated with therapeutic levels of [177Lu]DOTA-Octreotate (“Lutate”) (6-8 GBq per cycle) to determine when it is acceptable to allow subjects to leave the treating nuclear medicine facility from the radiation safety perspective. Lutetium-177 has predominant gamma emissions at 113 and 208 keV with relatively low abundances of 6% and 11% respectively.
Methods: Consecutive subjects with metastatic neuroendocrine tumours that demonstrated high somatostatin receptor expression on [68Ga]DOTATATE PET imaging and who were treated with Lutate were studied during an 8-week period in 2018. All subjects received 2L of amino acid infusion for reno-protection over 3-4 hrs during treatment. Exposure rates were measured using the recommended Ambient Dose Equivalent at 10 mm in tissue metric (H[asterisk](10)) in μSv/hr at the following distances from the ventral surface of the subject’s chest: 0,0.2,0.3,0.5,1,2,3 and 4 m. Measurements were made at the following time points after completion of administration: immediately after, 4, 24 and 120 hrs.
Results: Data were acquired in eleven subjects (6 female). The average amount of Lutate administered was 7640±140 MBq. The mean exposure rate from all subjects, normalised for amount administered, showed the combination of biological clearance and physical decay measured at 2m could be characterised by a bi-exponential function with a fast component of 0.79 hr-1 and slow component of 0.025 hr-1 in the approximate ratio of 2:1 respectively (R2=0.99). This equates to the exposure rate dropping to 50% of the initial value around 1hr after the end of the infusion, depending on when the subjects emptied their bladders. After approx.4 hrs the slow component of clearance starts to predominate with an average half clearance time of approx.28 hrs. The change in the measured exposure rate with distance in the range 1 - 4 m decreased empirically as a mono-exponential function with a constant of 0.75 m-1 (i.e., 50% decrease every 0.9 m). The upper limit for discharge into the community recommended by our local regulatory guidelines is 25 μSv/hr at 1m and this was achieved in all subjects studied by the 4 hr time point.
Conclusions: Small molecules such as the peptides used in radionuclide therapies are rapidly cleared by the kidneys. In our local environment, subjects can be treated with therapeutic amounts of 177Lu and routinely released into the community with suitable precautionary advice by 4 hours after treatment as long as they are not intending to undertake a long journey where they would be in close proximity to members of the public. Admission to the hospital or clinic for a prolonged stay (e.g., overnight) is only indicated for medical or other reasons.