Practical kidney dosimetry in peptide receptor radionuclide therapy using [(177)Lu]Lu-DOTATOC and [(177)Lu]Lu-DOTATATE with focus on uncertainty estimates

BACKGROUND: Kidney dosimetry after peptide receptor radionuclide therapy using (177)Lu-labelled somatostatin analogues is a procedure with multiple steps. We present the SPECT/CT-based implementation at Aarhus University Hospital and evaluate the uncertainty of the various steps in order to estimate the total uncertainty and to identify the major sources of uncertainty. Absorbed dose data from 115 treatment fractions are reported. RESULTS: The total absorbed dose with uncertainty is presented for 59 treatments with [(177)Lu]Lu-DOTATOC and 56 treatments with [(177)Lu]Lu-DOTATATE. For [(177)Lu]Lu-DOTATOC the mean and median specific absorbed dose (dose per injected activity) is 0.37 Gy/GBq and 0.38 Gy/GBq, respectively, while for [(177)Lu]Lu-DOTATATE the median and mean are 0.47 Gy/GBq and 0.46 Gy/GBq, respectively. The uncertainty of the procedure is estimated to be about 13% for a single treatment fraction, where the absorbed dose calculation is based on three SPECT/CT scans 1, 4 and 7 days post-injection, while it increases to about 19% if only a single SPECT/CT scan is performed 1 day post-injection. CONCLUSIONS: The specific absorbed dose values obtained with the described procedure are comparable to those from other treatment sites for both [(177)Lu]Lu-DOTATOC and [(177)Lu]Lu-DOTATATE, but towards the lower end of the range of reported values. The estimated uncertainty is also comparable to that from other reports and judged acceptable for clinical and research use, thus proving the kidney dosimetry procedure a useful tool. The greatest reduction in uncertainty can be obtained by improved activity determination, partial volume correction and additional SPECT/CT scans.