Dose Mapping After Endoradiotherapy with ¹⁷⁷Lu-DOTATATE/DOTATOC by a Single Measurement After 4 Days

Dosimetry of organs and tumors helps to assess risks and benefit of treatment with ¹⁷⁷Lu-DOTATATE/DOTATOC. However, it is often not performed in clinical routine because of additional efforts, the complexity of data collection and analysis, and the additional burden for the patients. Aiming at a simplification of dosimetry, we analyzed the accuracy of a theoretically substantiated approximation, which allows the calculation of absorbed doses from a single measurement of the abdominal activity distribution. Methods: Activity kinetics were retrospectively assessed from planar images in 29 patients with neuroendocrine tumors (NETs; n = 21) or meningioma (n = 8) after the administration of ¹⁷⁷Lu-DOTATATE (n = 22) or ¹⁷⁷Lu-DOTATOC (n = 7). Mono- or biexponential functions were fitted to measured data in 54 kidneys, 25 livers, 27 spleens, and 30 NET lesions. It was evaluated for each fit function how well the integral over time was represented by an approximation calculated as the product of the time t_l of a single measurement, the expected reading at time t_l, and the factor 2/ln(2). Tissue-specific deviations of the approximation from the time integral were calculated for time points t_l of 24, 48, 72, 96, 120, and 144 h. Results: The correlation between time integral and approximation improved with increasing time t_l Pearson r exceeded 0.95 for a t_l of 96 h or more in all tissues. The lowest maximum errors were observed at a t_l of 96 h, with deviations of the approximation from the time integral of median +5% (range, -9% to +17%) for kidneys, +6% (range, -7% to +12%) for livers, +8% (range, +2% to +20%) for spleens, and +6% (range, -11% to +16%) for NET lesions. Accuracy was reduced for measurements after 72 or 120 h. For measurements after 24, 48, and 144 h, the approximation led to large deviations for some of the patients, in particular unacceptable underestimates of the absorbed dose to the kidneys. Conclusion: A single quantitative measurement of the abdominal activity concentration by SPECT/CT 4 d after the administration of ¹⁷⁷Lu-DOTATATE/DOTATOC provides a 3-dimensional dose map and can be used to estimate the doses actually absorbed in the treatment cycle with minor additional resources and effort.