Simplified dosimetry for kidneys and tumors in (177)Lu-labeled peptide receptor radionuclide therapy

PURPOSE: To evaluate if satisfactory post-therapeutic image-based dosimetry can be achieved for Lu-177-DOTATATE treatments using a reduced number of image acquisitions to improve patient comfort and reduce economical costs. METHODS: 39 patients who underwent 147 treatment cycles of Lu-177-DOTATATE for neuroendocrine tumors were included in the study. A total of 291 and 284 absorbed doses were calculated to kidneys and tumors, respectively. Single-point dosimetry was performed using one SPECT/CT image acquired at 1 d or 7 d post-treatment using a fixed effective half-life (T(eff)) or using a patient-specific T(eff) determined for the initial cycle. Also, dose-per-activity values, (D/A)(1), were determined from the first cycle and used to calculate doses for subsequent cycles. All absorbed doses were evaluated against “true” doses calculated using both the 1 d and 7 d images. The relation between tumor grade and absorbed doses was also investigated. All dosimetry was performed on SPECT images. RESULTS: Absorbed doses to kidneys were most accurate when single-point dosimetry was performed using 1 d images with median ratios in relation to “true” doses in total dose of 1.00 (IQR: 0.97–1.03) when using fixed T(eff) and 1.01 (IQR: 0.98–1.04) when using T(eff) from the initial cycle. Calculations based on the 7 d image were most accurate for tumors with corresponding ratios in total absorbed dose of 0.98 (IQR: 0.96–1.00) and 1.00 (IQR: 0.99–1.01) when using a fixed T(eff) or T(eff) from the first cycle, respectively. The (D/A)(1) approach performed worse, as 2 of 77 total absorbed doses to the kidneys deviated with > 30%, and tumor-absorbed doses were increasingly overestimated with every cycle. Absorbed doses, T(eff) and 1 d uptake were higher for G1 tumors than G2 tumors. CONCLUSION: Dosimetry can be performed with satisfactory accuracy when using single SPECT/CT images acquired at 1 d for kidneys or at 7 d for tumors.