Personalized ¹⁷⁷Lu-octreotate PPRT: Cycle-to-cycle renal radiation dose prediction using quantitative SPECT/CT dosimetry

Objectives Peptide receptor radionuclide therapy (PRRT) with ¹⁷⁷Lu-octreotate is commonly administered at fixed activity per cycle despite the known huge inter-patient variability in absorbed radiation dose to the kidney. We aimed to assess the accuracy with which renal dose is predictive of the renal dose to be delivered during the subsequent cycle, with the goal to personalize administered activities.

Methods Eight patients underwent serial quantitative SPECT/CT (4, 24 and 72 h) after each of 4 ¹⁷⁷Lu-octreotate cycles (~7.4 GBq). Alignment of fused SPECT/CT volumes was performed through non-rigid registration of CT volumes. A GPU Monte Carlo code was used to generate a dose-rate map at each time-point. A 1-cm³ VOI was placed on the cortex of each kidney to sample the dose rates. These were fitted to a bi-exponential curve, which was integrated. The resulting renal dose was averaged between both kidneys and compared between each pair of consecutive cycles (difference expressed as % of later cycle renal dose). Positive and negative values indicated that the predicted dose overestimated or underestimated, respectively, the delivered renal dose.

Results The mean renal dose per administered activity was 0.48 ± 0.13 Gy/GBq (range: 0.30 to 0.71 Gy/GBq), which is consistent with published data. The mean cycle-to-cycle renal dose difference was near zero at -0.7 ± 9.9 % (range: -18.5 to 14.3 %).

Conclusions These results suggest that renal dose can be predicted from the precedent cycle with a good accuracy. This could enable prospectively personalizing administered activity at each subsequent cycle to deliver a prescribed renal dose. Personalized PRRT may improve clinical outcomes by maximizing per-cycle tumor doses, without exceeding the tolerable renal dose.