Effects of Increased Injected Activities Exceeding 9.0 Gbq [177lu]Lu-PSMA-I&t on Absorbed Doses to Lesions and Kidneys
Abstract
Purpose
Administration activities of 177Lu-PSMA were chosen cautiously in the treatment of advanced prostate cancer to refrain from inducing organ toxicity. The primary aim here was to investigate the association of increased injected activities and the respective absorbed doses (AD).
Methods
In this single center, retrospective analysis, patients with PSMA-avid metastatic castration-resistant prostate cancer treated with [177Lu]Lu-PSMA-I&T were included. Patients underwent multiple timepoint SPECT/CT imaging at 24, 48, and 72h post-injection during the first cycle of therapy. Patients included were either injected at the standard 7.4 GBq, or exceeding 9.0 GBq. Segmentations were completed manually for the kidneys (CT-based), while a 3.0 SUV threshold-based approach on SPECT was used for lesions. Dosimetry calculations were then performed using a multiple-timepoint protocol. Manual verification of segmentations coregistered between timepoints was completed. All time activity data was fit using a mono-exponential function. Rib and small volume (<3 mL) lesions were omitted due to partial volume effects.
Results
76 patients were included: 22 were injected with 9.1 ± 0.3 GBq and 54 patients injected with 7.4 ± 0.3 GBq. The AD to the kidney was comparable, 0.29 ± 0.08 Gy/GBq vs. 0.33 ± 0.13 Gy/GBq [p = 0.193] (9.1 GBq vs. 7.4 GBq). The tumor AD was comparable in the 9.1 GBq and 7.4 GBq cohorts: total tumor burden (0.95 ± 0.42 vs. 0.92 ± 0.52 Gy/GBq) [p = 0.302]; bone lesions 0.78 ± 0.42 vs. 0.74 ± 0.46 Gy/GBq [p = <0.001].
Conclusion
The AD across the disease burden, and individual lesions remained comparable even with increased injected activity. The AD to the kidney remained comparable between the two cohorts. Stratifying patients by tumor burden did not reveal greater AD occurred on lower volume patients (indicative of lack of tumor sink effect); on the whole disease burden, or at the individual lesion level.