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In this issue, Barna and colleagues present "Dose Calculations and Dose-Effect Relationships in 177Lu-PSMA I&T Radionuclide Therapy for Metastatic Castration-Resistant Prostate Cancer". They describe the dosimetry calculations and biomarker values correlations in 22 patients with mCRPC (metastatic castration-resistant prostate cancer) to 7416 ± 218MBq of 177Lu-PSMA I&T radionuclide therapy.

Planar post-injection images at several time points were used for evaluation of absorbed organ and doses. 68Ga-PSMA PET/CT follow-up imaging enabled the determination of individual tumor molecular volume (TMV) shrinkage. Changes in 7 biomarkers after the first treatment cycle were correlated with the calculated absorbed organ and TMV doses, resulting in a total number of 259 investigated correlations. Sixty-three TMVs were identified in bone, lymph nodes, and liver with an average volume reduction of 32.3%, 84.7%, and 72.9%, respectively.

Absorbed doses per unit of administered activity for organs and lesions show good agreement with previous works (0.77, 0.71, and 0.27 mGy/MBq for parotid gland, kidneys, and liver as well as 4.38, 5.47, and 4.95 mGy/MBq for bone, lymph node, and liver malignancies, respectively). Only 37 of 259 possible correlations were statistically significant, 26 of which are associated with the absorbed dose of an organ and decrease in alkaline phosphatase.

Authors concluded that although treatment with 177Lu-PSMA I&T leads to a big reduction of TMV in patients with mCRPC, the lack of correlations calls for studies using voxel-wise dosimetry based on SPECT/CTs.

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