Standardized Method for Quantitative Assessment of the Efficacy of Bone-Targeted Radionuclide Therapy with Radium-223 Chloride in Patients with Prostate Cancer.

Introduction: Radium-223 (²²³Ra) therapy improves survival in patients with metastatic castration-resistant prostate cancer (mCRPC), but response monitoring is challenging due to the lack of classic PSA response. Standardized assessment methods based on imaging are needed. Objective: To develop and evaluate a comprehensive diagnostic algorithm for quantitative monitoring of the efficacy of ²²³Ra therapy based on SPECT/CT and bone scintigraphy data. Materials and Methods: A prospective single-center study included 114 mCRPC patients who received 2 to 6 injections of ²²³Ra. All patients underwent planar bone scintigraphy with automated Bone Scan Index (aBSI) calculation, quantitative SPECT/CT with   standardized uptake value (SUV_max, SUV_mean) and percent injected dose (%ID) calculation, PSMA-ligand PET/CT, assessment of biochemical markers (PSA, LDH, ALP) and clinical status. Results: In patients who completed the full course (6 injections, n = 64), a statistically significant decrease in all quantitative parameters was revealed: SUV_max (from 51.9 ± 16.4 to 30.5 ± 11.2; p < 0.001), SUV_mean (from 2.87 ± 0.63 to 2.43 ± 0.58; p = 0.021),   %ID (from 12.4 ± 3.1 to 10.8 ± 2.6; p = 0.037), aBSI (from 2.15 ± 0.38 to 1.70 ± 0.31; p = 0.005), as well as levels of PSA, LDH and ALP. Imaging parameters significantly correlated with ALP levels, pain syndrome, functional status, and the development of hematological toxicity. High baseline SUV_mean, %ID, and aBSI values were associated with an increased risk of myelosuppression. Conclusion: Quantitative SPECT/CT parameters (SUV_max, SUV_mean, %ID) and scintigraphy parameters (aBSI) are objective markers of the efficacy of ²²³Ra therapy, reflecting the dynamics of the metabolic activity of bone metastases and allowing to predict the development of hematological toxicity. A comprehensive multiparameter approach improves the accuracy of treatment response assessment.

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