Evaluation of two-dimensional total bone uptake (2D-TBU) and bone scan index (BSI) extracted from active bone metastatic burden on the bone scintigraphy in patients with radium-223 treatment

Abstract

Objective: Radium-223 is a first-line alpha-emitting radionuclide treatment for patients with metastatic castration-resistant prostate cancer (mCRPC) with bone metastases, and the completion and prognosis of this treatment are intensively studied. Although the spread-based bone scan index (BSI) and the novel index of intensity-based twodimensional total bone uptake (2D-TBU) from anomalous hot spot counts of bone scintigraphy may provide useful input in radium-223 treatment, they have not been evaluated in detail yet. This study aimed to fill this gap by evaluating BSI and 2D-TBU in patients treated with radium-223.Methods: Twenty-seven Japanese patients with mCRPC treated with radium-223 were retrospectively analyzed. The patients were evaluated via blood tests and bone scans at baseline and 3 cycles intervals of treatment. BSI and 2D-TBU were analyzed via VSBONE BSI in terms of correlations, response to radium-223 treatment, and association with treatment completion, and the Kaplan-Meier survival analysis was performed.Results: Nineteen patients (70.4%) completed six cycles of radium-223 treatment, whereas eight patients (29.6%) did not complete the treatment regimen. A significant difference in baseline BSI and 2D-TBU was observed between these groups of patients.Both BSI and 2D-TBU were highly correlated with treatment response (r = 0.96, p < 0.001). Univariate analysis showed an association between radium-223 completion in median BSI and 2D-TBU values (p = 0.015) and completion percentage differences (91.7% vs. 45.5%; p = 0.027). The Kaplan-Meier product limit estimator showed that the median overall survival was 25.2 months (95% CI: 14.0-33.6 months) in the completion group and 7.5 months (95% CI: 3.3-14.2 months) in the without completion group (p < 0.001). Conclusions:Both BSI and 2D-TBU showed high correlations and strong agreement with treatment response. In particular, BSI could contain information not only on the spread but also on the comprehensive active intensity of the bone metastatic burden. Furthermore, both indices were associated with treatment completion with superior prognosis in the completion group. This study supports the necessity of preliminary assessment using BSI and 2D-TBU extracted from VSBONE BSI for radium-223 treatment decisions and monitoring.