Bone scan index as a new imaging biomarker in response assessment of prostate cancer patients with bone metastasis

Objectives: Bone scan index (BSI) is a reproducible, quantitative expression of tumor burden seen on bone scintigraphy. The objective of this study was to compare changes between serial scans of prostate cancer patients with bone metastasis, as assessed by visual analysis, lesion number, and automated BSI software.

Methods: We retrospectively reviewed 283 bone scans from 29 prostate cancer patients. Visual analysis for response assessment was done on a 4-point scale: 1 progression, 2 indeterminate (new uptake for which metastasis cannot be excluded nor confirmed), 3 stable disease, 4 regression. BSI and lesion number were calculated using EXINI bone^BSI. ΔBSI and Δlesion number were taken as the difference between two consecutive scans. The difference in ΔBSI and Δlesion number was evaluated by the Mann Whitney U-test. Survival differences according to response as assessed by the three methods were estimated by the Kaplan-Meier method. Regression analysis was done for survival with ΔBSI and Δlesion number.

Results: The mean BSI and lesion number at initial diagnosis of bone metastasis with a previously negative scan was 0.33 and 4, respectively. Four of the 17 cases with indeterminate visual response were true positive on follow-up, and had a significantly higher mean ΔBSI (p=0.030) and Δlesion number (p=0.004) than the false positive. For patients progressing by visual analysis, the mean ΔBSI for was significantly higher than for stable disease (see table). In all three methods of response assessment, Kaplan-Meier curves showed shorter survival for progression than stable disease, with the greatest difference being by ΔBSI (p=0.022). Regression modeling with ΔBSI for survival was also statistically significant (p=0.029).

Conclusion: BSI is a promising biomarker for disease progression or treatment response in prostate cancer patients with bone metastasis. Research Support: $$graphic_4F9AF000-C57D-476A-BC2E-18F84E79896C$$