Bone Scan Index predicts skeletal-related events in patients with metastatic breast cancer

BACKGROUND Bone Scan Index (BSI) expresses tumor burden in bone as a percentage of total skeletal mass, but its significance for metastatic breast cancer patients is unknown. We investigated whether baseline BSI is associated with skeletal-related events (SREs) or survival and identified the cut-off BSI score for predicting SREs in metastatic breast cancer patients. METHODS We retrospectively reviewed 144 patients with bone metastatic breast cancer. Bone scan examinations were performed and BSI was calculated using the Bonenavi(®) automated method. All patients received standard medical treatment for metastatic breast cancer. For bone metastasis prophylaxis, bisphosphonates were infused initially with analgesics as needed. We defined SRE as either bony, requiring intervention (surgery and/or radiotherapy) for pain or prevention of fracture, or spinal cord compression. The rates of SRE and overall survival (OS) were evaluated according to baseline BSI, and the cut-off score of BSI for predicting SRE in metastatic breast cancer patients was identified. RESULTS Thirty-three patients (25.6 %) had SREs. The median BSI was 1.08 % (inter-quartile range 0.50-3.23 %). To identify the cut-off BSI score for predicting SRE, we performed sensitivity analysis to check P-value at every 0.1 BSI interval (0.4-2.4) by multiple-variable proportional hazard analysis. A BSI cut-off point of 1.4 % showed the lowest P value. Patients with BSI scores ≥1.4 had a significantly higher rate of SRE than those with lower BSI (P = 0.022). However there was no significant difference in OS. CONCLUSION BSI may predict SRE in patients with metastatic breast cancer. A high BSI value (≥1.4) at diagnosis of bone metastasis may be a predictor of SREs in bone metastatic breast cancer patients.