Incidence of bone metastases and skeletal-related events in breast cancer patients: A population-based cohort study in Denmark

Background: Breast cancer (BrCa) is the most commonly diagnosed cancer among women in the industrialized world. More than half of women presenting with metastatic BrCa develop bone metastases. Bone metastases increase the risk of skeletal-related events (SREs), defined as pathological fractures, spinal cord compression, bone pain requiring palliative radiotherapy, and orthopaedic surgery. Both bone metastases and SREs are associated with unfavorable prognosis and greatly affect quality of life. Few epidemiological data exist on SREs after primary diagnosis of BrCa and subsequent bone metastasis. We therefore estimated the incidence of bone metastases and SREs in newly-diagnosed BrCa patients in Denmark from 1999 through 2007. Methods: We estimated the overall and annual incidence of bone metastases and SREs in newly-diagnosed breast cancer patients in Denmark from January 1, 1999 to December 31, 2007 using the Danish National Patient Registry (DNPR), which covers all Danish hospitals. We estimated the cumulative incidence of bone metastases and SREs and associated 95% confidence intervals (CI) using the Kaplan-Meier method. Results: Of the 35,912 BrCa patients, 178 (0.5%) presented with bone metastases at the time of primary breast cancer diagnosis, and of these, 77 (43.2%) developed an SRE during follow up. A total of 1,272 of 35,690 (3.6%) BrCa patients without bone metastases at diagnosis developed bone metastases during a median follow-up time of 3.4 years. Among these patients, 590 (46.4%) subsequently developed an SRE during a median follow-up time of 0.7 years. Incidence rates of bone metastases were highest the first year after the primary BrCa diagnosis, particularly among patients with advanced BrCa at diagnosis. Similarly, incidence rates of a first SRE was highest the first year after first diagnosis of a bone metastasis. Conclusions: The high incidence of SREs following the first year after first diagnosis of a bone metastasis underscores the need for early BrCa detection and research on effective treatments to delay the onset of SREs.