Prevalence and risk factors of bone metastasis and skeletal related events in patients with primary breast cancer in Japan

Bone metastasis (BM) is important for studying systemic spread of breast cancer. It often causes skeletal-related events (SREs) that worsen quality of life. We investigated the prevalence and risk factors for BM and SRE using a dataset from the Breast Oncology Research Network (BORN) in Japan. We collected data on primary breast cancer patients with node-positive or node-negative disease at intermediate to high risk of recurrence. The risk factors affecting the BM-free rate, SRE-free rate and overall survival were analyzed by using the Cox proportional hazard model. Data of 1,779 patients who were diagnosed with breast cancer during 2003-2005 were collected from the BORN and 1,708 cases were used for analysis. The median follow-up duration was 5.71 years. BM developed in 193 cases (11.3 %) and the BM-free rate at 5 years was 89.2 %. The annual hazard ratio of BM development differs remarkably according to the tumor subtype. SREs occurred in 133 (68.9 %) out of 193 patients and the SRE-free rate at 5 years was 92.6 %. In the multivariate analysis, clinical stage (P < 0.0001), number of lymph node (LN) metastases (P = 0.0029), tumor subtype (P = 0.034) and progesterone receptor status (P = 0.038) were independently significant risk factors for BM-free rate, but only clinical stage (P < 0.0001) and number of LN metastases (P = 0.0004) significantly correlated with SRE-free rate. This retrospective study clarifies the prevalence and risk factors for BM and SRE in Japanese breast cancer patients. Our results show the importance of considering subtype in the care of BM and SRE.