Most Common Site of Metastasis in Breast Cancer
Bone is unequivocally the most common site of metastasis in breast cancer, occurring in 51% of cases and affecting up to 70% of women with stage IV disease. 1, 2
Frequency of Metastatic Sites
The hierarchy of metastatic sites in breast cancer follows this pattern:
- Bone: 51% - the predominant site of distant spread 1
- Liver/soft tissue: 19% 2
- Pleura: 16% 2
- Lung: 14% 2
- Brain: 4% 2
Across all tumor types, patients with breast cancer have the highest incidence of skeletal complications compared to other malignancies. 3
Clinical Significance of Bone Metastases
Bone metastases carry substantial morbidity and mortality implications. The median survival for breast cancer patients with bone metastases is 16 months, but drops to only 7 months for patients who develop skeletal-related events (SREs). 3
Common Skeletal-Related Events
The five major objective complications include:
- Pathological fractures - the most common skeletal event 3
- Need for radiotherapy to bone 3
- Need for surgery to bone 3
- Spinal cord compression 3
- Hypercalcemia of malignancy 3
In the absence of bone-targeted treatments, the mean skeletal morbidity rate in breast cancer patients with bone metastases ranges between 2.2 and 4.0 SREs per year. 3
Variation by Molecular Subtype
The propensity for bone metastasis varies significantly by breast cancer subtype:
Luminal (ER/PR positive) cancers: Predominantly metastasize to bone, with 82% of patients who develop bone metastases having ER and/or PR positivity in the primary tumor 1, 2
HER2-enriched cancers: Preferentially spread to liver and lung rather than bone 1, 2
Triple-negative/basal-like cancers: Commonly metastasize to liver and brain, showing no predilection for bone metastases unlike other subtypes 1, 2
Critical Clinical Pitfall
Up to 13.6% of women diagnosed with early-stage breast cancer will develop bone metastasis within 15 years of diagnosis, even if the primary tumor is low grade. 2 This underscores that bone remains the primary metastatic site regardless of initial staging, making it the most important site to monitor during long-term follow-up.
The axial skeleton represents the most common anatomic location within bone for metastatic deposits. 3