Total adiposity measured by abdominal computed tomography (CT) at the third lumbar vertebrae (L3) has been associated with breast cancer survival, but most patients undergo chest CT. If adipose tissues at the thoracic level, including those surrounding the thoracic organs, are associated with survival, they could be used to inform care for significantly more breast cancer patients.

We included 2127 individuals aged 18-< 90, diagnosed with stage II-III breast cancer at Kaiser Permanente Northern California (2005–2019). Cross-sectional areas of adiposity were quantified at the fourth thoracic vertebrae (T4) and L3. Using multivariable Cox models, we estimated hazard ratios (HRs) and 95% confidence intervals to compare the strength of association of T4-level versus L3-level adiposity with all‐cause and cause-specific mortality.

Participants were on average 56.2 years old at diagnosis. During an average follow-up of 8.2 years, 593 deaths occurred, with 100 from heart disease and 429 from breast cancer. Positive, moderate-to-strong correlations were observed between adiposity at T4 and L3. After adjusting for covariates including body mass index and muscle, higher intrathoracic and intermuscular adiposity at T4 were associated with increased all-cause (intrathoracic: HR = 1.35[1.06–1.72]; intermuscular: HR = 1.26[1.01–1.59]) and heart disease-specific mortality (intrathoracic: HR = 2.23[1.31–3.78]; intermuscular: HR = 2.25[1.37–3.68]). Greater subcutaneous adiposity at T4 showed a non-significant trend toward increased mortality (overall: HR = 1.24[0.95–1.61]; breast cancer-specific: 1.27[0.93–1.73]). These associations were not observed at L3.

Despite strong correlations with L3, adiposity at T4 was significantly associated with overall and heart disease-specific mortality, while measurements at L3 were not, possibly due to the proximity of T4 to the breast tumor and heart.