Obesity increases the incidence of distant metastases in oestrogen receptor-negative human epidermal growth factor receptor 2-positive breast cancer patients.

BACKGROUND: Obesity is a major negative determinant of breast cancer outcome. However, there are contrasting data on the differential impact of obesity on specific breast cancer subtypes. In particular, very little is known on human epidermal growth factor receptor 2-positive (HER2+) tumours. PATIENTS AND METHODS: We assessed the prognostic role of increased body mass index (BMI) on a consecutive series of non-metastatic HER2+ patients treated at our institution before the introduction of adjuvant Trastuzumab. We separately analysed oestrogen receptor-positive (ER+) and -negative (ER-) HER2+ cases.
RESULTS: In ER-/HER2+ tumours we observed a significantly worse overall survival (Hazard ratio (HR) 1.79, p-value 0.041) and cumulative incidence of distant metastases (HR 2.03, p-value 0.019) in obese (BMI>30) versus normal/underweight (BMI<25) patients. Local relapses appeared to be non-significantly reduced in obese patients, masking the overall effect on disease-free survival. Outcome in ER+ tumours, instead, was not significantly different between BMI groups.
CONCLUSIONS: Obesity significantly correlates with worse overall survival and cumulative incidence of distant metastases in ER-/HER2 positive breast cancer. Differences in the biology of breast tumours may determine individual susceptibility to obesity. The biology of the underlying tumour should be taken into account in the design of dietary intervention trials in breast cancer.