Introduction: Aromatase enzyme activity is predominant in adipose tissue. This has led to speculation that aromatase activity is elevated in obese women and subsequently decreased the clinical activity of adjuvant aromatase inhibitors (AIs) in women with estrogen receptor positive (ER ) breast cancer (BC). We investigated the effect of obesity on the outcome of this population.

Patients and methods: Records of 320 consecutive post-menopausal (PM) women with ER BC starting single agent adjuvant letrozole between years 2005 and 2014 were retrospectively reviewed. Tumour and patients characteristic including body mass index (BMI) on the day of starting letrozole were extracted. Endpoints of main interest were: (1) Frequency of obesity; (2) relapse-free survival (RFS) in nonobese (G1; BMI < 30) and obese (G2; BMI ≥ 30) patients.

Results: Obesity (BMI: 30–34.99) and morbid obesity (BMI ≥ 35) were present in 105/320 (32.8%) and 115/320 (35.9%) women, respectively. Median follow-up of patients was 49 months; RFS at 5 years (G1: 69% versus G2: 78%) and at 8 years (G1: 69% versus G2: 71%). Median RFS is not reached in both groups (Log rank; P = 0.097). There was no correlation between BMI and RFS (correlation coefficient r = 0.075; P = 0.174).

Conclusion: In this cohort, more than two-thirds of PM women starting adjuvant AIs are obese. Obesity did not adversely affect the outcome of women on adjuvant letrozole.