Missing data on body mass index in a breast cancer register: how is it associated with patient characteristics and clinical outcomes?

AIM: To assess the completeness of data on body mass index (BMI) in a regional breast cancer register, and its association with patient characteristics and clinical outcomes. METHODS: This analysis used the data from the Waikato Breast Cancer Register and involved all women who were diagnosed with primary breast cancer in the Waikato District Health Board Region between January 2000 and June 2014. Patients with recorded BMI were compared with those with missing data in terms of demographics, disease factors and treatment factors. Cox regression modelling was performed, and hazards of specific outcomes associated with missing data on BMI were assessed. RESULTS: Of the 3,536 patients included in this analysis, 27.4% had missing data on BMI. Missing data was more frequent in older patients, rural dwellers, patients with comorbidities, screen detected patients, patients with early stage or low grade cancer and hormone receptor positive patients, but was minimal in patients who received chemotherapy. Patients with missing data were less likely to experience loco-regional recurrence (although not significant), metastasis and breast cancer specific mortality, but more likely to experience death from other causes even after demographic, disease and treatment factors were adjusted. CONCLUSIONS: Height or weight or both were not recorded for more than one quarter of the patients. Missing data was differential by specific patient characteristics and clinical outcomes.