Background Colorectal cancer is the third leading cause of death from cancer worldwide. It has been suggested that obesity may be a promoting factor in the growth of colorectal carcinoma. Although adiposity has been a recognised risk factor its effect on treatment success and prevalence of treatment-associated toxicities remains unclear.

Purpose To investigate the difference in relative dose intensity and treatment-induced toxicity in patients of normal BMI compared to overweight patients.

Materials and methods A retrospective study of patients receiving FOLFOX 6 for colorectal cancer between January 2006 and March 2010 at St. James's hospital, Dublin.

Results Patients of normal BMI (18.5 kg/m² to 25 kg/m²) had higher dose intensity at treatment initiation but received a lower dose intensity for the remaining cycles compared to overweight patients (BMI >25 kg/m²). The average relative dose intensity of FOLFOX was 64.78% (normal BMI group) and 67.05% (overweight group). The incidence of fatigue was significantly higher in patients with a normal BMI, (p=0.016) but there was no significant difference in the rate of hospital admission due to FOLFOX toxicity.

Patients with a ‘National Cancer Institute’s Common Terminology Criteria for Adverse Events' (CTCAE) grade 3/4 toxicity had their dose reduced to prevent such severe toxicity reoccurring. CTCAE grade 3/4 was prevalent in 41% of overweight, and in 65% of normal weight patients. Subsequent dose reductions occurred in 53% of overweight and 65% of the normal weight patients.

Conclusions The overweight group experienced less severe toxicities than the normal BMI group indicating that they may be capable of tolerating doses based on actual body weight rather than capping the BSA which is common practice. The low % RDI (relative dose intensity) received by both study groups may highlight a need to gain better control of toxicities. Future studies should investigate the impact of pharmacist counselling on supportive medication on % RDI-related toxicities.