Variability in physical function for patients living with breast cancer during a 12-week exercise program.

To describe the variability during weekly performance on physical function tests during a 12-week individualized exercise program for patients with breast cancer and to test if the expected improvements in physical function surpass the minimally clinically important difference (MCID), after accounting for week-to-week variance. Twenty-five participants, 19 years and older living with breast cancer within 2 years of their initial diagnosis, were recruited. Some participants were undergoing active treatment, while others completed their treatment. The intervention was an individualized exercise session twice a week, for 1 h each session, for a total of 12 weeks. Main outcomes tested included the 6-min walk test and chair stand test. A significant average improvement was observed in the 6MWT (p < .01) and the chair stand test (p < .001) following the intervention. Individual confidence intervals were wide across all testing measures with only 28% and 8% of participants meeting or surpassing the MCID for the 6MWT and chair stand test, respectively. Despite a significant improvement in physical function during the program, the majority of patients did not reach the MCID, which could be due to large variability resulting from treatment-related side effects or measurement error.