INTRODUCTION: An optimal relative dose intensity (RDI) of adjuvant chemotherapy is associated with better survival in patients with breast cancer. Little is known about the role of physical fitness in attaining an adequate RDI in patients with early stage breast cancer. We investigated the association between pre-treatment physical fitness and RDI in this population.

METHODS: We pooled individual patient data from two randomized exercise trials that studied exercise programs in early breast cancer: the PACES (n = 230) and the PACT (N = 204) study. Logistic regression models were used to evaluate the association between pre-treatment fitness and achieving an optimal RDI (≥85%). In addition, we added an interaction term to the model to explore the potential moderating effect of participating in an exercise program.

RESULTS: Data were available for 419 patients (mean age at diagnosis 50.0 ± 8.6 years). In the total sample, lower pre-treatment physical fitness was associated with significantly lower odds of achieving ≥85% RDI: age-adjusted OR 0.66 [95%CI 0.46-0.94]. In patients allocated to the supervised exercise intervention during chemotherapy (n = 173), the association between pretreatment physical fitness and RDI was almost completely mitigated (OR 0.95 (95%CI 0.54-1.56)), while it was more pronounced in patients who received care as usual (n = 172, OR 0.31 (95%CI 0.13-0.63) pinteraction: 0.022).

CONCLUSION: Early stage breast cancer patients with relatively lower levels of pretreatment physical fitness have lower odds of achieving an optimal dose of chemotherapy. Given that physical fitness is modifiable and our results suggest that following a moderate-to-high intensity exercise training during chemotherapy could improve treatment completion, clinicians should not refrain from referring patients to supportive exercise programs because of low fitness.