What is the adherence to an exercise intervention during (neo-)adjuvant chemotherapy among Swedish patients with breast cancer? Data from the Phys-Can randomised controlled trial

OBJECTIVES: While exercise adherence is known to vary during cancer treatment, little is known about what predicts these changes during chemotherapy or within individual treatment cycles for breast cancer. We examined changes in adherence and its predictors (1) across chemotherapy treatment and (2) within treatment cycles in women undergoing (neo-)adjuvant chemotherapy for breast cancer. DESIGN: This study is based on data from the Phys-Can multicentre parallel randomised trial. SETTING: The exercise intervention was conducted at public gyms in three Swedish university cities. PARTICIPANTS: 178 women undergoing (neo-)adjuvant chemotherapy without any chemotherapy treatment delays and had any adherence data were included in the analysis. INTERVENTIONS: Participants in the Phys-Can trial were randomised to either high or low-to-moderate intensity combined endurance and resistance training. PRIMARY OUTCOME: The primary outcome variable for this secondary analysis of Phys-Can trial data was adherence to endurance and resistance training. Bayesian multilevel growth curve models were used to examine adherence to resistance and endurance training throughout the chemotherapy treatment period and within chemotherapy cycles. Potential predictors of adherence included exercise intensity, chemotherapy dose, muscle strength, body mass index, cardiorespiratory fitness, fatigue and age. Results are reported with 95% credibility intervals (CrIs). RESULTS: =-1.2, 95% CrI -2.2, 0.2). No significant predictors were found for adherence to resistance training over time. CONCLUSIONS: Women with breast cancer may require additional support to maintain exercise adherence during the later stages of chemotherapy and during the second and third weeks of their chemotherapy cycles. Those with lower pretreatment fitness levels may benefit from more intensive support to sustain engagement in exercise.The Phys-Can trial was registered in Clinical trials: ClinicalTrials.gov NCT02473003.