Body Composition Analysis in Postoperative Breast Cancer Patients Undergoing Chemotherapy and Its Association with Physical Activity and Quality of Life: A Longitudinal Pilot Study

<b>Background/Objectives</b>: Breast cancer survivors often experience adverse body composition changes and reduced quality of life (QoL) after chemotherapy. This study aimed to assess changes in body composition in postoperative breast cancer patients undergoing chemotherapy and to examine their associations with physical activity and QoL. <b>Methods</b>: This longitudinal observational pilot study included two repeated assessments (after surgery and before the third chemotherapy cycle - six weeks period). Sixty women (mean age 57 ± 10 years) who had undergone breast cancer surgery and were scheduled for chemotherapy were assessed twice: after surgery and prior to the third chemotherapy cycle. Body composition was analyzed using anthropometric and bioelectrical impedance methods. Physical activity was evaluated with the International Physical Activity Questionnaire-Long Form (IPAQ-L), while QoL was measured with the World Health Organization Quality of Life-bref version (WHOQOL-BREF) questionnaire as well. <b>Results</b>: During chemotherapy, participants showed significant increases in body weight (<i>p</i> = 0.001), BMI (<i>p</i> = 0.001), and muscle mass (<i>p</i> = 0.001), with stable fat percentage. Physical activity levels improved overall, particularly in moderate activity (<i>p</i> = 0.001), while sedentary time decreased (<i>p</i> = 0.020). QoL remained generally stable, with significant improvement in the environmental domain (<i>p</i> = 0.028). Higher fat percentage correlated negatively with physical (<i>p</i> = 0.040) and social (<i>p</i> = 0.049) QoL, while BMI correlated inversely with psychological well-being (<i>p</i> = 0.020). Waist-to-hip ratio was also negatively associated with psychological QoL (<i>p</i> = 0.017). <b>Conclusions</b>: Vigorous activity showed an association with more favorable body composition, whereas sedentary behavior correlated with higher BMI and muscle mass.