Association between Daily Step Counts and Frailty Trajectory Classes in Lung Cancer Patients Undergoing Radiotherapy: A Prospective Longitudinal Study

Purpose To characterize frailty-trajectory categories in patients receiving radiotherapy for lung cancer, identify factors associated with trajectory membership, and examine the association between daily step counts during radiotherapy and frailty trajectories. Methods In this prospective longitudinal study, daily step counts were collected using the WeChat Step Mini Program from the start to the end of radiotherapy. Frailty was assessed using the Frailty Phenotype Scale at 3 time points: before radiotherapy, at the end of radiotherapy, and 1 month after radiotherapy. A latent class growth model (LCGM) was used to identify distinct frailty-trajectory classes. Multinomial logistic regression was performed to examine factors associated with class membership and to evaluate the association between step counts and frailty-trajectory class. Results Among 327 patients, LCGM identified 3 frailty trajectories: prefrailty declining (56.58%), frailty stable increasing (27.83%), and severe frailty increasing (15.60%). Factors associated with frailty-trajectory membership included older age, poorer nutritional status, more advanced disease stage, poorer sleep status, and oral intake impairment. Lower daily step counts during radiotherapy were associated with membership in trajectories characterized by persistent frailty worsening. Conclusions Frailty trajectories from before radiotherapy to 1 month after radiotherapy were heterogeneous. Step-count gradients were observed around 3000, 6000, and 9000 steps/day, suggesting exploratory cut points corresponding to different frailty-risk intervals and providing a quantitative reference for early risk identification and individualized activity recommendations. Continuous step monitoring combined with routine frailty screening may help identify high-risk patients early and support multidimensional assessment and proactive management during radiotherapy care. Implications for Cancer Survivors: Daily step counts provide a scalable, real-world metric to support early risk stratification and guide targeted frailty screening and management during early survivorship after lung cancer radiotherapy.