Levels and patterns of self-reported and objectively-measured free-living physical activity among prostate cancer survivors: a prospective cohort study

Background: No prior study has measured or compared self-reported and objectively-measured physical activity trajectories in prostate cancer survivors before and after treatment. Methods: Clinically-localized prostate cancer patients treated with radical prostatectomy were recruited from 2011-2014. Of the 350 participants enrolled at the main site, 310 provided self-reported physical activity at baseline before radical prostatectomy, and 5 weeks, 6 months, and 12 months post-radical prostatectomy. A subset of participants (n=81) provided objectively-measured physical activity at all study time points by wearing an accelerometer for seven days each. Changes in activity over time were compared using Friedman’s test. Agreement between self-reported and objective measures was evaluated using Spearman’s rank correlation coefficient. Results: Self-reported moderate-to-vigorous physical activity was high at baseline (median=32.1 minutes/day), followed by a decline at 5 weeks (15.0 minutes/day) and a recovery at 6 and 12 months (32.1-47.1 minutes/day). In contrast, objectively-measured moderate-to-vigorous physical activity was low at all four time points (median=0.0-5.2 minutes/day), with no overall change across study assessments (global p=0.29). Self-reported moderate-to-vigorous physical activity tended to be more closely related to objectively-measured light intensity physical activity (rho=0.29-0.42) than to objectively-measured moderate-to-vigorous physical activity (rho=0.07-0.27, p=0.009-0.32). Conclusions: In our population of prostate cancer survivors with critically low moderate-to-vigorous physical activity levels, self-reported measures greatly overestimated moderate-to-vigorous physical activity and may have been more reflective of light intensity physical activity. As cancer survivor guidelines were derived from self-reported data, our findings may imply that intensities of physical activity below moderate, such as light intensity, still have health benefits.