Examining Mediators and Moderators of Yoga for Women With Breast Cancer Undergoing Radiotherapy

Abstract

Hypothesis This study examines moderators and mediators of a yoga intervention targeting quality-of-life (QOL) outcomes in women with breast cancer receiving radiotherapy.Methods Women undergoing 6 weeks of radiotherapy were randomized to a yoga (YG; n = 53) or stretching (ST; n = 56) intervention or a waitlist control group (WL; n = 54). Depressive symptoms and sleep disturbances were measured at baseline. Mediator (posttraumatic stress symptoms, benefit finding, and cortisol slope) and outcome (36-item Short Form [SF]-36 mental and physical component scales [MCS and PCS]) variables were assessed at baseline, end-of-treatment, and 1-, 3-, and 6-months posttreatment. Results Baseline depressive symptoms (P = .03) and sleep disturbances (P < .01) moderated the Group × Time effect on MCS, but not PCS. Women with high baseline depressive symptoms in YG reported marginally higher 3-month MCS than their counterparts in WL (P = .11). Women with high baseline sleep disturbances in YG reported higher 3-months MCS than their counterparts in WL (P < .01) and higher 6-month MCS than their counterparts in ST (P = .01). YG led to greater benefit finding than ST and WL across the follow-up (P = .01). Three-month benefit finding partially mediated the effect of YG on 6-month PCS. Posttraumatic stress symptoms and cortisol slope did not mediate treatment effect on QOL. Conclusion Yoga may provide the greatest mental-health-related QOL benefits for those experiencing pre-radiotherapy sleep disturbance and depressive symptoms. Yoga may improve physical-health-related QOL by increasing ability to find benefit in the cancer experience.

Keywords: QOL; breast cancer; mediation; moderation; yoga.

Figure 1.

The least-squared means of mental health–related QOL (SF-36 MCS) are from a multilevel modeling analyses controlling for baseline MCS score, baseline SF-36 general health subscale, and randomization factors. Figures illustrate a Group × Time interaction for those with (A) high and low baseline depressive symptoms (mean ± ½SD) on the Center for Epidemiologic Studies (CES-D) and (B) high and low baseline sleep disturbances (mean ± ½SD) on the Pittsburgh Sleep Quality Index (PSQI). Higher SF-36 MCS scores represent greater QOL. Abbreviations: CES-D, Centers for Epidemiological Studies–Depression; QOL, quality of life; SF-36, 36-item Short Form; MCS, mental component scale; YG, yoga group; ST, stretching control group; WL, or waitlist control group; XRT, radiotherapy treatment; PSQI, Pittsburgh Sleep Quality Index.

Figure 2.

Group differences on posttraumatic response across time: these figures represent the least-squared means (adjusted for the baseline level of the outcome variable, baseline SF-36 general health subscale, and randomization factors) of a multilevel modeling analysis for each of the mediators over time. Abbreviations: SF-36, 36-item Short Form; IES, Impact of Event Scale; XRT, radiotherapy treatment.

Figure 3.

Yoga indirectly affects physical health–related QOL (SF-36 PCS) at the 6-month follow-up via increased benefit finding (BFS) at 3 months. Values on each path are unstandardized path coefficients taken from bootstrapping analyses controlling for age, stage of disease, time since diagnosis, type of surgery, chemotherapy type, and baseline benefit finding (BFS), physical health–related QOL (SF-36 PCS), and the SF-36 general health subscale. The a₁ and a₂ paths correspond to the mean differences in 3-month BFS between YG relative to WL and ST relative to WL, respectively. Thus, YG resulted in 3-month BFS scores that were a₁ = 5.55 units higher than WL (P = .04), and ST resulted in BFS that were a₂ = 0.54 points higher than WL (P = .84). The b pathway corresponds to the relation between 3-month BFS and 6-month SF-36 PCS when group is held constant. Thus, for every 1 point increase in 3-month BFS, individuals reported an average b = 0.14-point increase in 6-month SF-36 PCS (P = .04). The relative indirect effects of group can be determined by multiplying the a and b paths. Thus, relative to WL, YG resulted in SF-36 PCS scores that were a₁b = 0.77 units higher as a result of the positive effect of YG on BFS, which corresponds to a significant indirect effect of YG versus WL on 6-month SF-36 PCS via 3-month BFS (B = 0.77; SE = 0.58; 95% bias-corrected and accelerated [BCa] CI = 0.01 and 2.58). Conversely, there was no significant indirect effect of ST versus WL on 6-month SF-36 PCS via 3-month BFS (B = 0.07; SE = 0.43; BCa CI = −0.84 and 0.88). Abbreviations: QOL, quality of life; SF-36, 36-item Short Form; PCS, physical component scale; BFS, Benefit Finding Scale; YG, yoga group; ST, stretching control group; WL, or waitlist control group. *P < .05.