In this large cohort of breast cancer survivors, we found some evidence for an association between social connectedness and overall mortality. Whereas participation in community and religious activities was significantly associated with lower overall mortality, such interactions had no material influence on breast cancer specific mortality. The number of close friends, relatives and living children, the frequency of contacts with significant others, and marital status had no significant impact on subsequent breast cancer-specific or overall survival rates.
The current findings are consistent with some, but not all, previous studies that addressed the impact of social networks on breast cancer outcome, as reviewed by Falagas et al
]. Of nine published studies, four reported a survival advantage associated with one or more index of social networks [4
] whereas five reported no association [8
]. All of the null studies had limited power to detect associations (<250 participants). A recent meta-analysis combining data from 87 studies of social support, social networks, and marital status and cancer outcomes reported stronger inverse associations with cancer mortality among breast cancer survivors compared with other cancer sites [2
]. Possible mechanisms suggested included that those with social networks would be more inclined to seek and/or follow through with treatment. Social networks may also favorably change hormone concentrations, thereby preferentially acting on hormonal cancers [2
]. An alternative explanation is that more studies have been conducted in breast cancer relative to other sites, thereby providing sufficient power to detect weak associations between social networks and survival relative to other sites.
An analysis of Nurses’ Health Study (NHS) data by Kroenke et al
] also based on the Berkman-Syme Index found that the number of close friends and relatives as reported prior to breast cancer diagnosis was associated with improved survival subsequent to breast cancer diagnosis even after adjusting for breast cancer stage at diagnosis. In contrast to present findings, participation in religious and community activities had no important influence on breast cancer or overall survival. Differences in results in the NHS and current study could be related to differences in the age (NHS women were about 10 years older) or economic levels (the NHS was comprised of nurses who were presumably more educated) between the study populations, and the timing of collection of social network data with respect to the breast cancer diagnosis (data collected two years post diagnosis in the NHS versus six years for the current study). For example, having larger numbers of friends and close relatives could be associated with better treatment compliance, accounting for the inverse association with number of friends and relatives noted in the NHS. In contrast, numbers of friends and relatives may be less important after treatment is completed. As with our results, Kroenke et al
reported religious and community participation was significantly associated with improved all-cause survival after diagnosis.
Randomized controlled trials assessing the impact of improved social support on breast cancer survival have produced mixed results. An initial randomized controlled trial that enrolled 86 women with metastatic breast cancer reported a mean doubling in survival time after weekly group therapy for one year [13
]. However, subsequent trials in similar populations have suggested that while group therapy may improve quality of life, it does not prolong survival [14
One possible explanation for the mixed findings from previous studies is measurement error in the assessment of social networks. Previous research, including the current study, ranked women on the number of close relationships and marital status. A recent study found that the quality, rather than quantity, of close relationships was associated with improved survival among ninety breast cancer survivors; a composite measure of marital confiding and close relationships had a strong inverse effect (RR= 0.41 (95% CI 0.21–0.80)) [6
]. In measuring the quality of social networks, it may be important to account for specific informational, instrumental, and emotional support being provided by members of the network [29
Taken together with previous studies, the available evidence suggests that social networks, as measured by the number of close relationships, may have an influence on survival after a breast cancer diagnosis. After accounting for confounders, women in the highest category of social connectedness had a 28% (95% CI, -4%-51%, p-trend 0.02) lower risk of overall mortality compared to those in the lowest category of social connectedness. There was an observed benefit for increased community and religious participation, a finding that may reflect better health and extended longevity of women able to engage in activities outside the home (e.g. reverse-causation). Moreover, the suggestions of higher death rates observed in women that had regular visits from larger numbers of their children (p=0.05), which may be surrogate for declining health or physical incapacity, is also consistent with a confounding influence of general health on the results, despite adjustment for smoking, BMI, and other predictors of mortality.
The reduced risk of overall mortality with increasing levels of social connectedness was likely driven by cardiovascular disease-specific mortality. In analyses restricted to mortality from cardiovascular disease in this cohort (n=123), there was a suggestion of reduced risk of cardiovascular disease-specific mortality among those in the highest category of overall social connectedness (scores=21–26) compared to those with low scores (5–12) (Adjusted HR=0.41, 95% CI 0.16 to 1.04, p-trend 0.01). This is consistent with recent reports that increasing levels of social engagement confer protection against stroke in women [37
] and cardiovascular mortality in men [38
The present analysis had some limitations. Not all eligible women responded to the study invitation, and respondents were generally more highly educated and demonstrated a healthier profile; i.e. fewer current smokers, greater proportion of BMI values in the normal range than those who did not participate [39
]. Thus, although this is the only study to target a population-wide cohort of breast cancer survivors, results may not be applicable to all breast cancer survivors. In addition, women were enrolled several years after breast cancer diagnosis and those with more rapidly fatal, aggressive breast cancers would have been underrepresented in the study cohort. Finally, few women reported having no friends or close relatives, and the majority were married. This may have reduced power to detect associations at extreme levels of social isolation.
Identification of factors that improve the health and longevity of breast cancer survivors is an important public health challenge. Our study supports the hypothesis that social networks reduce the likelihood of death from any cause among breast cancer survivors…