However, the study's lead author, Dr Östen Helgesson, warned that his findings should be treated with caution as the design of the study meant that it was not possible to assess how much stress was needed to increase the risk of breast cancer. Furthermore, the questionnaire used in the study had not been finally validated as an accurate way of recording stress and relating it to the chances of developing breast cancer.
Although the association between stress and breast cancer has been investigated, there have been few studies that have examined the link prospectively, and these have produced contradictory findings. Dr Helgesson, a general practitioner from Gothenburg, Sweden, and a doctoral student at the Sahlgrenska Academy, believes his study is another stepping stone along the way to discovering whether stress can be used as a predictor of subsequent breast cancer.
He said: "This study is prospective and is based on information that is unbiased with respect to knowledge of disease. Therefore it can be regarded as more valid than results drawn from case-control studies. It showed a statistically significant, positive relationship between stress and breast cancer, and this association was independent of potential confounding factors, such as alcohol, body mass index, education, maternal history of breast cancer, smoking, age at which the women started their periods, age at which they had their first baby, the interval between the two, and their age at menopause."
Dr Helgesson and his colleagues from the Sahlgrenska Academy followed 1,462 Swedish women, aged between 38 and 60, for 24 years. The women had a physical examination and answered a simple questionnaire in 1968/91. They were asked by a physician whether they had had a feeling of stress for a month or longer, including tension, fear, anxiety, or sleep disturbances connected with conflicts in the family or at work. There were six degrees of stress varying from "no stress experience at all", up to "experienced mental stress constantly during the last five years".
The women had follow-up examinations in 1974/5, 1980/81 and 1992/93, but they were not asked about stress again, and therefore the study relates to stress experienced over a particular five-year period as reported at one point in time i.e. 1968/69.
The researchers found that women who said they had experienced stress during the five years before the first examination had double the risk of developing breast cancer during the following 24-year period, compared to women who had reported no stress. Out of 1,350 women for whom there were complete data, 24 who had experienced stress went on to develop breast cancer and 432 women who had experienced stress did not develop breast cancer. Amongst the women who had not experienced stress, 23 developed breast cancer and 871 did not.2
The mean average age at which the women were diagnosed with breast cancer was 60.25 which is in line with statistics from Sweden's National Board of Health and Welfare that show the highest incidence of breast cancer occurs in the group of women aged between 60 and 69 (23.6% of total breast cancer incidence as measured in 1992), while other sources indicate a median age of 65 at which breast cancer is diagnosed. Therefore, the study did not show that women who suffered stress developed breast cancer at an earlier age than other women.
Dr Helgesson said: "Although our study does show a significant association between stress and breast cancer, I would emphasise that more research needs to be carried out before it can be said that stress definitely increases a woman's risk of breast cancer. Ours is one of only a very small number of prospective studies, and although our findings are significant, more and larger prospective studies need to be done. Our work takes us one step forward to discovering whether stress really is a predictor of subsequent breast cancer."