Question

BACKGROUND & METHODS

The relationship between stress and breast cancer has been examined in many epidemiological studies, the results of which have yielded mixed results. One reason for these differences is the use of different measures of stress. For example, death of a spouse or close relative is a major acute stressor, whereas stress experienced in daily life is more moderate and chronic in nature. As these different types of stress likely have different physiological and psychological impacts, the observed relationship with breast cancer risk may differ.

While the relationship between the acute stress of major life events and the risk of breast cancer has been assessed in several studies, less attention has been paid to perceived daily stress. A number of studies have shown that prolonged, low-level stress of daily life results in a persistent activation of stress hormones, which may impair estrogen synthesis, and may thereby be related to a lower risk of breast cancer. Finally, daily stress may also indirectly affect the risk of breast cancer through variations in health related behavior.

A study was launched in Copenhagen, Denmark to assess the relationship between self reported intensity and frequency of stress and first time incidence of primary breast cancer. The study recruited a population-based sample of n=7,018 Danish women. All women were asked about their perceived level of stress at baseline (2000-2001). Women were excluded if they had been diagnosed with breast cancer before the start of the study (n=120) or if they provided no information on stress or other variables (n=209). The remaining 6,689 women were followed until 2015, with n=26 (< 0.1%) lost to follow-up.

Women in the study were asked about their level of stress in terms of intensity and frequency. In the questionnaire, stress was defined as the sensation of tension, nervousness, impatience, anxiety, or sleeplessness. The women were asked to report their stress intensity as none (0), light (1), moderate (2) or high (3) and the frequency as never/hardly ever (0), monthly (1), weekly (2) or daily (3). The responses of the two questions were then added together to obtain a composite score ranging from 0 to 6. This stress score was also categorized as low (0-1), medium (2-4) and high stress (5-6). Participants were followed from the date of the baseline examination until the end of the study period in December 2015, during which time there were n=251 first diagnoses of primary breast cancer.

Q: What other explanations should be considered before concluding that stress is causally related to a decreased risk of breast cancer? How likely are these explanations?

Answer 1

Stress is casually related to a decreased risk of breast cancer

1. Before hypothesising this relationship the investigators have to study the extraneous variables which can affect stress among the women such as age, occupation, education, marital status, contraceptives , no of children, smoking, alcoholism, substance abuse. Extraneous variables that could threaten the internal validity of the results. Internal validity is the extent to which a study establishes a trustworthy cause-and-effect ( here level of stress and primary breast cancer). So we have to avoid the confounders.

2. The relationship should be statistically tested and this explanation as also needed

A) Explanation on Correlation analysis

Risk association can be revealed that is degree and direction of relationship between level of stress and primary breast cancer revealed.

Pearson's correlation coefficient gives information about the magnitude of the association, or correlation as well as the direction of the relationship.

The value of r always lies between -1 and +1, +1 indicates perfect positive correlation and -1 indicates perfect negative correlation.

Positive correlation incidence of primary breast cancer increases with increase in level of stress.

3. Relative risk also need to be explained as it is a prospective cohort study

Relative risk is the direct measure of strength of association between suspected cause and effect.

Relative risk greater than 1 suggest positive relationship between cause and effect

It is calculated by divide incidence of primary breast cancer among those who have dress by incidence of primary lung cancer with those who are not having stress( Incidence among exposed ÷ Incidence among non exposed)