Women with schizophrenia are 30 percent more likely to develop breast cancer, according to a new meta-analysis, but the risk varies significantly from one study to another.
The meta-analysis, “Association of Schizophrenia With the Risk of Breast Cancer Incidence,” was published in the journal JAMA Psychiatry.
Patients with schizophrenia tend to lead unhealthier lifestyles, which include smoking, alcohol and substance abuse, obesity, and lack of exercise, all of which are risk factors for cancer. However, whether schizophrenic patients have a higher risk of developing cancer is not fully known.
In addition to the effects on lifestyle, some researchers have also hypothesized that genetic factors regulating schizophrenia could also be involved in cancer. But the disorder has also been associated with a reduced risk of colorectal cancer, malignant melanoma, and prostate cancer.
The association between schizophrenia and breast cancer is uncertain. A previous meta-analysis showed an increased risk of breast cancer in schizophrenic patients compared with the general population. However, a subsequent study suggested this might not be the case. One of the issues with conducting a meta-analysis on this topic is the significant variation in results among studies, making it hard to reach a consensus.
This variation — termed heterogeneity — between studies is measured using a variable called the I2 statistic, which describes the percentage of variation across studies. It has recently been suggested that the I2 statistic is not the best way to measure heterogeneity, and another method called prediction interval (PI) should be used instead.
To investigate this, researchers conducted an updated meta-analysis to determine the association between schizophrenia and breast cancer risk, as well as report the I2 and PI.
They searched the literature for studies that reported the standardized incidence ratio (SIR) for the risk of breast cancer in women with schizophrenia versus the general population. The meta-analysis included 125,760 women from 12 cohorts.
Results showed that women with schizophrenia had a 1.31 times higher risk of developing breast cancer than the general population. However, there was significant heterogeneity among the studies, as demonstrated by both the I2 statistic (I2 = 89%) and the PI (0.81-2.10). The wide range of the PI indicates significant heterogeneity among the studies.
Therefore, while this particular meta-analysis shows that women with schizophrenia have a higher risk of developing breast cancer, the large heterogeneity points to the possibility that a future study could show less risk of breast cancer in women with schizophrenia than the general population.
Researchers suggest that one of the reasons behind this increased association is that many clinical conditions common in patients with schizophrenia, such as obesity, are risk factors for breast cancer. Women with the disorder also tend to have higher levels of a molecule called prolactin, which has been found to increase breast cancer risk.
“The results of this meta-analysis demonstrated that women with schizophrenia are at a higher risk for the incidence of breast cancer, compared with the general female population, although substantial between-study variance is present,” the investigators said.
Researchers add that intensive screening for breast cancer in women with schizophrenia is important.