A meta-analysis has found an increased risk of breast cancer in women with schizophrenia, but its authors noted significant diversity of results across the included studies.

In the meta-analysis, Chuanjun Zhuo, MD, PhD, and Patrick Todd Triplett, MD, presented the results of 12 cohort studies involving 125,760 women that showed the risk of breast cancer in women with schizophrenia, compared with the general population.

They found that women with schizophrenia had a 31% higher standardized incidence ratio of breast cancer (95% confidence interval, 1.14-1.50; P less than .001). However, significant heterogeneity was found between studies, with the prediction interval ranging from 0.81 to 2.10. The report was published in JAMA Psychiatry .

“Accordingly, it is possible that a future study will show a decreased breast cancer risk in women with schizophrenia compared with the general population,” said Dr. Zhuo of Tianjin Medical University, China, and Dr. Triplett, of Johns Hopkins University, Baltimore.

As it turns out, one of the subgroup analyses showed that the association between schizophrenia and breast cancer was significant only in studies that excluded women who were diagnosed with breast cancer before they were diagnosed with schizophrenia (standardized incidence ratio, 1.34; 95% CI, 1.20-1.51; P less than .001).

The same was seen in studies where there were more than 100 cases of breast cancer (SIR, 1.31; 95% CI, 1.18-1.46; P less than .001), while the association was not significant in studies with fewer than 100 cases.

The authors said their findings contradict a hypothesis that schizophrenia might be protective against cancer.

“These results, together with our recent meta-analysis results showing no association with lung cancer risk but a reduced hepatic cancer risk in schizophrenia, indicated that the association between schizophrenia and cancer risk may be complicated and depend on the cancer site,” wrote Dr. Zhuo and Dr. Triplett.

In terms of possible mechanisms underlying the increased risk of breast cancer seen in this study, the authors suggested that people with schizophrenia could experience other clinical conditions such as obesity that might increase their risk of breast cancer.

“As breast cancer may be a hormone-dependent cancer, a significant positive association between plasma prolactin levels and the risk of breast cancer has been observed; in addition, increased prolactin levels have been documented in women with schizophrenia, particularly for those receiving certain antipsychotics,” they wrote.

While the incidence of cancer in people with schizophrenia might not necessarily differ from that of the general population, the authors said studies have found that people with schizophrenia have higher cancer mortality. Because “breast cancer prevention and treatment options are less optimal in women with schizophrenia, our results highlight that women with schizophrenia deserve focused care for breast cancer screening and treatment,” they wrote.

The Tianjin Health Bureau Foundation and the Natural Science Foundation of Tianjin, China, supported the study. No conflicts of interest were declared.

SOURCE: Zhuo C et al. JAMA Psychiatry. 2018 Mar 7. doi: 10.1001/jamapsychiatry.2017.4748 .