(ChemotherapyAdvisor) – Rates of ovarian cancer among survivors of breast cancer and the general population “have declined in sync since the late 1970s,” perhaps due to common risk factors such as shared genetic and hormonal exposures, an epidemiological study concluded in the Journal of Clinical Oncology, published online ahead of print on January 2, 2013.

That’s the good news, reported William F. Anderson, MD, of the National Cancer Institute, Rockville, MD, and colleagues. “However, we also demonstrate that there is greater risk for ovarian cancer after estrogen receptor (ER)-negative than ER-positive breast cancer from 1990 forward,” he added.

“The distinct age-specific patterns by ER expression likely reflect differences in the risk factor profiles for second primary ovarian cancers in these two distinct types of breast cancer.”

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The investigators examined second primary ovarian cancers after first primary breast cancer using 1973 to 2008 data from the Surveillance Epidemiology and End Results (SEER) program. “Standardized incidence ratios (SIRs) were calculated as the observed numbers of ovarian cancers among survivors of breast cancer compared with the expected numbers in the general population,” they wrote. Incidence rates for second primary ovarian cancer by year of diagnosis of the first primary breast cancer adjusted for age of breast cancer diagnosis and years since diagnosis comprised the absolute rates.

Between 1973 and 2007, a total of 1,543 second primary ovarian cancers were diagnosed after a first primary breast cancer and were observed through 2008. They found SIRs for second primary ovarian cancer to be elevated over the entire study period (SIR, 1.24; 95% CI: 1.2 to 1.3). Absolute rates declined, with an estimated annual percentage change near 1% (−1.34% to −0.09% per year).

Although secular trends for second ovarian cancers were similar after ER-positive and ER-negative breast cancers, age-specific patterns were found to vary significantly by ER expression (P for interaction <0.001). Women younger than 50 years of age with ER-negative breast cancer had the largest SIR, at 4.35 (95% CI: 3.5–5.4).

“To our knowledge, this is the first study to examine both the SIRs and absolute rates for second primary ovarian cancers among survivors of breast cancer overall and stratified by ER expression,” Dr. Anderson reported. “This unique study was feasible because SEER is one of the few large-scale and population-based cancer registries that records both multiple primary cancers and ER expression for breast cancer with meticulous data collection and standards.”

To investigate these descriptive results, analytic studies with treatment information, hormonal exposures, genetic risk factors, and ER data are needed, the authors concluded.