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Frequent aspirin use was associated with a decreased risk of ovarian cancer, even in the presence of risk factors, according to a recent meta-analysis.

Overall, frequent aspirin use was associated with a 13% reduction in ovarian cancer risk (relative risk [RR] 0.87, 95% CI 0.80-0.94) across 17 studies, with no difference between cohort and case-control study results (P=0.48), reported Lauren Hurwitz, PhD, MHS, of the National Cancer Institute, and colleagues.

Furthermore, risk reductions were especially notable for high-grade serous ovarian cancer (RR 0.86, 95% CI 0.78-0.94), they wrote in the .

This latter finding is important since "most established risk factors are more weakly associated with high-grade serous ovarian cancers," they noted.

"These results suggest that primary prevention of ovarian cancer is an added benefit of frequent aspirin use that could be incorporated into composite risk-benefit calculations," Hurwitz and colleagues concluded. "Given that women with elevated ovarian cancer risk because of epidemiologic risk factors also benefit and that the [number needed to treat] to prevent one ovarian cancer is lower for higher-risk women, future work should explore how chemoprevention programs with aspirin could complement existing preventive strategies, which are currently limited to women with the highest risk ... and target additional high-risk subgroups to maximize population-level impact and minimize risks."

When looking at associations within subgroups defined by risk factors for ovarian cancer, Hurwitz and team observed a possible effect modification by history of endometriosis.

Frequent aspirin use was associated with reduced ovarian cancer risk in women without endometriosis (RR 0.82, 95% CI 0.73-0.92), but not those with endometriosis (RR 1.15, 95% CI 0.80-1.65, P=0.08). This inverse association, the authors suggested, "was likely driven by the small number of women with endometriosis and the limited power to detect associations within this subgroup."

Otherwise, risk reductions associated with frequent aspirin use were consistent across subgroups defined by factors that either increase or decrease cancer risk.

For example, frequent aspirin use was associated with a lower ovarian cancer risk regardless of obesity, with the association slightly stronger among women with obesity (RR 0.79, 95% CI 0.67-0.93) versus those without it (RR 0.91, 95% CI 0.80-1.03, P=0.18), as well as among women with a family history of breast or ovarian cancer (RR 0.86, 95% CI 0.79-0.94) and without such a history (RR 0.88, 95% CI 0.72-1.06, P=0.88).

Consistent risk reductions were also seen in subgroups defined by protective factors for ovarian cancer, including parity (P=0.71), duration of oral contraceptive use (P=0.79), and tubal ligation (P=0.27).

Furthermore, a risk reduction was seen with frequent aspirin use among women with two or more ovarian cancer risk factors (RR 0.81, 95% CI 0.73-0.90).

This meta-analysis included nine cohort studies involving 491,651 women at risk for ovarian cancer. Mean age at baseline ranged from 46.0 to 68.2 years, mean follow-up ranged from 4.6 to 14.3 years, and the prevalence of frequent aspirin use ranged from 9.8% to 38%.

During follow-up, 2,600 women were diagnosed with incident ovarian cancer (56% high-grade serous, 2% low-grade serous, 9% endometrioid, 5% clear cell, 4% mucinous, and 23% other/unknown epithelial).

Across the included eight case-control studies, there were 5,726 cases of cancer (54% high-grade serous, 4% low-grade serous, 15% endometrioid, 9% clear cell, 5% mucinous, and 13% other/unknown epithelial) and 8,027 controls. Median age of the cases ranged from 56.2 to 60.7 years, and the prevalence of frequent aspirin use ranged from 5.6% to 29.8%.

Overall, frequent aspirin use was associated with a 10% reduction in ovarian cancer risk in the cohort studies (HR 0.90, 95% CI 0.81-1.01) and a 16% reduced risk in the case-control studies (OR 0.84, 95% CI 0.72-0.98).

Hurwitz and team noted that the use of observational data may have introduced some bias, and that they were unable to examine associations for low-dose aspirin, which has been more strongly associated with reduced risk for ovarian cancer in previous studies.

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