Associations of reproductive factors with ovarian cancer may differ by histologic type. Data from a multiethnic, population-based, case-control study, conducted in Hawaii and Los Angeles, California, between 1993 and 1999, were used to assess this hypothesis. A structured questionnaire was administered to 558 histologically confirmed epithelial ovarian cancer cases and 607 population controls. Factors suppressing ovulation, including pregnancy and oral contraceptive use, were inversely associated with the risk of all histologic types. Nonmucinous but not mucinous tumors were significantly associated with menstruation years (odds ratio = 1.5 for the highest vs. the lowest quartile) and lifetime ovulatory cycles (odds ratio = 2.8 for the highest vs. the lowest quartile). Duration of breastfeeding (odds ratio = 0.4 for the highest vs. the lowest quartile) was significantly and inversely related to nonmucinous tumors but not to mucinous tumors. Among all tumor types, endometrioid tumors were the most strongly related to pregnancy and tubal ligation, while clear cell tumors were the only type that was associated with noncontraceptive hormone use. The risk factors were similar for borderline and invasive tumors, except for age at diagnosis. Mucinous tumors, both borderline and invasive, were more common in Asian women than in Caucasian and other women. Our data suggest that histologic types of epithelial ovarian cancer are etiologically distinct.

Epidemiologic findings have been inconsistent regarding the association of dietary fat, dairy products, and lactose with risk of ovarian cancer. The authors conducted a case-control study in Hawaii and Los Angeles, California, to examine several dietary hypotheses regarding the etiology of ovarian cancer in a population with a broad range of dietary intakes. A total of 558 patients with ovarian cancer diagnosed in 1993-1999 and 607 controls were interviewed regarding their diet. Consumption of all dairy products, all types of milk, and low-fat milk, but not consumption of whole milk, was significantly inversely related to the odds of ovarian cancer. Similar inverse gradients in the odds ratios were obtained for intakes of lactose and calcium, although these nutrients were highly correlated (r = 0.77). The odds ratio for ovarian cancer was 0.46 (95% confidence interval: 0.27, 0.76) among women in the highest quartile of dietary calcium intake versus the lowest (p for trend = 0.0006). The significant dietary association was limited to dairy sources of calcium (p for trend = 0.003), although a nonsignificant inverse gradient in risk was also found in relation to calcium supplement intake. These results suggest that intake of low-fat milk, calcium, or lactose may reduce the risk of ovarian cancer.