Mathematical Models of Ovarian Cancer Incidence

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Abstract
Pike has proposed “protected time” as one summary method for modeling reproductive risk factors in relation to ovarian cancer incidence. We evaluate this and other approaches to summarizing risk for ovarian cancer. We identified 472 incident cases of ovarian cancer during 2,298,068 person-years of follow-up of 24- to 55-year-old premenopausal women at cohort inception. Reproductive exposures, use of oral contraceptives, and history of tubal ligation were evaluated. Age at menopause is directly related to cumulative risk of ovarian cancer up to age 70 years (age 55 vs. age 45, risk increase = 62%; 95% confidence interval = 36 to 96%) and age at menarche is inversely related to risk (age 15 vs. 11, risk reduction = 31%; 27–34%). Use of oral contraceptives for 5 years before age 30 decreases risk of ovarian cancer to age 70 by 37% (32 to 41%). Tubal ligation reduces risk up to age 70 by 21% (−2 to 38%). Parity reduces risk, independent of age at first birth and age at last birth. A model summarizing years of ovulation offers a fit comparable to a more complex modeling of reproductive variables. The model fit is good, with a concordance statistic of 0.60 (0.57 to 0.62) indicating reasonable ability to differentiate those who will develop ovarian cancer from those who will remain disease free. This model may be applied in the identification of women at high risk for ovarian cancer, for example, in selecting candidates for prevention trials.