Abstract 866: The proportion of breast and gynecological cancers in Australian women that can be attributed to the use of oral contraceptives and hormone replacement therapy
Authors: Jordan Susan J. , Wilson Louise , Whiteman David , Webb Penelope ,   PAF Project Group
Published: August 2015

Introduction: In 2008 The International Agency for Research on Cancer concluded that combined oral contraceptives (OCs) are a cause of cancer of the breast and cervix but decrease the risk of ovarian and endometrial cancer. They also concluded that estrogen-only hormone replacement therapy (HRT) increases the risk of both endometrial and ovarian cancer, while use of estrogen plus progestogen (combined) HRT increases the risks of breast cancer. The level of risk associated with each cancer type varies by duration and recency of use of hormones. Our aim was to calculate the proportion of breast and gynecological cancers arising in Australian women (population attributable fractions - PAF) that could be attributed to these commonly used hormonal medications.

Methods: We calculated PAFs using the standard formula, which requires an estimate of the relative risk for each exposure-cancer association and the prevalence of the causal factor in the population. Relative risks were obtained from published pooled or meta-analyses of the associations and prevalence estimates were derived from Australian National Health Survey estimates or other large national studies. For factors inversely associated with cancer we calculated the “prevented fraction” and estimated the number of cancers that were prevented due to exposure to the factor.

Results: The estimated PAFs suggested that fewer than 5% of breast and gynecological cancers in Australia in 2010 could be attributed to oral contraceptive use and estimates were similar for HRT use. However, the calculated preventive fractions for oral contraceptives suggested that the numbers of endometrial and ovarian cancers may have been 46% and 24% higher respectively if no Australian women had used OCs. Given combined HRT increases breast cancer risk and breast cancer is much more common than endometrial or ovarian cancer we also assessed the likely effect if all women taking combined HRT instead took estrogen-only HRT. Under this scenario, we estimate the absolute numbers of ovarian and endometrial cancers would increase slightly but the number of breast cancers attributable to HRT use would decrease substantially.

Conclusions: Our analyses suggest that the use of combined OCs confers more benefit than harm with respect to cancer incidence, with a small increase in breast/cervical cancer diagnoses weighed against a much larger decrease in numbers of endometrial and ovarian cancers. In contrast, our results indicate that there would be fewer cancers overall if women did not use HRT. When HRT is required, most guidelines recommend that women with a uterus take combined HRT to prevent endometrial cancer. However, our findings suggest that, in terms of total cancer burden, it might be safer for women to avoid progestogens and use estrogen-only HRT for symptom relief. Future studies should examine this possibility.

Citation Format: Susan J. Jordan, Louise Wilson, David Whiteman, Penelope Webb, PAF Project Group. The proportion of breast and gynecological cancers in Australian women that can be attributed to the use of oral contraceptives and hormone replacement therapy. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 866. doi:10.1158/1538-7445.AM2015-866