Large breasts account for nearly half of breast cancer cases in Singapore

High body mass index (BMI) and breast density are major risk factors for breast cancer, according to a recent Singapore study.

“Multiple risk factors for breast cancer are well-established; these include breast density, reproductive, menstrual and modifiable lifestyle factors such as BMI, hormone replacement therapy (HRT) use and alcohol consumption,” said researchers. “In this study, we examined the impact of known breast cancer risk factors … on the incidence of breast cancer.”

Of the 28,130 enrolled women, 474 developed breast cancer. The resulting incidence rate was 1.7 percent. Majority of the participants were of Chinese ethnicity (84.2 percent). Almost all (92.7 percent) had at least one child, and most had breastfed (63.8 percent). Rates of contraceptive use and HRT were low. Almost 90 percent were postmenopausal. [Sci Rep 2020;10:503]

Established breast cancer risk factors that were common in the sample included high breast density (65.2 percent), BMI ≥25 kg/m² (43.4 percent) and early age at menarche (≤13 years; 35.1 percent).

Calculating for the population attributable risks (PARs), researchers found that up to 16.2 percent of all breast cancer cases could be avoided by keeping BMI below 25 kg/m². Moreover, the combination of BMI and ethnicity (51.4 percent) and BMI and breast density (45.9 percent) resulted in large PARs, accounting for around half of the breast cancer cases.

Even when taken alone, breast density emerged as an important risk factor for breast cancer. Almost half (48.6 percent) of all cases could be prevented if breast density was kept to 12.29 percent at most.

“Emerging evidence has shown that prophylactic treatment with drugs such as Tamoxifen can induce a reduction in breast density, which is in turn linked to decreased breast cancer risk,” said the researchers. “Hence, we also examined breast density as a potentially modifiable risk factor.” [J Natl Cancer Inst 2011;103:744-752)

Notably, having a family history of breast cancer explained only 3.8 percent of all cases, while having a personal history of benign disease had a PAR of only 4.3 percent.

In the present study, participants were enrolled from the Singapore Breast Cancer Screening Project (SBCSP), a prospective, population-based trial for screening mammography. Identification of incident breast cancer was performed through linkage with the Singapore Cancer Registry. Variables of interest were collected from the SBCSP questionnaire. Women who had malignancies in other sites or were pregnant were ineligible for the analysis.

“Risk-stratification strategies to increasing screening in women with high risk may be a potential strategy to improve breast screening guidelines,” the researchers said.

“Understanding the attributable risk of known risk factors on breast cancer incidence may motivate health policy changes, possibly through personalized risk-reduction by introducing target intervention such as Tamoxifen (as a prevention) to reduce their risk,” they added.