The wealthiest Canadian women are about 15 per cent more likely to develop breast cancer than the poorest ones, new data from Statistics Canada reveal.
The most likely explanation is that poorer women have markedly more children and do so at an earlier age, the researchers said. Missing menstrual periods means less exposure to estrogen, a female sex hormone that can fuel the growth of cancerous tumours.
A secondary factor is that wealthier women undergo screening mammography in larger numbers, meaning their cancers are more likely to be detected early.
The research team, led by Marilyn Borugian of the British Columbia Cancer Agency, noted that "breast-cancer incidence is one of the few adverse health outcomes consistently associated with higher socio-economic status."
Earlier research showed that low-income women are also about 12 per cent less likely to die of breast cancer. In part, that is due to the fact that the poor tend to die young, and breast cancer occurs principally in postmenopausal women.
The study, published on Wednesday in Health Reports, examined all breast-cancer cases and divided them by age and income (based on the household income in their neighbourhood).
The data show that, in women under the age of 39, the incidence of breast cancer ranges from 10.2 to 13.5 per 100,000, depending on their income. Women in the highest income quintile had a 25-per-cent greater risk than those in the lowest income group.
Similarly, the cancer rate ranged from 129 to 141 per 100,00 in women aged 40 to 49; from 239 to 255 in those 50 to 59 and; from 305 to 348 in those in the 60-to-69 age group. In women over the age of 70, breast-cancer incidence ranged from 355 to 432 per 100,000. The wealthiest women had a 20-per-cent higher risk.
Over all, the difference between the highest and lowest income group was 15 per cent, but the disparity increased with age.
The researchers noted that the number of children born per 1,000 women aged 15 or older was inversely related to the neighbourhood income quintile. In other words, the wealthier families are, the fewer children they have.
Similarly, the higher a woman's income, the more likely she is to undergo screening mammography.
A second study, also published in Health Reports, shows that survival rates vary dramatically depending on the patient's primary cancer site.
One-year survival ranges from 98 per cent for thyroid cancer to a dismal 6 per cent for pancreatic cancer.
The data reveal that, once a patient survives one year - and an initial course of treatment - survival rates improve markedly.
For example, a person with stomach cancer has only a 24-per-cent survival initially, but that jumps to 92 per cent at year five. However, for some cancers, such as chronic lymphocytic leukemia, the survival rate remains virtually unchanged over five years at about 77 per cent.