Breast cancer is the leading cause of cancer-related death among women worldwide. There are different types of breast tumors according to their responsiveness to various hormones and expression of proteins. Most breast cancers are estrogen-receptor (ER) positive and grow in response to the hormone estrogen. About two-thirds of these are also progesterone-receptor (PR) positive. In about a fifth of breast cancers, the cells make too much of a protein known as HER2. Between 10-20% of breast cancers are “triple-negative”, which means there are no estrogen and progesterone receptors and no expression of the HER2 protein. Some geographic variations of breast cancer incidence suggest that environmental factors may also play a role in its development.
Radon is a naturally-occurring radioactive gas found in air, soil, rocks, and water. The main source of indoor radon is from soil gas entering homes through foundation cracks. Approximately 6% of US homes have radon levels above the Environmental Protection Agency recommended levels. Radon decay products enter the human body mostly by inhalation and deliver radiation doses to various organs including the breast. This could cause DNA damage.Although this is a possible mechanism by which radon exposure could bring about cancerous changes in the breast, there have been very few studies looking at whether the two are linked. Researchers in the US have conducted a study to look at the association between environmental radon exposure and breast cancer incidence in US women. They recently published the results in the journal Environmental Health.
The Nurses’ Health Study II (NHSII) is an ongoing study of over 116,000 US female registered nurses who were aged between 25 and 42 years at the start of the study in 1989. Participants reside in all 50 states. They completed questionnaires every two years giving information about any illness, medical history, diet, lifestyle factors, and health behaviors. They also provide address information. The response rate to these questionnaires is over 90%.
The researchers examined data from this questionnaire to identify participants who reported breast cancer. In these cases, their medical records were reviewed to confirm breast cancer and find out more about the type of tumor. The researchers also reviewed the residential addresses of the participants and used a National Laboratory computer model to calculate each participant’s overall radon exposure.
During the period of the study, almost 4,000 invasive breast cancers occurred. Careful statistical analysis showed that environmental radon exposure was not associated with invasive breast cancer risk overall. Looking in more detail at the various tumor types, radon exposure was also not associated with ER-positive or ER/PR-positive breast tumors. However, there was a suggestive association between higher levels of radon exposure and risk of ER-negative, ER/PR-negative and triple-negative tumors.Triple negative tumor types are usually found in patients who have a gene mutation which prevents repair of damaged DNA (BRCA1). The researchers suggest that radon exposure may be involved in similar DNA damage mechanisms.
This is the first study of its kind looking at environmental radon exposure and incidence of invasive breast cancer. Although there was no association found between radon exposure and risk of overall or hormone-receptor-positive breast tumors, there was a suggestive association with the risk of hormone-receptor-negative breast tumors. Further research is needed to clarify this possible association.
Written by: Julie McShane, Medical Writer