BONE density refers to the amount of mineral density per square meter of bone - bone mineral density (BMD). It is used as an indirect indicator of osteoporosis and fracture risk in a woman.
The most widely used test for BMD is the dual-energy xray absorptiometry (DEXA) scan. Results are scored by two measures - the T-score and the Z-score. These scores indicate the amount one's BMD varies from the mean.
The T-score is the relevant measure when screening for osteoporosis, and the definitions are standardised internationally by the World Health Organisation (WHO), and corrected for age and ethnicity.
A normal BMD is a T-score of -1.0 or higher, while osteoporosis is defined as -2.5 or lower.
Besides age, BMD also differs in different ethnic groups. Asians have a lower BMD compared to Caucasians, but when adjusted for height and weight, no differences are found.
Risk factors for developing osteoporosis are increasing age, menopause, low oestrogen levels, low body weight, family history, use of steroids, and heavy alcohol consumption. Oestrogens increase bone density, hence, after the menopause (when oestrogen levels drop), the bones become less dense, and hence, fracture easily.
The benefit of oestrogen replacement therapy after menopause has been the reduction in the risk of osteoporotic fractures. However, oestrogen replacement therapy can increase the risk of breast cancer.
It has been suggested that BMD is a surrogate marker for lifetime exposure to oestrogen, which is a risk factor for developing breast cancer. Studies have shown an association between a normal and high BMD in post-menopausal women and breast cancer risk.
What is breast density?
Dense breasts have less fatty tissue compared to breasts that are not dense. Breast density is assessed on mammography, and cannot be felt by palpation.
Dense breasts can make it harder to detect breast cancer on mammographic screening. Research has shown that dense breasts are four to six times more likely to develop breast cancer.
Younger women have dense breasts, which should become less dense as they age, and hence mammograms are more sensitive (more likely to detect cancer) in older women. Because of this, mammogram screening is not recommended for women below the age of 40 years old.
Oestrogen replacement therapy may lead to an increase in the density of the breast.
Because of the increased risk of breast cancer with dense breasts, several states in the US have passed a new law that requires doctors to inform women that they have an increased risk of breast cancer if a mammogram shows that they have dense breasts (the Breast Density Inform Bill). This has led to controversy among doctors as there is no standard method of measuring breast density on a mammogram film alone, unlike bone mineral density, where there is a more objective measurement of density.
A new method of objectively measuring breast density is the volumetric measurement that is still in the process of evaluation.
Women with dense breasts who have an increased risk of breast cancer (based on a strong family history or previous benign breast biopsy which shows increased risk of breast cancer such as atypical hyperplasia), are advised to have further imaging such as an adjunct ultrasound or magnetic resonance imaging (MRI).
Breast and bone mineral density in women's health
Mammographic breast density and bone mineral density are important measurements in women's health.
Epidemiological studies have demonstrated that both dense breasts and dense bones increase the risk of breast cancer, and it is likely that both are correlated to a lifetime exposure to oestrogen. It is possible that it is the exposure to oestrogens which are responsible for the increased risk of breast cancer.
However, a small study has shown no correlation between mammographic breast density and bone mineral density - that is, a woman with dense breasts need not necessarily have dense bones. Thus there is a component of breast density which is not related to the effect of oestrogens.
Will reducing mammographic breast density lead to reduction in breast cancer risk? There are chemopreventive drugs such as tamoxifen and aromatase inhibitors which have been shown to reduce breast cancer risk, and at the same time reduce breast density.
Will reducing bone mineral density reduce breast cancer risk? This hypothesis can never be tested, because reducing bone mineral density increases the risk of osteoporotic fractures, which is a major cause of morbidity and mortality in the elderly.
In conclusion, more research into mammographic breast density, including developing an international standardised objective measurement of breast density, is required, and in this era of personalised medicine, the addition of mammographic breast density and bone mineral density to known models of breast cancer risk may identify a cohort of women at increased risk of breast cancer for preventive therapy.
This article is contributed by The Star Health & Ageing Panel, which comprises a group of panellists who are not just opinion leaders in their respective fields of medical expertise, but have wide experience in medical health education for the public. The members of the panel include: Datuk Prof Dr Tan Hui Meng, consultant urologist; Dr Yap Piang Kian, consultant endocrinologist; Datuk Dr Azhari Rosman, consultant cardiologist; A/Prof Dr Philip Poi, consultant geriatrician; Dr Hew Fen Lee, consultant endocrinologist; Prof Dr Low Wah Yun, psychologist; Datuk Dr Nor Ashikin Mokhtar, consultant obstetrician and gynaecologist; Dr Lee Moon Keen, consultant neurologist; Dr Ting Hoon Chin, consultant dermatologist; Prof Khoo Ee Ming, primary care physician; Dr Ng Soo Chin, consultant haematologist. For more information, e-mail firstname.lastname@example.org. The Star Health & Ageing Advisory Panel provides this information for educational and communication purposes only and it should not be construed as personal medical advice. Information published in this article is not intended to replace, supplant or augment a consultation with a health professional regarding the reader's own medical care. The Star Health & Ageing Advisory Panel disclaims any and all liability for injury or other damages that could result from use of the information obtained from this article.