Breast density on mammography can significantly vary between individuals. The density is a function of the relationship between radiolucent fat and radiodense glandular tissue.
Breast density varies with age and generally younger women have denser breasts (i.e. more glandular tissue relative to fat) and post menopausal women have progressively less gland and more fat as involution of the breast takes place with increasing age.
Density can vary in an individual. Women on estrogen therapy have denser breasts and a women with significant mass loss has denser breasts compared with images done previously.
There is a historical note in the density of breasts 4; Wolfe initially postulated that breast densities could be divided into four categories which he called N1, P1, P2 and DY (or dysplastic) This correlates with the classification of four groups below. Wolfe suggested that the incidence of breast cancer was higher in the DY group than in the N1 group. The delay in diagnosis in DY breasts is likely a function of the complex appearance and not the parenchymal pattern itself.
There are at least four BI-RADS categories for breast density
- breast is almost entirely fat: < 25% fibro-glandular tissue
- breast has scattered fibro-glandular dense tissue: 25-50% fibroglandular tissue
- breast tissue is heterogeneously dense: 51-75% fibroglandular tissue
- breast tissue is extremely dense: >75% fibroglandular tissue
- the incidence of IDC is higher in this group than in other groups of breast density likely as a combination of the amount of gland present as well as possible observation error
- it is in this group where screening ultrasound is potentially beneficial
Many factors can alter breast density
Factors that can increase breast density
- hormone replacement therapy: the effect is greater with combination hormone therapy than with estrogen therapy alone
- weight loss: from reduction of breast fat
- breast cancer: especially inflammatory breast cancer
- inflammation: mastitis
Factors that can decrease breast density
- medications: e.g. Danazol 3
- vitamin D and calcium intake in pre-menopausal women
- increasing age
- weight gain
- acromegaly 2
The use of density determination is now mandatory in California, Connecticut 7, Texas, New York and Virginia. Patients have to be informed of the fact that they have "dense" breasts 5 with the implication that the dense breast has a potentially higher chance of developing a breast cancer 6. One caveat isthe intra observer variation in what constitutes "dense".