Density of breast tissue can directly influence tumour development

New research suggests that there is a direct relationship between the density of breast tissue, and the risk of developing tumours in dense areas of the breast.

The results of the study, published today in the Open Access journal Breast Cancer Research, strongly suggest that some aspects of dense breast tissue directly influence tumour development in breast tissue.

Mammographically dense tissue - areas of the breast consisting of glands, milk ducts and fibrous tissue - has already been established as a risk factor for breast cancer. The risk of developing breast cancer is approximately five times higher in women in whom sixty percent or more of the total breast tissue is dense, compared to women with less than ten percent of dense breast tissue. Despite this knowledge it is not known whether tumours tend to arise in areas of mammographically dense tissue, or if mammographic density is simply a marker of breast cancer risk.

In order to address this question, Giske Ursin and colleagues, from the University of Southern California in the United States, studied the mammograms of 27 women, all diagnosed with the same type of tumour - ductal carcinoma in situ (DICS). These are tumours inside the milk ducts that have the potential to become invasive and spread to other tissues, and is the most common form of non-invasive breast cancer. These types of tumour are usually visible as tiny flecks of calcium in a mammogram and can be easily differentiated from dense breast tissue. For each woman, the mammograms of the affected breast or breasts at diagnosis were studied by two independent expert mammography radiologists. The radiologists then coded the DCIS tumour according to whether or not it was found in an area of mammographically dense tissue.

Of the 22 DCIS tumours about which both the radiologists were in agreement, 21 tumours were classified as being in dense areas of the breast. Furthermore, the locations of the tumours in the breast correlated strongly with the area of the breast with the highest average percent density - the lateral-superior mammographic quadrant.

Overall, the results of the study show that DCIS occurs overwhelmingly in the areas of the breast that are mammographically dense, and occurs in the part of the breast that has the highest percent density on the mammogram. This relationship is not brought about by the presence of DCIS, and according to the authors this "suggests that some aspect of mammographically dense tissue directly influences the carcinogenic process in the local breast glandular tissue".

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