Determination of breast tissue composition for improved accuracy in estimating radiation doses and risks in mammographic screening

Statistical analysis of mammographic breast composition measurements: towards a quantitative measure of relative breast cancer risk

OBJECTIVE

A number of studies have identified the relationship between the visual appearance of high breast density at mammography and an increased risk of breast cancer. With the advent of digital mammography and the promise of routine measurements of parameters associated with breast composition, the possibility arises of using breast composition in a quantitative manner to predict relative breast cancer risk. Previous measurements have shown that the average proportion of glandular and adipose tissue within the breast varies with both age and breast size. In order to be able to identify individual women with an unusually high volume of glandular tissue, it will therefore be necessary to make comparisons with a disease-free population matched for age and breast size.

METHODS

A large number of breast glandular thickness measurements were analysed to investigate the statistics of breast composition across a disease-free population as a test of a suitable methodology for relative risk estimation. The large data set is also used to revisit the trends in breast composition used in the current UK method of breast radiation dosimetry.

RESULTS

It is demonstrated that a non-linear transformation can be used to produce normal statistical distributions, suitable for producing a standardised "Z-score" for breast composition.

CONCLUSION

A standard "Z-score" approach to identify women with unusually glandular breasts is recommended and so provide a basis for cancer risk estimations.

Dosimetric implications of age related glandular changes in screening mammography

The UK National Health Service Breast Screening Programme is currently organized to routinely screen women between the ages of 50 and 64, with screening for older women available on request. The lower end of this age range closely matches the median age for the menopause (51 years), during which significant changes in the composition of the breast are known to occur. In order to quantify the dosimetric effect of these changes, radiographic factors and compressed breast thickness data for a cohort of 1258 women aged between 35 and 79 undergoing breast screening mammography have been used to derive estimates of breast glandularity and mean glandular dose (MGD), and examine their variation with age. The variation of mean radiographic exposure factors with age is also investigated. The presence of a significant number of age trial women within the cohort allowed an extended age range to be studied. Estimates of MGD including corrections for breast glandularity based on compressed breast thickness only, compressed breast thickness and age and for each individual woman are compared with the MGD based on the conventional assumption of a 50:50 adipose/glandular composition. It has been found that the use of the conventional 50:50 assumption leads to overestimates of MGD of up to 13% over the age range considered. By using compressed breast thickness to estimate breast glandularity, this error range can be reduced to 8%, whilst age and compressed breast thickness based glandularity estimates result in an error range of 1%.