DETERMINATION OF AVERAGE GLANDULAR DOSE WITH MODERN MAMMOGRAPHY UNITSFOR 2 LARGE GROUPS OF PATIENTS

Until recently, for mammography Mo anode-Mo filter x-ray tube assembli es were almost exclusively used. Modem mammography units provide the p ossibility to employ a variety of anode-filter combinations with the a im of adapting the x-ray spectrum to compressed breast thickness and c omposition. The present contribution provides information on the radia tion exposure of two large groups of patients (one of 1678 and one of 945 women) who were mammographed with modern x-ray equipment, and on t he dosimetry necessary for the evaluation. For dosimetric purposes spe ctral information is essential. X-ray spectra have been determined for various anode-filter combinations from measurements with a Ge detecto r. Based on these spectra, conversion factors from air kerma free in a ir to average glandular dose (fi factors) have been calculated for dif ferent anode-filter combinations, compressed breast thickness ranging from 2 to 9 cm and breast compositions varying from 0 to 100% glandula r tissue. Determinations of various quantities, including entrance sur face air kerma (ESAK), tube output, tube loading (TL), fraction of gla ndular tissue (FGL) and compressed breast thickness, were made during actual mammography. Average glandular dose (AGD) was determined using g factors corrected for tissue composition as well as g values for sta ndard breast composition, i.e. 50% adipose tissue and 50% glandular ti ssue by mass. It is shown that, on average, the influence of the actua l breast composition causes variations of the order of about 15%. For group 1 and group 2, the mean values of average glandular dose (using g factors corrected for tissue composition) were 1.59 and 2.07 mGy res pectively. The number of exposures per woman was on average 3.4 and 3. 6 respectively. The mean value of compressed breast thickness was 55.9 and 50.8 mm respectively. The mean age of group 1 was 53.6 years (for group 2 the age was not recorded). The fraction by mass of glandular tissue FGL decreases with increasing compressed breast thickness and a ge of patient (from 75% at 25 mm to 20% at 80 mm, and from 65% at 20 y ears to 30% at 75 years). For a medium-sized breast, i.e. a compressed breast thickness of 55 mm, FGL is about 35%, indicating that the stan dard mix (FGL = 50%) might need some modification, particularly becaus e of additional evidence from another investigation with similar resul ts on FGL.