In-phantom depth-dose measurements in mammography utilizing a Compton spectrometer

Mammography, using X-rays, is routinely utilized for screening of patients for prevention of breast cancer. Nevertheless, there is a small but significant risk of radiation induced carcinogenenesis associated with radiation exposure of the breast, being glandular tissue the most sensitive one among the tissues composing breast. Risk estimation in mammography may be determined by the mean absorbed dose in the glandular tissue, . It is impossible to measure directly. Conversion factors are used to derive from measurements of incident exposure or kerma in free air, which may be obtained by Monte Carlo calculations in mathematical breast models, or from depth-dose measurements in homogeneous breast phantoms. In this work, X-ray beams from two apparatuses have been used in connection with a BR-12 mammography phantom: a GE Senographe 700T clinical unit, Mo anode/Mo filter, and a Philips constant potential equipment, W anode/Mo filter. values have been determined by two procedures: (a) by measurement of the incident air kerma, using an ionization chamber, multiplied by conversion factors from the literature, determined by Monte Carlo calculations on the basis of breast models; (b) by obtaining the incident air kerma and the dose, in several depths in a BR-12 phantom, by means of X-ray energy spectra measured with a Compton spectrometer. In this spectrometer, the radiation scattered by a small cylindrical Lucite probe, located over or inside the phantom, is detected, at an angle of about 90o with the direction of the primary beam, by a CdTe semiconductor detector. The air kerma and in-phantom depth-doses, in the X-ray primary beam, are obtained by utilizing the Klein-Nishina cross-section and parameters of the experimental setup. Depth-dose curves and results normalized for unit incident air kerma were obtained for both systems and are presented for 30 kV. (author)