PATIENT DOSE AND RISK IN MAMMOGRAPHY

Patient dose in mammography is estimated by two different methods which are compared and which give good agreement. A mean tissue dose of about 1 mGy per film is found for a breast of 4.5 cm compressed thickness. Variables which affect dose are then considered quantitatively, including compressed breast thickness, tube potential, grids, magnification and beam collimation. The variables having the greatest effect are breast thickness and magnification. For example, an 8 cm breast receives about four times the mean tissue dose of a 3.5 cm breast. Similarly, using a magnification factor of about 2, dose is increased about four times in the primary beam, but the adverse consequences may be largely removed with conventional collimation to part of the breast only. Finally, the dose estimates are combined with existing data on breast cancer induction to predict the risk of carcinogenesis in a breast screening programme. For example, in a screening centre performing 15 000 examinations per year, only one induced cancer is predicted in about 7 years of screening under average UK conditions of age and breast thickness.