ASSESSMENT OF MEAN GLANDULAR DOSE IN MAMMOGRAPHY

The routine assessment of patient dose in the National Health Service Breast Screening Programme is performed as part of the quality assuran ce protocol recommended by the Institute of Physical Sciences in Medic ine. The mean glandular dose to a standard breast is deduced from meas urement of the air kerma at the entrance surface of a 4 cm Perspex pha ntom by applying a series of conversion factors. The exposure factors far this measurement are those used clinically. The measured mean glan dular dose is then compared with nationally accepted action levels. In some centres the assessment of mean glandular dose using Perspex is s upplemented by patient dose surveys. The mean glandular dose to a seri es of patients attending a breast screening unit may be estimated from a knowledge of the exposure factors and compressed breast thickness, using a knowledge of the X-ray tube output. Measurements made on units in the Northern Region of England and in Scotland using both methods are presented. The implication of these measurements with regard to pa tient dose surveys in mammography and quality assurance programmes are discussed. An analysis of the uncertainties associated with the measu rement techniques is presented.