Consistency of film optical density in mammographic screening programmes.

Maintaining a constant and appropriate mean film optical density in mammography is an essential part of quality assurance in a breast screening programme. It depends on stability in X-ray exposure controls, film supplies, intensifying screens and film processing. A programme of weekly monitoring is described which helps to trace the source of any irregular or step-wise changes in any of the relevant parameters. Its results are also used to investigate by statistical methods the degree of film optical density variation achieved in periods of several weeks of apparent general stability. Results are presented for 20 X-ray sets and a total of 32 record periods of 8-10 weeks each. The mean standard deviation for density of a single film of a Perspex block was found to be 3.5%. For processor speed index it was the same while for X-ray set variations it was less than 2%. Neither of these last two variables showed any strong correlation with block film optical density.