Checking the consistency of sensitometers and film processors in a mammographic screening programme

Quality control procedures in relation to speed and contrast in X-ray film processors, both in X-ray departments and breast screening centres, are based on the use of film sensitometers. Calibrations of these instruments, or simple checks of their constancy with time, are not readily available. A procedure is described in which a reference sensitometer is compared on a 3-monthly basis with the local sensitometer in seven breast screening centres (13 processors) in a region. This demonstrated long-term stability of speed and contrast in both sensitometers and processors. The results may also be used to compare speed and contrast between processors in centres using film of a single type. Standard deviations for single measurements are estimated to be approximately 1% for step density on a single sensitometer, less than 3% for comparison of two sensitometers, about 6% for comparison of processor speed, and between 4 and 5% for processor contrast.