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Radiation doses in mammography

FAQs for health professionals

» Should we be adjusting our mammographic techniques to produce images with the lowest possible radiation dose to the patient?

No. 

While it is possible to select mammography imaging components (e.g. film, intensifying screens) and technique factors that will produce images with very low X-ray exposure, the images would have very low quality. They will not provide the necessary diagnostic results. The correct action is to optimize each imaging procedure or technique so that the mammography equipment and imaging factors are selected and adjusted to provide the necessary image information and visualization with the lowest radiation exposure.

» What determines the radiation dose to the breast of my patients in mammography?

The dose to the breast of an individual patient is determined by a combination of three factors: 

  • the characteristics of the equipment being used; 
  • the technique factors selected for the examination;
  • the size and density of the patient’s breasts. 

The radiation beam of each mammography machine is usually calibrated so that it delivers a specific and known absorbed dose to a reference (average size) breast when imaged with a specific set of technique factors. The major factors determining this dose are the sensitivity of the receptor (film- screen combination, characteristics of digital receptors), and the setting of the automatic exposure control (AEC) level to produce a specific film density. 

The dose generally increases with increased breast size and density, for a given optical density. With regard to the technique, the selected kV (typical range of 24kV to 32kV) and anode/filter combination are the major factors that determine the dose. Lower kV values are used to enhance contrast but do not provide sufficient penetration through thick or dense breast tissue where the higher kV values are required. 

When the kV is reduced in a specific examination, the mAs (in the units of mAs) must be increased to produce the necessary receptor exposure. The combination of reduced kV and increased mAs results in a higher dose. Dose generally increases with increased breast size and density because more radiation is required to penetrate the breast and expose the receptor.

» Which quantity is used to express the exposure to radiation of a mammography patient?

The quantity that is generally used is the mean glandular dose (MGD). This is based on the assumption that it is the glandular tissue in the breast, and not the adipose tissue or fat that is most sensitive to radiation effects. The MGD is defined as the mean (average) dose to the glandular tissue. It is considered to be a reasonable quantity for comparing relative risk from different mammography procedures.

» Is it possible to measure the mean glandular dose to the breast of my patient in mammography?

The MGD cannot be measured directly because it occurs within the breast. Estimates of the MGD for a specific patient can be calculated (usually by a physicist) from the tube output, which is directly measurable, and the exposure parameters to obtain the image. However, the exact composition of the breast is not known and it also influences the MGD. Some digital mammography systems automatically calculate and display the MGD for each patient.

» What is the best kV value to use in mammography to reduce the absorbed dose to the breast?

The better question would be ‘what is the best kV value to provide the necessary image contrast and visibility with the lowest possible radiation absorbed dose to the breast?’ The consideration for image information must come first. 

The kV value that provides the optimum balance between image information (visibility of the signs of cancer) and absorbed dose depends on the compressed size and density of the breast and will generally be in the range of 24 to 32. If the kV is set too low for a specific breast size or density the breast will not be properly penetrated and the dose will be higher than necessary. If the kV is too high then the contrast and also  the visibility, will be reduced.

To ensure the necessary image quality with the lowest possible dose, mammography should be performed by highly qualified radiographers with the ability to evaluate each patient’s breast and then select the optimum kV and other exposure parameters.

» Should I tell my patients the radiation exposure for their mammographic procedures?

There are several factors to consider. First, you will not know the dose for each patient unless there has been a special effort to calculate it from the exposure parameters and the equipment calibration data (beam output).
What should be known in each mammography facility is the absorbed dose for a reference average breast size and for the range of breast sizes encountered. These data are determined by a physicist. The output is directly measurable and MGD is estimated from output and the exposure parameters.

What should be avoided is any encouragement for patients to go 'dose shopping' by searching the facility that does mammography with the lowest dose. The appropriate approach is to provide patients with professionally prepared information covering the benefits and risk of mammography.

» Is compression necessary, even if the patient has sensitive breasts?

Yes. 

Each breast is gently but firmly compressed using a clear plastic device just for a few seconds. It is important that the breast is compressed to obtain the clearest possible picture and the lowest radiation dose. Compression stops the breast moving during the exposure. Some women find mammography uncomfortable, similar to have the blood pressure taken.