The mean glandular dose (MGD) is an estimate of the average absorbed dose to the glandular tissues of a breast during mammography. It is measured in Gray (Gy).

The most commonly accepted method of calculating the mean glandular dose is described by Dance et al (2000):

                                              MGD = Kgcs

• K = entrant surface air kerma

• g = conversion factor for 50% glandular breast based on thickness and half-value layer

• c = correction factor based on non-standard glandularity/thickness

• s = correction factor based on non-molybdenum anode/filter combination

The MGD scan provides an indication of the radiation risk to the breast during exposure. Regional and national radiology safety guidelines will use the MGD as a measure to determine diagnostic reference levels. 

To enable quality control and comparisons between different equipment, MGD is calculated to the standard breast which is defined as a 4.2 cm thick ACR phantom. It is a legal requirement that the MGD to the ACR phantom not exceed 3 mGy. Typical MGDs are lower than this limit.

For a standard breast (defined as 4.2 cm thick when compressed, with a 50:50 ratio of glandular tissue to fat), the MGD is typically 3.0 mGy per view.

The risk of contracting fatal cancer after being exposed to a glandular dose of 2 mGy in a woman aged between 50 and 65 is approximately 1 in 50,000 (0.002%). 

Factors that affect radiation dose received by the breast include:

• breast thickness and composition

• beam energy (kVp)

• target/filter combination

• use of anti-scatter grid

• use of magnification mode

• image receptor technology