Figure 22.2a shows compression force (daN) versus contact area (dm²) for both data sets. In this figure, four pressure values (5, 10, 15, 20 kPa) are indicated as straight lines: Force (daN) = Pressure (kPa) × Contact area (dm²). Figure 22.2b shows contact pressure (kPa) versus contact area (dm²) for the same data. In this figure four force values (5, 10, 15, 20 daN) are indicated as hyperbolas: Pressure (kPa) = Force (daN)/Contact area (dm²).

It is clearly visible that the applied forces and pressures were considerably higher in the Netherlands, however in both countries similar trends exist as a function of contact area: Smaller breasts received lower forces than larger breasts in both countries, and pressures were higher for smaller breasts compared to larger breasts.

Comparing the number of high compression forces in both data sets, we see that the Dutch set has 18.6 % (n = 2,528) compressions higher than 15 daN, versus 1.9 % (n = 139) in the U.S.. In terms of high pressures we find 10.7 % (n = 1,458) compressions higher than 20 kPa in the Netherlands, versus 1.7 % (n = 119) in the U.S. On the other side of the scale we can compare the number of low compression forces. The U.S. data set contains 23.5 % (n = 1,688) compressions that received less than 5 daN, versus practically none, 0.04 % (n = 6), in the Netherlands. Lastly, we counted 21.7 % (n = 1,555) compressions below 5 kPa of pressure in the U.S., versus only 0.8 % (n = 114) in the Netherlands.

Figure 22.3a shows breast thickness (mm) versus contact area (dm²) for both datasets. The average thickness is nearly identical for both datasets, but the standard deviation in the U.S. data is on average 16 % larger. Figure 22.3b shows absorbed glandular dose (mGy) versus contact area (dm²). All dose values were recalculated with Volpara’s comprehensive dose model, which enables inter-manufacturer comparison. The U.S. data has a higher mean value and a much larger standard deviation compared to the Dutch data. These differences are possibly influenced by the larger variation of the breast thickness in the U.S. set. Another source of variation is that U.S. images were made on mammography machines with various target- and filter materials, whereas the Dutch screening only used machines with Tungsten target and Rhodium or Silver filter.

Figure 22.4 illustrates modelled compressions following a strict force protocol (F = 14 daN ± 5 % standard deviation) and a strict pressure protocol (P = 10 kPa ± 5 % standard deviation). In Fig. 22.4a, the force values for the 10 kPa-protocol are proportional to the contact area until reaching the 20 daN guideline upper limit. The modelled 14 daN-protocol is constant around 14 daN. In Fig. 22.4b the pressure values for the 10 kPa-protocol are constant around the target value of 10 kPa, and again limited to 20 daN of force for contact areas larger than 2.0 dm². The strict 14 daN-protocol extends far beyond the scale with a maximum pressure of 120 kPa (900 mmHg) for the smallest breast found in these data sets.