Breast compression and radiation dose in two different mammographic oblique projections: 45 and 60 degrees

Standard mammography includes two views, craniocaudal and medio-lateral oblique. Depending on patient's body constitution, central beam angle in mediolateral oblique projection may vary, with 45 degrees being suitable for the majority of patients in routine daily practice. With continuous improvement in X-ray technology and radiographers' training, the risk of radiation induced cancerogenesis is considerably reduced and acceptable when compared to benefit. However, the risk still exists, being cumulative and directly related to absorbed glandular dose. There is no minimal dose of radiation which is absolutely harmless, and every effort to reduce the dose is welcome. In this retrospective study two different angles (45 vs. 60 degrees) of mediolateral oblique view were compared according to radiation dose and efficacy of breast compression.

In 52 women, additional 60 degrees oblique films were done after craniocaudal and mediolateral oblique 45 degrees-films, with the same kVp and positioning technique. Breast thickness, time-current products (mA s) and absorbed doses were compared between 45 degrees- and 60 degrees-films. Subgroups of women with large, small, prominent and pendulous breasts were analyzed separately, following the same methodology as for the whole group.

mA s were 11.5% lower and compression 7% better with an angle of 60 degrees than with 45 degrees. In the subgroup of women with small breasts, mA s values were 13% lower and compression 9% better with 60 degrees than with 45 degrees, while in the subgroup with large breasts, mA s were 9% lower and compression 5% better. In the subgroup of patients with pendulous breasts, mA s values were 12% lower and compression 10% better with 60 degrees than with 45 degrees, while in the subgroup with prominent breasts, mA s values were 4% lower and compression 3% better. Absorbed glandular dose was estimated to be approximately 20% lower when an oblique mammogram was done with 60 degrees instead of 45 degrees. The compression with 15 kp was well tolerated by the majority of patients.

Mammograms of excellent quality should be done with as low a radiation dose as possible. Adequate breast compression is fundamental in mammography due to immobilization of the breast, shortening of the exposure times, reduction of motion and geometric blur and prevention of overpenetration by means of equalizing breast thickness. As the absorbed glandular dose cannot be accurately measured, it is convenient to estimate the dose approximately, on the basis of its linear proportionality with exposure dose. With constant technical properties of X-ray machines, exposure dose is determined only by mA s. Hence, the absorbed glandular dose in our study was influenced only by changes of mA s and breast thickness. As the absorbed dose reduction is proportional to the product of the reduction of mA s and thickness, we estimated that absorbed dose was 7-22% lower if 60 degrees is applied instead of 45 degrees. Breast compression and mA s were more favourable in women with pendulous breasts, possibly because of elongation of the glandular disc in the lateroascending direction, with its longer axis directed more perpendicularly. Fibroglandular tissue in the 60 degrees-view is thus projected onto a larger film area, with less effect of superimposition. In conclusion, because of lower mA s values and better compression, which finally result in a 25% lower absorbed dose, we recommend the oblique view be done with an angle of 60 degrees, especially for small and pendulous breasts.