Entrance skin exposure and mean glandular dose: effect of scatter and field gradient at mammography.
OBJECTIVE
To investigate the effect of scatter and x-ray field gradient on entrance skin exposure and mean glandular dose by using the measurement protocol of the American College of Radiology.


METHODS
The exposure difference between the off-axis geometry of the American College of Radiology protocol and the central-axis geometry was measured by using eight common ionization chambers and two mammographic units (Senographe DMR; GE Medical Systems, Milwaukee, Wis; and Mam-CP II; Transcontinental X-ray, Charlotte, NC) at 25, 28, and 30 kVp. The field gradient was also measured on film with a scanning densitometer.


RESULTS
The central-axis exposure was 2.0%-3.6% higher than the off-axis exposure for the Senographe DMR unit and 3.1%-8.0% higher for the Mam-CP II unit. The maximum phantom scatter was 0.7% (Senographe DMR) and 1.3% (Mam-CP II).


CONCLUSIONS
Entrance skin exposure varies, depending on the type of ionization chamber and the characteristics of the mammographic unit. Determination of mean glandular dose with the American College of Radiology protocol may lead to underestimates of the actual measured dose by 4%-8% for the x-ray machines used in this study.
Ng
K H
KH
Department of Medical Physics, University of Wisconsin-Madison 53706, USA.
Aus
R J
RJ
DeWerd
L A
LA
Vetter
J R
JR
D001940:Breast / Q000528:radiation effects*; D005260:Female; D006801:Humans; D008327:Mammography; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D012542:Scattering, Radiation; D012867:Skin / Q000528:radiation effects*