Distribution of Absorbed Dose in Cone-Beam Breast Computed Tomography: A Phantom Study With Radiochromic Films

摘要

Cone-Beam Breast Computed Tomography (CBBCT) of the pendant breast with dedicated scanners is an experimental 3D X-ray imaging technique for breast cancer diagnosis under evaluation in comparison to conventional two-view 2-D mammography of the compressed breast. In CBBCT it is generally assumed that a more uniform distribution of the radiation dose to the breast volume can be obtained, with respect to mammography, at equal Mean Glandular Dose (MGD) levels. In fact, in CBBCT the X-ray beam rotates for 360 deg around the breast, while in each mammography view the breast is irradiated from one side only. Using a CBBCT laboratory scanner developed by our group, we have measured the distribution of the radiation dose in a hemi-ellipsoidal PMMA breast phantom of 14 cm diameter simulating the average uncompressed breast, using radiochromic films type XR-SP inserted at mid-plane in the phantom. The technique factors were 80 kVp (5.6 mm Al Half Value Layer), tube load in the range 23–100 mAs, for an air kerma at isocenter in the range 4.7–20 mGy, for a calculated MGD in the range 3.5–15 mGy for a 14 cm diameter breast of 50% glandularity. Results indicate that the dose decreases from the periphery to the center of the phantom, and that along a transverse profile, the relative dose variation $Delta ={}$ ((edge-center)/center) is up to $(25 pm 4)hbox{%}$ at a distance of 80 mm from the nipple. As for the relative dose variation along the phantom longitudinal axis, the maximum value at middle of the phantom measured is $delta ={}$ ((nipple-chest wall)/chest wall) ${}= -(15 pm 4)hbox{%}$, indicating that the dose decreases from the chest wall toward the nipple. The values of the paramet-n-ners $Delta$ and $delta$ depend also on the height of the X-ray tube focal spot with respect to the phantom vertex (nipple). Results are in rough agreement with similar previous determinations using thermoluminescence dosimeters.