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Distribution of absorbed dose in cone-beam breast computed tomography: A phantom study with radiochromic films

Cone-Beam Breast Computed Tomography (CBBCT) of the pendant breast with dedicated scanners is an experimental X-ray 3-D 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 a 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 23-100 mAs, for an air kerma at isocenter in the range 4.7-20 mGy, corresponding to a MGD of 3.3-15.8 mGy for a 14 cm diameter breast of 50% glandularity. Results indicate a relative dose variation Δ = ((edge-center)/center) up to 25% at a distance of 50 mm from the nipple. As for the relative dose variation along the phantom longitudinal axis, the maximum value measured is δ = ((nipple-chest wall)/chest wall) = -15%, indicating that the dose decreases from the chest wall toward the nipple. The values of the parameters Δ and δ depend also on the height of the X-ray tube focal spot with respect to the phantom vertex (nipple).