Dose distrubution in cone-beam breast computed tomography: an experimental phantom study

We measured the spatial distribution of absorbed dose in a 14-cm-diameter PMMA half-ellipsoid phantom simulating the average uncompressed breast, using an X-ray cone-beam breast computed tomography apparatus, assembled for laboratory tests. Thermoluminescent dosimeters (TLDs) were placed inside the phantom in six positions, both axially and at the phantom periphery. To study the distribution dose inside the PMMA phantom two experimental set-ups with a different copper filtration (50 μm and 200 μm) were adopted. Irradiations were performed with X-ray tube voltages of 50 to 80 kVp in 10 kVp increments. In a case (50 μm Cu), irradiations were at constant 73.75 mAs; in the other case (200 μm Cu) irradiations were delivered at mAs values such to obtain the same delivered dose. The values obtained by TLDs (in nC) were normalized to the one obtained in the central position inside the phantom. Results with 50 μm Cu show that maximum increment of dose (up to 63%) is obtained, at 50 kVp, along the axial direction (from the ‘chest wall’ to the ‘nipple’), and, in the radial direction, up to 82% from the breast axis toward the ‘skin’. With 200 μm Cu a distribution of dose inside PMMA more uniform was obtained (the maximum variation is 33% at 50 kVp) in comparison to a thinner Cu filtration.