Breast dose reduction options during thoracic CT: influence of breast thickness

Little is known about the effectiveness of dose reduction options according to breast thickness. The purpose of this phantom study was to compare the effects on dose and noise of bismuth shielding versus a low kilovoltage for different breast thicknesses.

CT acquisitions were performed first at 120 kVp (reference acquisition), then at 120 kVp with shielding and at 100 kVp without shielding on a phantom with three different prosthetic breast thicknesses, corresponding to the minimum, median, and maximum values first measured in a sample of 30 female thoracic CT examinations, which were randomly selected. Breast doses were measured with optically stimulated luminescence dosimeters placed on and beneath the prosthetic breast. For noise evaluation, the CT number SDs were measured within six ROIs at increasing depths.

Taking into account all breast thicknesses, the average breast dose was reduced by 42.1% with shielding and by 33.0% at 100 kVp (p=0.009). In-depth noise increased less with shielding (19.0% vs 32.1%, p<0.0001). For 1-cm breast thickness, the breast dose fell by 46.5% and 29.7% with shielding and 100 kVp, respectively (p=0.01), and in-depth noise increased by 19.5% and 33.9% (p=0.01). The corresponding values for 2-cm breast thickness were -38.5% and -30.1%, (p=0.02) and 16.5% and 33.5% (p=0.001), whereas those for 4-cm thickness were -40.6% and -40.5% (p=0.95) and 20.7 and 29.2% (p=0.02).

Greater breast dose reduction is achieved by shielding for breast thicknesses less than 4 cm. Regardless of breast thickness, shielding leads to a smaller increase in in-depth noise.