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Monte Carlo and Phantom Study of the Radiation Dose to the Body from Dedicated CT of the Breast¹

¹ From the Department of Radiology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, Ga (I.S., S.V., S.S., C.J.D., A.K.); and Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Ga (I.S., A.K.). Received June 19, 2007; revision requested August 22; revision received August 29; final version accepted September 27. Supported in part by National Institutes of Health grant RO1-EB002123 from the National Institute of Biomedical Imaging and Bioengineering and a Distinguished Cancer Clinicians and Scientists Award and a Cancer Research Award from the Georgia Cancer Coalition. Address correspondence to A.K. Department of Radiology, University of Massachusetts Medical School, 55 Lake Avenue North S2-836, Worcester, MA 01655 (e-mail: Andrew.Karellas{at}umassmed.edu).

Purpose: To prospectively determine the radiation dose absorbed by the organs and tissues of the body during a dedicated breast computed tomography (CT) study by using Monte Carlo methods and a phantom.

Materials and Methods: By using the Geant4 Monte Carlo tool kit, the Cristy anthropomorphic phantom and the geometry of a dedicated breast CT prototype were simulated. The simulation was used to track x-rays emitted from the source until their complete absorption or exit from the simulation limits. The interactions of the x-rays with the 65 different volumes representing organs, bones, and other tissues of the phantom that resulted in energy deposition were recorded. These data were used to compute the radiation dose to the organs and tissues during a complete dedicated breast CT scan relative to the average glandular dose to the imaged breast (relative organ dose [ROD]), by using the x-ray spectra proposed for dedicated breast CT imaging. The effectiveness of a lead shield for reducing the dose to the organs was investigated.

Results: The maximum ROD among the organs was for the ipsilateral lung with a maximum ROD of 3.25%, followed by ROD for the heart and the thymus. Of the skeletal tissues, the sternum received the highest dose with a maximum ROD to the bone marrow of 2.24% and to the bone surface of 7.74%. The maximum ROD to the uterus, representative of that of an early-stage fetus, was 0.026%. These maxima occurred for the highest-energy x-ray spectrum (80 kVp) that was analyzed. A lead shield does not substantially protect the organs that receive the highest dose from dedicated breast CT.

Conclusion: Although the dose to the organs from dedicated breast CT is substantially higher than that from planar mammography, it is comparable to or considerably lower than that reached by other radiographic procedures and much lower than that of other CT examinations.

© RSNA, 2008