A Comprehensive CT Organ Dose Database for Weight-Specific Adult and Pediatric Patients

Introduction and Innovation As a result of recent concerns from the radiology and regulatory communities about the increasing use of CT and the potential health risk to patients [1-2], a project was proposed and was funded by the NIBIB to develop a modern CT dose reporting and tracking software, called VirtualDose. Literature reviews up to the year of 2011 suggested that many existing software packages, such as ImPACT, ImpactDose, CT-Dose and CT Expo, were based on anatomically crude phantoms and ignore pediatric, pregnant and obese patients. When compared with advanced voxel, NURBS and mesh phantoms, the stylized phantoms are known to result in significantly dose discrepancies up to 100% in certain cases [3]. It is also important to adopt new tissue weighting factors in effective dose calculations using the recent ICRP 103 recommendations [4] and the latest multi-detector CT (MDCT) scanners in future CT dose software tools that are designed using modern programming features. At the core of this VirtualDose software is a comprehensive pre-calculated organ dose database. This paper describes the joint effort by two groups to perform Monte Carlo slice-by-slice organ dose calculations for various CT scanners using a library of adult and children phantoms. Innovation: A comprehensive library of patient phantoms of different ages, genders, and body weights. Material and Methods Two validated CT scanner models (GE LightSpeed Pro 16 and Siemens SOMATOM sensation 16) operated at different tube voltages (80kVp, 100kVp, 120kVp and 140kVp) and beam collimations (1.25 mm, 5 mm, 10 mm and 20 mm) using both the head and body bowtie filters were used to calculate the organ dose. Detailed CT scanner X-ray sources and bowtie filter geometry were modeled by an iterative method using the MCNPX v2.6 Monte Carlo code, as shown in Figure 1. The x-ray source moved along a single axial trajectory to allow for the calculations of organ doses from a single CT slice. This simulation process was repeated to cover from the top of the head to the feet for one phantom. This approach was applied to a set of voxel phantoms covering 50th percentile of adults and children at different ages (1-, 5-, 10-, and 15-year-old), a pregnant female at three gestational stages, and obese patients of different body weights, as shown in Figure 2-5. These meshor NURBS-based anatomical phantoms were adjusted to desired anatomical parameters, before being converted to voxel geometry for use in the MCNPX code to calculate organ doses. Results A detailed database of slice-by-slice organ doses was established for a total of 27 phantoms at RPI and the University of Florida. All simulations were run on 5 Linux servers with 16 cores and 8GB RAM memory each and it approximately took more than 200 days to finish ~60000 simulation cases. All the related organ doses are incorporated into a standardized database compiled using Microsoft SQL server 2008. Organ doses from a contiguous axial scans defined by a specific protocol can be easily obtained from the database. For the helical scan covering the same scan length in a pitch setting of 1, the contiguous axial scans give approximately the same radiation dose with the same technical factor. For scans of pitch > 1, or overlapping helical scan with pitch <1, radiation dose decreases as pitch increases the given scan parameters. The effective dose can be derived using both the ICRP-60 and ICRP-103 tissue-weighting coefficients for easy comparison. The CT organ doses calculated in this study for a GE and Siemens scanner can be used to estimate dose from other scanners using organ-specific adjustment factors or CTDIvol normalization factor to account for the different organ doses from different scanner types [6, 7]. Conclusion This set of new and comprehensive organ dose database is expected to improve both the accuracy and usability in CT dose reporting and tracking in the future. Combined with the additional knowledge that can be extracted from the DICOM file (the axial scan range, the scan pitch, the mAs, the kVp, patient age, gender, and weight) the software package can be a helpful tool in the management of CT dose. Acknowledgement This project was supported by the National Institute of Biomedical Imaging and Bioengineering (R42EB010404). References [1] ICRP 87: MANAGING PATIENT DOSE IN COMPUTED TOMOGRAPHY. (2001) [2] AAPM Report 96,"The Meas, Reporting, and Management of Radiation Dose in CT". (2008). [3] Liu HK, Gu J.W, Caracappa P.F and Xu X.G. Phys. Med. Biol. Vol.55, PP.1441-1451(2010). [4] ICRP 103: Recommendations of the ICRP ( 2008). [5] C. Lee, D. Lodwick, J. Hurtado, D. Pafundi, J. L. Williams, and W. E. Bolch, The UF family of reference hybrid phantoms for computational radiation dosimetry. Physics In Medicine And Biology, 2010. 55(2): p. 339-63. [6] Caracappa P, Gu J and Xu X Organ-Specific Adjustment Factors for Calculating Dose from Any CT Scanner Med. Phys. 37 3100(2010). [7] A. C. Turner, M. Zankl, J. J. DeMarco, C. H. Cagnon, D. Zhang, E. Angel, D. D. Cody, D. M. Stevens, C. H. McCollough, and M. F. McNitt-Gray, The feasibility of a scanner-independent technique to estimate organ dose from MDCT scans: Using CTDIvol to account for differences between scanners. Medical Physics, 37(4): p. 1816-1825(2010).