K-Factors for Overweight and Obese Patients in CT Examinations as a Function of Body Mass Index
Y Gao1*, A Ding2, P Caracappa3, X Xu4, (1) Rensselaer Polytechnic Institute, Troy, NY, (2) Rensselaer Polytechnic Institute, Troy, NY, (3) Rensselaer Polytechnic Institute, Troy, New York, (4) Rensselaer Polytechnic Inst., Troy, NYMO-D-134-1 Monday 2:00PM - 3:50PM Room: 134
Purpose: To generate Dose Length Product to Effective Dose conversion factors (k-factors) for a series of overweight and obese patients and to establish a functional form for k-factors in relation to body mass index.
Methods: Using the Monte Carlo code MCNPX 2.5.0 and a validated model of the CT source, CT scans were simulated on a series of overweight and obese adult male and female phantoms with fixed height and body mass index (BMI) ranging from 23 to 45. For several CT scan protocols simulations generated the organ doses and effective doses using ICRP 103 tissue weighting factors. The k-factors were then derived using the dose length product (DLP) for each protocol and the effective dose from pairs of phantoms with the same BMI. For each protocol, a regression analysis was performed for k-factors to BMI.
Results: The calculated k-factors showed a consistent trend of decreasing with increasing BMI for each of the protocols. In the pelvic scan protocol, for example, the k-factor for patients with a BMI of 45 is 42% of that for an average sized individual. The k-factor equations in terms of BMI fit an exponential function regression for all protocols with R-squared values greater than 0.99 in every case.
Conclusion: Where patient height and weight are available, the BMI-dependent k-factor equations can provide more accurate effective dose estimation for overweight and obese patients. While they do not provide the patient-specific dose information that is the purpose of some modern CT dose tracking software, they provide easy access to more appropriate risk estimation and a better metric for performance tracking of a diverse patient population than DLP alone.
Funding Support, Disclosures, and Conflict of Interest: One member was funded by the 2012-2013 J. Newell Stannard Fellowship Award from Health Physics Society.
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