ESR News February 2015
How to estimate patient doses from CT
John Damilakis, Professor of Medical Physics
The advent of multi-detector computed tomography scanners, offering fast image acquisition and improved image quality, has led to a significant increase in the use of CT. However, radiation dose associated with CT examinations and the potential of developing cancer due to radiation is an issue of great concern. Several methods have been described in the literature on the assessment of effective and organ dose from CT examinations.
Methods for estimating dose from CT examinations
Effective dose estimation using Dose Length Product (DLP)
Computed Tomography Dose Index (CTDI) has been the pillar of CT dosimetry for about 30 years. CTDI measurements are performed at the periphery and at the center of standard head (16cm in diameter) and body (32cm in diameter) polymethyl-methacrylate (PMMA) cylindrical phantoms using a 100mm long pencil ionization chamber. The weighted value of CTDI (CTDIw) is a weighted sum of CTDI values at the central and peripheral locations of the phantom and provides an indication of the average dose over the central slice of a series of contiguous slices. CTDIvol is calculated by dividing CTDIw by pitch. Thus, for spiral examinations with a pitch factor of 1.0, the CTDIvol is equal to CTDIw. DLP is calculated by multiplying the CTDIvol value by the scan length. There is a linear relationship between DLP and effective dose. DLP can be used for the estimation of the patient effective dose using body region-specific coefficients. These coefficients are published values of (Effective Dose)/DLP that can be used to convert calculated values of DAP into patient effective dose (1). Although this method is simple and quick, it has its limitations. Thus, if the CT scan is not a typical scan, for example, if it starts at a different anatomical level than a typical scan, considerable uncertainties are introduced. Moreover, the DLP method is not scanner-specific i.e. it uses a single value for all CT scanners. Therefore, this method provides only a rough estimate of effective dose and should be used with caution.
Monte Carlo Simulation
Several Monte Carlo codes have been successfully employed to assess radiation dose from CT examinations. Mathematical phantoms are used to represent the average patients during simulations. Over the last few years, a shift has been observed from simple mathematical phantoms to tomographic patient models. Patient-specific and scanner-specific Monte Carlo simulations have been used to accurately estimate radiation dose from CT examinations (2). Monte Carlo software packages can be used to a) develop voxelized models based on image data from patients who underwent MDCT examinations and b) calculate patient-specific radiation doses. Uncertainties associated with Monte Carlo methods include errors related to modelling, for example, the effect of patient table attenuation should be taken into consideration otherwise errors in effective dose of about 5% will result. Monte Carlo simulation is a powerful tool that allows accurate estimation of patient dose. However, it is time consuming and operators must have the required information to simulate CT scanners and the expertise to perform the experiments.
To calculate patient effective and organ doses, normalised dose data based on simulation measurements published in the literature can also be used (3,4). Moreover, commercially available software packages have been developed to calculate organ and effective dose based on pre-tabulated data derived from simulated CT exposures on phantoms.
Physical anthropomorphic phantoms and thermoluminescence dosimetry
Physical anthropomorphic phantoms representing adult patients or paediatric patients and thermoluminescent dosimeters (TLDs) have been used to measure patient doses from CT. These phantoms are usually cut into sections, which contain holes for the position of dosimeters. Accurate calibration of TLDs is very important before dose measurements. Instrumentation of thermoluminescence dosimetry includes an annealing oven to anneal all crystals before use and a TLD reader system to read the dosimeters. Verification of Monte Carlo simulation results is usually performed using this method.
References
1. Deak P et al. Multisection CT protocols: Sex- and age-specific conversion factors used to determine effective dose from dose length product. Radiology 2010, 257:158-166
2. Damilakis J et al. Radiation dose to the conceptus from multidetector CT during early gestation: A method that allows for variations in maternal body size and conceptus position. Radiology 2010, 257:483-489
3. Tzedakis A et al. The effect of z overscanning on patient effective dose from multidetector helical computed tomography examinations. Medical Physics 2005 32:1621-1629
4. Mazonakis M et al. Thyroid dose from common head and neck CT examinations in children: is there an excess risk for thyroid cancer induction? European Radiology 2007 17:1352-1357