Answer to Question #9800 Submitted to "Ask the Experts"

Category: Radiation Basics

The following question was answered by an expert in the appropriate field:


I want to use the Monte Carlo dosimetry package sponsored by ImPact Scan. The narrative on the website says we have to purchase the NRPB SR-250 in order to use the dosimetry system.

Can you shed any light on this?

I have not had the occasion to use the National Radiological Protection Board (NRPB) ImPACT software, but as I understand it, the software allows calculations of specific dose quantities, perhaps most notably effective dose, resulting from particular CT (computerized tomography) scans. The effective dose requires knowledge of the absorbed doses to all the significantly irradiated organs. The respective organ absorbed doses are converted to equivalent doses by multiplying by the radiation weighting factor, which is 1.0 for x rays, and by the respective tissue weighting factor for the specific tissue/organ. The tissue weighting factors are available in International Commission on Radiological Protection (ICRP) documents (either ICRP Publication 60 or the more recent ICRP Publication 103, depending on which reference is being used). The weighted equivalent doses are then summed to obtain the effective dose.

The ImPACT software does not itself generate the dose distribution that results from a particular CT procedure. Rather, it uses the dose-distribution results that were obtained separately and are available as NRPB-SR250: Normalised Organ Doses for X-Ray Computed Tomography Calculated Using Monte Carlo Techniques, the source that you mention. This electronic data source, available on a small disc, contains the normalized doses to all irradiated tissues from a number of procedures. The description available on the United Kingdom Health Protection Agency Order page states: "The computer data package contains the result of 23 series of Monte Carlo calculations, described in an NRPB report. These model the conditions of exposure for a range of common makes of CT scanner. Data for each series of calculations are provided as separate data files in compressed form on a 3.5-inch microdisk for use with computers operating under PC/MS-DOS. When expanded, each data file contains 208 sets of normalised dose data for 27 organs or regions of a mathematical phantom, corresponding to irradiation under specific CT conditions of every 5-mm-thick transverse slab of the phantom. Full details of the format of the normalized organ dose data are given, together with details of how these data can be used in assessing typical patient doses from individual CT scanners. A useful and freely available specific tool for the manipulation of these CT dose coefficients has been developed by ImPACT (the UK CT evaluation centre)."

The newest version 1.0.4 of the ImPACT software is available through the link directly above in the quotation. This allows calculations of effective dose consistent with recommendations of ICRP Publication 103 as well as ICRP Publication 60. The software includes appropriate input information for a wide variety of different CT scanners and uses the normalized doses from the NRPB-SR 250 data to calculate absolute doses to affected tissues and effective doses from specific scans.

Good luck in your pursuits.

George Chabot, PhD, CHP

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