Monte Carlo calculations of organ doses to patients

NRPB has carried out computer simulations of the passage of diagnostic x-rays through a patient's body (called Monte Carlo calculations) in order to calculate doses to the radiosensitive organs in the patient. The patient's body is represented by a mathematical phantom which describes the body outline and the internal layout of the organs in terms of simple geometrical shapes as shown in the picture below. Phantoms are available representing an adult patient and children of five ages - newborn, 1, 5, 10, and 15 years old.

Mathematical phantom for Monte Carlo
calculations of patient dose

The exposure of the phantom to x-ray beams typical of a wide range of examinations (including computed tomography) has been modelled. Organ doses and the effective dose have been calculated and related to the practical dose measurements routinely made outside the patient (for example, ESD and DAP).

A number of software reports, containing files of organ doses normalised to the appropriate measurable dose quantity, are available.

NRPB-SR250 for CT examinations on adults
NRPB-SR262 for conventional x-ray examinations on adults
NRPB-SR279 for conventional x-ray examinations on children

A further software report, containing normalised organ doses for computed tomography examiniations on children, is under development.

Effective doses normalised to the dose in free air on the axis of rotation of the CT scanner are found to increase as the age (size) of the patient decreases, as shown in the following figures.

CT scans of head and neck

CT scans of trunk

Normalised doses to newborn babies are seen to be about 2.5 times higher than those for adults for CT scans of the head and neck and between 1.4 and 2.3 times higher for scans of the trunk, depending on the type of scanner used. More detailed results are given in a published paper (Khursheed A, Hillier MC, Shrimpton PC and Wall BF (2002). Influence of patient age on normalised effective doses calculated for CT examinations. Brit. J. Radiol, 75, 818-30).

Last reviewed: 25 May 2010