Influence of Patient Size on Organ Doses in Diagnostic Radiology

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Abstract
Since organ doses are very difficult to measure in diagnostic radiology, they are mainly determined by means of Monte Carlo calculations using anthropomorphic phantoms. These calculations provide so-called conversion factors from measurable quantities (such as air kerma free-in-air at a reference point or entrance dose) to the organ doses of interest. These can then be derived by multiplying the dose conversion factors by the measured values of the corresponding reference quantity. However, in reality there are limitations mainly due to the great variation in patient size. Strictly speaking, these conversion factors can only be applied to the 'standard person' having the same body dimensions and masses as the respective phantom. This is true for adults and children regardless of the phantom, which can be MIRD type like the MIRD-5 phantom, the adult GSF phantoms ADAM and EVA and the child phantoms of Cristy or voxel-type like the paediatric GSF phantoms BABY and CHILD that are based on computed tomographic (CT) data. As there is no complete set of phantoms of all body forms in every age group available, one has to use approximations. Using the voxel-phantom BABY a simple method is presented to vary the body dimensions by changing the voxel size in one or more dimensions. Simulating four common examinations in paediatric radiology it is shown that only the dimension in the direction of the irradiaton (in the present study this is the AP diameter) has a relevant effect on the organ dose conversion factors. The size of the influence is very different for the various organs, depending mainly on the type of examination, the photon energy (the tube voltage), the position of the organ in the relevant direction and the percentage of the organ being directly irradiated.

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