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Correspondence |
With regard to the paper "The effect of source to image-receptor distance on effective dose for some common X-ray projections" by Poletti and McLean [1], published in the September 2005 issue of this journal, the authors have performed the work competently and have presented some valuable results. There are, however, some points we would like to raise.
We believe that the description of the dose reductions demonstrated in the paper as "modest" undervalues to an extent the contribution that increasing the focus to image receptor distance can have on optimization of radiation doses; especially in light of their findings. In their paper, reductions of up to 46% and 68% were shown for thoracic exposures following consistent collimation to a central region of interest and the entrance surface, respectively, with increased distance. Whilst we accept that the latter measurement is clinically unimportant, since the structure of interest is rarely positioned at the entrance surface, we believe that the reductions shown at the central region of interest are impressive and compare very favourably with other, more expensive methods of dose-reduction. Admittedly, when collimation was fixed to the image receptor, little or no reduction was shown at increased distances. However, under optimized radiographic protocols, this is of no clinical relevance since a region of interest can never be coincident with the image receptor. The data provided therefore demonstrate that good radiographic practice is critical in that the level of collimation and entrance surface markings employed for a specific body part should change depending on the position of the structure along the anteroposterior axis and the distance between the focus and the image receptor (unless the region of interest is at the entrance surface, which as stated above is highly unlikely). In practice this appears to be rarely done.
On a separate note, the authors highlight the importance of theoretical studies to examine a topic of this nature and describe some of the problems encountered in experimental verification of theory. We acknowledge that computer simulation can generate an array of valuable data and presents answers to difficult scientific questions in a very precise way, but rarely do patients demonstrate such a high level of precision. If dose-reducing measures are going to be useful clinically (which presumably is the main reason for doing these types of investigations), a thorough understanding of the effect of specific measures on patients of varying size, state of health, gender and so on must be established before widespread implementation. Mathematical modelling alone can rarely achieve this.
The data presented by Poletti and McLean are important and of much use to the radiographic and radiological communities. We believe that their results, like those of some previous workers [2], have shown that increasing source to image receptor distance can reduce stochastic radiation risk for common X-ray examinations, and further studies investigating more widespread usage should be encouraged.
Yours etc.,
School of Medicine and Medical Sciences, University College Dublin, Belfield, Dublin 4, Ireland
References
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