Dosimetry for the Lens of the Eye, Applications for Medical Staff Involved in Interventional Radiology Procedures

Sammanfattning: "Background: Cataract (opacity of the lens of the eye) has always been thought of as a deterministic effect. That is, that there is a threshold dose below which damage does not occur. However, recent studies suggest a lower threshold and the question whether there is any threshold at all has been raised. One situation that leads to special concern, if this turns out to be the case, is during interventional radiology (IR) and cardiology procedures where the medical staff, being exposed to scattered radiation from the patient, can receive relatively high absorbed doses to their eyes. Aim: When the dose to eye is measured it is usually done by placing a dosimeter at the side of the head near the eye. The aim of this study is to investigate how good an approximation this is by means of experimental studies and Monte Carlo calculations. Another aim is to study the distribution of absorbed dose inside the skull including that to the lens, and to investigate if there is any relationship between the output from the x-ray tube and the absorbed dose to the lens. Material & Methods: Measurements were carried out with a thorax phantom placed on an operating table and a head phantom placed where a physician would be expected to stand. The head phantom contained TLDs and Gafchromic film. The measurements were performed both with and without protective goggles. Measurements were also performed on two senior physicians wearing headbands with TLDs during procedures. The simulations were made in the Monte Carlo program PENELOPE. The geometry contained a detailed head, and eyes, and a patient lying on the operating table. Results and Discussion: The phantom measurements show that the absorbed dose to the lens is higher than that at the forehead for both eyes. This means that a dosimeter at the forehead underestimates the lens dose. A clear relationship between lens dose and DAP value could also be seen. This is not as pronounced in the measurements on the physicians although there is still a relation. When protective goggles were used in the phantom measurements a reduction of the absorbed dose to the lens could be seen for the right eye. However, film measurements show that radiation can slip through the goggles in some angles. The simulations show that the absorbed dose in the lens is lower compared to the absorbed dose at the forehead, i.e. a dosimeter at the forehead would overestimate the lens dose. From the simulations there could be seen that the energy of the radiation reaching the lens have a small but pronounced shift towards lower energies, compared to the energy leaving the x-ray tube. Conclusions: Dosimeters used for measuring the dose to the lens of the eye may underestimate the dose with up to 25 %. This indicates that a better way of estimating the dose to the lens may have to be found. It seems to be a relationship between dose to the patient and dose to the physicians’ eyes and perhaps a factor could be found so that estimations can be made without specific point dose measurements. The film measurements show that the design of protective goggles has to be improved, so that no radiation can slip through at any angle."