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Letters to the Editor |
Department of Radiology, University Hospital Bonn, Sigmund-Freud-Strasse 25, D-53127 Bonn, Germany; e-mail: wilhelm@uni-bonn.de
Editor:
My colleagues and I read with great interest the article by Dr Ilgit and colleagues in the October 2000 issue of Radiology (1) concerning the absorbed radiation dose to the ocular lens during fluoroscopically guided dacryocystoplasty. In this article, the authors determined that the mean dose to the lens of the treated eye was 4.6 mGy and to that of the untreated eye, 38.5 mGy.
Dr Ilgit and colleagues may not have been aware of a previous study (2) of this problem performed by our group and whose results appeared in a German-language journal in 1998. In addition to dose measurements in the ocular lens, we also measured the dose to other radiosensitive head and neck tissues, including the parotid and thyroid glands, during dacryocystography and fluoroscopically guided dacryocystoplasty. Our dacrocystoplasty technique was further described in an article that appeared in Radiology (3), with accompanying editorial (4), in August 1999. The radiation doses were measured in 13 consecutive patients and also in a phantom model. Our results revealed much lower doses to the lens than those in the study by Dr Ilgit and colleagues, despite apparently comparable measurement techniques. Their reported radiation dose to the lens of the treated eye was three and one-half times higher than our measurement and seven times higher than that of the contralateral untreated lens.
The difference in the measured dose to the treated lens and some of the difference to the untreated lens may be accounted for by the greater number of digital images obtained by Dr Ilgit and colleagues during the procedure. However, the larger discrepancy between the measurements of the dose to the untreated lens cannot be explained on this basis alone. We believe that this discrepancy may be related to differences in procedural technique, specifically the beam collimation used during the procedure. Figure 1a in the article by Dr Ilgit and colleagues is a posteroanterior image that shows the focus of the beam to be centered on the nasal septum during dilation. With this technique, the contralateral lens is also directly exposed to the beam during the intervention. All of our procedures and measurements were performed by using tight beam collimation to the eye being treated during posteroanterior fluoroscopy and digital image acquisition. Findings from our phantom studies showed that the dose to the untreated lens was only approximately 25% of the dose to the treated lens when the procedure was performed with collimation during posteroanterior imaging. Because much of the interventional procedure is performed by using lateral fluoroscopy and digital image acquisition, some radiation dose to the contralateral lens cannot be avoided.
Although Dr Ilgit and colleagues conclude that their measured dose is well below the threshold dose for lens opacity and cataract formation, we believe that every effort should be made to reduce the applied radiation dose. The addition of beam collimation to the eye being treated during posteroanterior imaging can significantly reduce the overall dose to the treated and untreated lens and to other radiosensitive head and neck tissues.
REFERENCES
and
Niyazi Meric, PhD
Department of Radiology, School of Medicine, Gazi University, Besevler 06510, Ankara, Turkey* e-mail: erhanti@med.gazi.edu.tr; Department of Engineering Physics, Faculty of Science, University of Ankara, Turkey
We appreciate the comments of Dr Wilhelm regarding our article (1), in which we conclude that even the highest measured radiation dose to the lens of the eye during balloon dacryocystoplasty is well below the deterministic threshold and that balloon dacryocystoplasty, as an intervention, has no more risk than that of diagnostic radiologic procedures. In his letter, Dr Wilhelm compares our results with those of his studies that appeared in a German-language journal (2). We were not aware of this study, since it was not cited in our literature search, which was mainly in English.
Although the balloon dacryocystoplasty procedures are similar and radiation dose measurement techniques are comparable (1–4), our results revealed that the radiation dose to the lens of the eye was three to seven times higher than their measurements. In these two articles (1,2), the mean dose values to the lens of the eye closer to the image intensifier was 4.6 and 1.37 mGy, and to the contralateral lens (closer to the x-ray tube), 38.5 and 5.43 mGy. The treated eye was always close to the image intensifier in all our patients.
We agree with Dr Wilhelm that every effort should be made to avoid unnecessary lens irradiation, since no dose of ionizing radiation is safe (mentioned in the discussion section of our article). We also believe that beam collimation is an important factor and should be used in posteroanterior projection to reduce the lens dose.
In our phantom study (Ilgit E, Meric N, Bor D, unpublished data, 1997), dose measurements revealed that approximately 90% of the overall radiation dose to the lens of the untreated eye (which is closer to the x-ray tube) results from lateral projection. For the treated contralateral lens, this ratio is approximately 65%. This information shows that posteroanterior projection has an apparently small effect on the overall radiation dose, thus the sole use of beam collimation for this projection cannot reduce it significantly. However, decreasing the fluoroscopy time and the number of digital images in lateral projection is the most important factor for reducing the lens dose. Unfortunately, fluoroscopy in lateral projection, which results in direct exposure of the lens of the closer eye, is mandatory for the intervention. After the dosimetric study, we abandoned digital image acquisition in lateral projection, which we believe to be the major source of higher lens dose, to reduce the lens dose. Now we are also using tight collimation in posteroanterior projection.
REFERENCES
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