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Medical Physics |
1 From the Department of Radiology, School of Medicine, Gazi University, Besevler 06510, Ankara, Turkey (E.T.I., I.O., O.K., S.I.); the Departments of Physics (N.M.) and Engineering Physics (D.B.), Faculty of Science, the University of Ankara, Turkey. Received November 3, 1999; revision requested December 15; revision received March 16, 2000; accepted March 20. Address correspondence to E.T.I. (e-mail: erhanti@med.gazi.edu.tr).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: The radiation dose to the lens of the eye during balloon dacryocystoplasty (which includes pre- and postintervention dacryocystography) was measured in 10 consecutive patients by using thermoluminescent dosimeters on the lids of both eyes as close as possible to the lenses. A C-arm angiographic unit coupled with a digital imaging system was used, with similar exposure and geometric parameters in all cases.
RESULTS: The mean radiation dose to the lens of the treated eye was 4.6 mGy ± 2.2 (dose range, 1.9–9.1 mGy) and to that of the untreated eye was 38.5 mGy ± 17.5 (dose range, 14.7–67.8 mGy).
CONCLUSION: The lens of the untreated eye receives a higher dose than that of the treated eye because of its closer proximity to the x-ray tube in a lateral projection. In the lens, even the highest measured radiation dose (67.8 mGy) still was well below the deterministic threshold for lens opacity and cataract formation.
Index terms: Lacrimal gland and duct, 223.1295, 223.1296 • Lacrimal gland and duct, interventional procedures, 223.1295, 223.1296 • Radiation, exposure to patients and personnel, 223.1295, 223.1296
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The purpose of this study was to measure the absorbed radiation dose to the lens of the eye in balloon dacryocystoplasty and to evaluate the possibility of deterministic radiation effect on the lens.
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MATERIALS AND METHODS |
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Imaging of the LDS with digital subtraction dacryocystography was the first step of the procedure to determine the site and degree of obstruction in the patient with obstructive epiphora at presentation. It was also performed immediately after balloon dacryocystoplasty to verify the result. Digital subtraction dacryocystography in a posteroanterior and/or lateral projection was performed in all cases at a rate of one to two frames per second by using a 1024 x 1024 acquisition matrix; fluoroscopy was limited to positioning the patient’s head for each projection. Interventions using fluoroscopic guidance were performed in a lateral projection in all cases. In four patients, a posteroanterior projection also was used to aid in repeat canalization. The distance between the under-couch x-ray tube and the image intensifier was fixed at 65 cm for posteroanterior projections and was fixed at 76 cm for lateral projections. The image intensifier was placed as close as possible to the patient’s head. Fluoroscopy was performed in the pulse-progressive mode. For lateral projections, the image intensifier always was placed on the treated side of the patient. The system was operated at 80 kVp during fluoroscopy and at 85 kVp during digital image acquisition. Total x-ray beam filtrations were 2.77 and 2.83 mm aluminum for fluoroscopy and for digital image acquisition, respectively. Mean values for fluoroscopic milliampere and milliampere second settings per frame for digital image acquisition were 2 and 20, respectively. The total number of frames and the fluoroscopy time as determined from the timer of the control panel of the system were obtained. Exposure and geometric parameters were similar in each projection for all cases and were kept constant during diagnostic imaging and intervention.
Lithium fluoride thermoluminescent dosimeter (TLD) chips (3.7 x 3.7 x 0.9 mm) (Model 100; Harshaw Chemical, Solon, Ohio) in plastic handling packets (two per packet) were used to measure the radiation dose. TLD packets were placed on the eyelids below and above the superior and inferior margins of both orbits, respectively (Figure). All TLD measurements were obtained by using a model 4000 reader (Harshaw Chemical, Solon, Ohio). TLDs were calibrated by using an ionization chamber (Rad Check Plus; Victoreen, Cleveland, Ohio) in the same radiation beam, and the calibration factor was established to determine the local exposure to the patients’ TLDs. The mean of the exposures, measured by using four TLDs, was used to calculate the dose to the lens in each eye. Measured exposures were converted to absorbed doses by using the conversion factor f = 0.89(cGy · kg/mC) from the literature (13).
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RESULTS |
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Irradiation of the eye can cause damage to the proliferating cells in the anterior epithelium of the lens by free radical formation, oxidative effects, and permeability changes. These damaged cells and their breakdown products accumulate at the posterior pole of the lens and form subcapsular opacities, which are responsible for the local alterations in the index of refraction as characteristics of cataracts (10,14,15). The other parts of the eye are relatively less sensitive to radiation damage. The threshold for x-ray–induced cataract formation as a deterministic response in humans, which occurs after some delay, seems to be 2–10 Gy for acute exposure to low linear energy–transfer radiation (11,12,14–16). It is considerably higher for fractionated radiation and for chronic exposure (10,11,16).
Dose measurements in diagnostic radiologic procedures that involve the head often have been focused on the irradiation of the eye. The use of TLDs for radiation dose measurement in superficial organs or tissues such as the eye lenses is an accurate dosimetric technique (9). TLDs placed on both orbits were used to measure the radiation dose to both eye lenses in our 10 patients who underwent balloon dacryocystoplasty. During balloon dacryocystoplasty, mean absorbed radiation doses of 4.6 mGy ± 2.2 to the lens of the treated side and 38.5 mGy ± 17.5 to the contralateral lens were measured. Even the highest measured radiation dose of 67.8 mGy in the contralateral lens still was well below the deterministic threshold for visual impairment.
In the literature, there are some reports regarding the lens radiation dose in diagnostic x-ray examinations, but to our knowledge only a few reports are available regarding interventional procedures. In studies by Casselden (17), Mustafa and Janeczek (18), and Maclennan and colleagues (19), the mean absorbed doses to the lens of the eye during digital subtraction cerebral angiography were 3.27 mGy, 4.7 mGy, and 30.9 mGy, respectively. Galloway and colleagues (20) reported a 1.2-mGy mean radiation dose to the lens of the eye during digital subtraction dacryocystography. The use of eye shields and an oblique-lateral projection, which allows simultaneous high-quality bilateral dacryocystography with a lens radiation dose of less than 1 mGy, was described by Jackson and colleagues (14). It is also noteworthy that in diagnostic radiology, more commonly used modalities such as computed tomography of the cranium may result in an estimated radiation dose to the lens of the eye of 5–75 mGy, depending on technical parameters and on whether or not the orbit is within the primary beam (16,21,22).
Interventional radiologic procedures often require much more fluoroscopy time and radiographic image acquisition than diagnostic x-ray examinations (9,12,23,24). Furthermore, repeated interventions often are necessary; thus, accumulated radiation doses may be substantial. Repeated dilations also have been reported in the LDS (1–7). Berthelsen and Cederblad (16) measured doses to the eye of 22–139 mGy (mean dose, 90.4 mGy) in five cases during the embolization of intracerebral arteriovenous malformations. Bergeron and colleagues (23) measured the entrance skin dose in eight patients who underwent neurologic interventional procedures with TLDs placed directly on patients’ heads and noted the maximum dose as 1,335 mGy (mean dose, 615 mGy). Kuwayama and colleagues (24) reported transient alopecia in two patients who underwent endovascular neurologic interventions; in each patient, the measured dose at the temporal area was approximately 4.2 Gy.
We attempted transluminal balloon dilation in 142 LDSs in 128 patients, and dosimetric evaluation was performed in 10 consecutive patients after 95 dilation procedures in 89 patients. We think that, although the balloon dilation procedure has a steep learning curve, we measured the radiation dose after having obtained considerable experience. The radiation dose to the lens of the eye may be substantially higher in inexperienced hands.
In our technique of balloon dacryocystoplasty, fluoroscopic monitoring in a lateral projection, with the treated eye close to the image intensifier and with the contralateral eye close to the x-ray tube, was used during the passage of the guide wire through the obstructed LDS and for the correct positioning and the confirmation of the inflation of the balloon at the obstruction. In our opinion, the negotiation of the guide wire through the lumen of the LDS is the most important part of the procedure and is facilitated by using fluoroscopic guidance in a lateral projection but resulted in a higher radiation dose to the lens of the contralateral eye, which had a normal LDS. We believe that to avoid a higher radiation dose to the otherwise healthy eye, the treated eye can be placed close to the x-ray tube in a lateral projection, and the procedure still can be performed, with satisfactory fluoroscopic image quality.
It should be stated that, in our opinion, the use of a lateral projection is mandatory, as we failed in a few cases in which we attempted repeat canalization of the LDS under fluoroscopic monitoring in posteroanterior or oblique-lateral projections, which might reduce the dose to the lens. It should also be remembered that repeated and/or bilateral balloon dilations of the LDS might be necessary in some cases and might result in a higher dose to the lens. Moreover, due to several other diagnostic, follow-up, and interventional radiologic procedures that may be performed during the lifetimes of patients who have undergone dacryocystoplasty, the dose to the lens may be considerable.
The use of modern x-ray equipment, ideal techniques of fluoroscopic and digital imaging, collimation, protective eye shields, and the avoidance of unnecessary fluoroscopy and image acquisition are suggested for reduction of the dose to the lens. The limitation of fluoroscopy time is not practical during interventional procedures; instead, x-ray systems with the capability of low-rate pulsed fluoroscopy are preferred. In our opinion, the total number of frames obtained in our 10 patients during dacryocystography was too high; limitation of the number of frames obtained still can allow reliable evaluation of the LDS.
Eye shields and tight collimation are another alternative for the reduction of the dose to the lens unless they impede imaging and intervention in the LDS. For posteroanterior projections, it is possible to adjust the collimators so that the LDS remains in the smallest radiation field necessary while lenses are outside the primary beam. It is unfortunate that, because of overlapping in a lateral projection, it is difficult to work with a field size in which the LDS remains in the radiation field, but the lenses do not. We advise that digital image acquisition in a lateral projection be reserved for complex cases. Posteroanterior projection is sufficient for the evaluation of the LDS before and after the intervention, which results in a decrease in the total number of frames and, consequently, in a reduction of the dose to the lens. In response to the results of this study, we abandoned the use of a lateral projection for dacryocystography, and a lateral projection was used during only fluoroscopy for the intervention.
While no dose of ionizing radiation is safe, and any unnecessary lens irradiation should be avoided, even the highest measured radiation dose to the lens of the eye during balloon dacryocystoplasty with our technique is well below the deterministic threshold. Balloon dacryocystoplasty as a minimally invasive and reliable intervention has no more risk than diagnostic radiologic procedures in terms of cataract formation due to ionizing radiation exposure.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, E.T.I.; study concepts, E.T.I., N.M., D.B., S.I.; study design, E.T.I., D.B.; definition of intellectual content, E.T.I., N.M., D.B.; literature research, E.T.I., N.M., D.B., O.K.; clinical studies, E.T.I., I.O., O.K.; data acquisition, N.M., I.O.; data analysis, E.T.I., N.M.; manuscript preparation, E.T.I., N.M., D.B.; manuscript editing, E.T.I., D.B.; manuscript review, E.T.I., D.B., S.I.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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  K. E. Wilhelm, E. T. Ilgit, D. Bor, and N. Meric Radiation Dose in Fluoroscopically Guided Dacryocystoplasty Dr Ilgit and colleagues respond: Radiology, May 1, 2001; 219(2): 577 - 578. [Full Text]
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