Reduction of ocular lens dosage in dacrocystography

ClinicalRadiology(1989) 40, 615-618

Reduction of Ocular Lens Dosage in Dacrocystography A. JACKSON, M. P. H A R D C A S T L E , * A. SHAW'~ and W. W. GIBBON

Department of Diagnostic Radiology, The Medical School, Manchester, * Department of Diagnostic Radiology, Manchester Royal Infirmary, Manchester and 4fDepartment of Medical Physics, Christie Hospital, Manchester Irradiation of the lens of the eye in dacrocystography is largely avoidable. We have measured the lens dosage in bilateral dacrocystography in vivo and in a phantom. We describe simple protective measures which can reduce the lens dosage by over 95% to less than one mGy.

Several successful surgical procedures have been developed for the relief of epiphora due to lacrimal duct obstruction. The choice of the appropriate procedure in individual cases is dependent on both anatomical demonstration o f the site of obstruction and confirmation of the physiological significance of incomplete obstructions (Lloyd and Welham 1974; Campbell, 1964; Huninck et al., 1988). Isotope dacroscintigraphy has recently been shown to be a simple and precise method for determining the significance of a partially obstructed lacrimal duct (Brown et al., 1981; Rose and Clayton, 1985). Nevertheless dacrocystography remains the method of choice for demonstrating the anatomical site of any obstruction. Most authors now recommend the use of both techniques in the routine pre-surgical assessment of patients with epiphora (Chaudhuri, 1976; Rose and Clayton, 1985). Radiological investigations involving the orbit inevitably carry a risk of radiation exposure to the lens of the eye. The lens dose in dacroscintigraphy has recently been calculated at less than 6 m G y even in a completely obstructed system (Robertson et al., 1979). In dacrocystography the lens dosage is dependent on several factors including the number of exposures, the exposure factors and the degree of magnification. In fact, the irradiation of the lens in dacrocystography is incidental, and could largely be avoided (Fig. 1). It is the aim of the present study to measure the radiation dose to the lens of the eye in dacrocystography and to describe methods for its reduction by the use of simple techniques which could easily be applied routinely.

Thermoluminescent dosemeters (TLDs) in the form of lithium fluoride crystals in polythene handling packets were used to measure dosages. The equipment used for all exposures was a Siemens Orbix isocentric skull unit with a 0.6 mm focal spot. All exposures were made with a nominal magnification of 1.5. A standard radiographic head phantom was used for the measurement of ocular lens dosage under varying exposure conditions. T L D packets were taped on both eyes and remained in position throughout each series of exposures. Six series of exposures were made to assess the effect of various modifications in technique on the lens dosage involved. Groups 1 and 2 duplicated the film sequence involved in traditional dacrocystography using mento-occipital and straight lateral projections, where each lacrimal system is intubated and imaged separately. Group 3 reproduced the exposures involved in simultaneous bilateral dacrocystography using a mento-occipital and the lateral oblique projection described below. Groups 4, 5 and 6 involved the same sequence of exposures as group 3 but with protection of the eyes using the cones in the Orbix (group 4), the lead eye shields (group 5) or both (see Tables 1 and 2).

METHODS A skull phantom was constructed from a dry skull in which the positions of the cornea and lacrimal ducts were indicated by radio-opaque markers. The phantom was used to determine the optimal radiography projections for simultaneous demonstration of both lacrimal ducts and to design lead eye shields which provided complete coverage of the lenses in both radiographic projections. Eye shields were constructed from 3 mm lead sheet and were attached to a head band using Velcro. Separate shields were used in the mento-occipital and lateral oblique projections to provide optimal lens protection (see Fig. 2a, b). Correspondence to: Dr A. Jackson, Department of Diagnostic Radiology, North ManchesterGeneral Hospital, Delauneys Rd, Manchester M8 6RB.

Fig 1-Dacrocystogram using the 40° mento-occipital projection without eyeshields.The positionof the ocular lenshas beenmarkedby a white circleto illustratethe separationbetweenthe lensand the lacrimal system.

616

CLINICAL RADIOLOGY Table I - Projections used in each group of dose measurements performed on the head phantom. Radiographic details of the various projections are described in the text

Group

1& 2 3 4 5 6

Number of projections

Eye protectzon

AP

Lat

Oblique

5 3 3 3 3

4 0 0 0 0

0 2 2 2 2

None None Orbix cones Eye shields Both

Table 2 - Exposure values employed for each projection in each of the various exposure groups

Projection

Group

40"Mentooccipital

1

Lateral 30" Lateral oblique

(a)

2-6 1 2 3 6

kV

mAs

73

63

117 66 117 117

32 30 22 2

Eye dosages were measured in four patients undergoing bilateral dacrocystography. Prior to the procedure the eye shield head band was fitted but eye shields were applied only during the actual exposures. One per cent amethocaine eyedrops were instilled in each eye and the lower canaliculus was intubated bilaterally using polythene cannulae designed for sub-mandibular sialography (Meadox Surgimed A/S). The patient lay supine on the Orbix table with the anatomical base line and the median sagittal plane both vertical. The isocentre was positioned in the mid-line 1.5 cm caudal to the interpupillary line. For antero-posterior (AP) views the 40 ° mento-occipital projection described by Campbell (1964) proved optimal and a 30 ° right lateral oblique was employed as the other standard projection (see Figs 2 and 3). Exposures were made in each projection prior to and during injection of contrast (Niopam 370, Merck) and a further AP exposure was taken 5-10 minutes later to confirm complete drainage. Eye shields were used in all four patients. In two patients the cones in the Orbix were used in addition. T L D dosemeters were fixed on both sides of the eye shields to allow estimation of eye dosage both with and without eye shields. After the procedure the patients were warned of the risks of corneal anaesthesia. RESULTS

(b)

Fig 2 (a) and (b) showing the lead eye shields m place for AP and oblique projections respectively.

Use of the skull phantom confirmed the observation of Campbell (1964) that a 40 ° mento-occipital projection provides the minimum degree of foreshortening in the frontal plane. The use of an oblique lateral projection allowed separation of the naso-lacrimal ducts in the phantom with any angle of obliquity greater than 15°. Use of a 15° oblique projection, however, failed to fully separate the naso-lacrimal ducts in patients where significant naso-lacrimal duct distension was present. The use of the 30 ° oblique lateral projection overcame this problem

LENS IRRADIATION

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IN DACROCYSTOGRAPHY

Table 4 - The effect of using various forms of eye protection during bilateral dacrocystography in four patients Patient

Eye

Without eye protection

With orb& cones

With eye shields

With both

1

R L R L R L R L

----5.5 6.5 8.5 9.5

1.5 1.8 1.9 1.4 -----

----1.3 0.9 1.4 0.9

0.6 0.9 0.5 0.6 -----

2 3 4

reductions in eye dosage were found when maximum available kilovoltage was employed (group 2), when simultaneous bilateral dacrocystography was performed (group 3) and when tight collimation of the primary beam (group 4) or eye shields were used. The use of all these measures simultaneously (group 6) resulted in a reduction of the measured eye dose of 97% for bilateral dacrocystography. The results of the in vivo measurements in four patients are shown in Table 4. These measurements confirm that the use o f lens protection allows routine bilateral dacrocystography to be performed with ocular lens dosages of less than one mSv.

(a)

DISCUSSION

(b) Fig. 3 Bilateral dacrocystogram u s i n g e y e s h i e l d s . (a) 40 ° m e n t o o c c i p i t a l p r o j e c t i o n u s i n g s u b t r a c t i o n t e c h n i q u e . (b) 30 ° r i g h t l a t e r a l oblique projection. Bilateral subtotal obstruction of both lacrimal ducts a t t h e level o f t h e m e d i a l p a l p e b r a l l i g a m e n t is s h o w n .

even when both ducts were significantly dilated (Fig. 3a, b). The individual exposures and exposure parameters used for the phantom studies are shown in Tables 1 and 2. Table 3 shows the measured dose equivalents to the eye in each of the phantom measurement groups. Significant

Table 3 - Individual eye dosages recorded in each group using the head phantom Group

One Two Three Four Five Six

Measured dose equivalent (mSv) Right eye

Left eye

31.8 21.9 24.1 9.6 2.1 0.7

26.5 20.3 12.5 7.9 1.9 0.8

Simultaneous bilateral dacrocystography must demonstrate both naso-lacrimal ducts on a single lateral or oblique radiograph and the original description of the technique (Campbell, 1964) relied on the use of high kilovoltage to allow distinction between superimposed ducts on a straight lateral projection. The use of a 15° oblique lateral projection to allow separate demonstration of the ducts has recently been described (Lewis, 1988). In the current study we found a 30 ° oblique projection to be more satisfactory for ensuring complete separation of the duct systems when one or both are dilated. The standard technique of dacrocystography involves irradiation of the lens of the eye even though this lies outside the area of interest on the images obtained. As we have shown this dose can be markedly reduced by simple measures and there is no reason not to routinely employ eye protection in dacrocystography. Irradiation of the eye can cause damage to the proliferating cells in the anterior epithelium of the lens. These damaged cells and their breakdown products accumulate at the posterior pole of the lens forming dot like sub-capsular opacities (Merriam et al., 1972). The remainder of the eye is relatively insensitive to radiation damage (Rubin and Casarett, 1968). Occupationally induced cataracts have been seen after only 0.7-1.0 Gy of mixed gamma radiation in cyclotron physicists (Ham, 1953) and at similar levels in other radiation workers (Lvovskaya, 1974). However, doses of 1.5-4.0 Gy of low linear energy transfer (LET) radiation accumulated over 10 to 20 years have been documented without causing vision impairing cataract (Lvovskaya, 1974, 1976). Although cataract formation is a non-stochastic effect, the lens appears to be more susceptible to damage from protracted exposures than other organs (Merriam et al.,

618

CLINICAL RADIOLOGY

1972; U p t o n , 1987). The threshold for cataract formation from occupational exposure to protracted low L E T radiation is estimated to be between 8-10 G y ( I C R P 41, 1984). On this basis the current r e c o m m e n d e d dose equivalent limit for the eye in occupationally exposed workers is 0.15 Sv/year ( I C R P 41, 1984). However, M e r r i a m et al. (1972) suggested that doses as low as 150 c G y could, if accumulated over a short period o f time be sufficient to cause cataract formation. It is apparent that the radiation dose to the eye in d a c r o c y s t o g r a p h y is relatively small, even without eye protection. Nevertheless, m a n y patients will undergo d a c r o c y s t o g r a p h y as part o f a series o f investigations which m a y involve isotope dacroscintigraphy, facial radiographs, t o m o g r a p h y or C T scanning and postoperative dacrocystography. These patients m a y accumulate a significant lens dosage over a relatively short period and any unnecessary lens irradiation should certainly be avoided in keeping with the A L A R A principle ( I C R P 26, 1977). The use o f a carefully collimated, high kilovoltage primary beam combined with eye shields allows significant reductions in lens dosage without compromising the diagnostic quality o f the images. We r e c o m m e n d the routine use o f these measures in dacrocystography. In addition the use o f an oblique lateral projection allowing simultaneous bilateral d a c r o c y s t o g r a p h y provides further reduction in lens doses when bilateral studies are required. In s u m m a r y the routine use o f the protective techniques described here enables the p r o d u c t i o n o f high quality bilateral dacrocystograms with a radiation dose o f less than 1 m G y to the lens o f the eye. This represents approximately 3 % o f the lens dosage using conventional techniques.

REFERENCES

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