British Journal of Radiology 75 (2002),685-688 © 2002 The British Institute of Radiology
Absorbed dose behind eye shields during kilovoltage photon radiotherapy
C R Baker, PhD
F Luhana, MSc
and
S J Thomas, MA, MSc
Medical Physics Department, Box 152, Addenbrooke's NHS Trust, Hills Road, Cambridge CB2 2QQ, UK
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Abstract
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The absorbed dose at the position of the lens of the eye under lead or tungsten eye shields during kilovoltage photon radiotherapy is critically dependent not so much on the thickness of the eye shield itself as on the size of the treatment field and the diameter of the shield used. Whilst dose from primary photons is easily attenuated to relatively insignificant levels by a few millimetres of lead or tungsten, scattered photons from outside the shielded area can provide over 25% of the prescribed dose. Since backscatter factors do not increase monotonically with photon energy, it is not safe to assume that the highest photon energy used will provide the highest dose. A simple method to estimate the dose under an eye shield based on tabulated backscatter factors is shown. Measurements under commercially available eye shields were made to verify the expression and to determine the attenuation of primary photons. Predicted and measured absorbed dose under the eye shields were found to agree to within 1% of the prescribed dose. The relative dose due to primary photons beneath the eye shields was found to be less than 0.1% and 0.5 (±0.1)% for the 150 kV and 260 kV beams, respectively. This is considerably less than the dose from backscattered radiation.
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Introduction
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The attenuation of kilovoltage photons from a Pantak DXT300 unit (Pantak Bramford, CT) through commercial tungsten eye shields (MT-T-45-S, M; Med-Tec Ltd, Orange City IA.UK supplier: Oncology Systems Limited) has been investigated for 150 keV and 260 keV beams (8 mm Al and 2.5 mm Cu half-value layers, respectively). These eye shields are designed for use in megavoltage electron beams and are recommended for use up to 9 MeV [1]. Each eye shield consists of 2.5 mm tungsten (composition by mass: 90% tungsten, 6% nickel, 2% iron, 2% copper) coated with 2 mm of dental acrylic on thebeam entrance side to reduce the dose to the eyelid from backscattered electrons (when used during electron treatments).
The eye shields were found to provide adequate attenuation of primary photons at both 150 kV and 260 kV. Measurement of primary transmission, however, is not sufficient to estimate the dose beneath the shields, which is due largely to photons scattered from the unshielded area of the treatment field. The degree of backscattering will depend both on the dimensions of the irradiated area and on the energy of the beam. Backscatter factors published by Grosswendt [2] indicate a maximum scatter contribution for beams with qualities around 8 mm Al (approximately 150 kV).
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Predicted dose
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For an eye shield placed at the centre of a treatment field, the dose received under the shield, Ds, relative to the dose at the same position in an unshielded beam, D0, can be estimated by: 
where p is the fraction of primary beam transmitted through the shield, s is the shield diameter and f is the diameter of the applied treatment field; BSF(s) and BSF(f) are backscatter factors corresponding to the dimensions and energy of the incident beam [2, 3]. The geometry considered is shown in Figure 1a.
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Figure 1. Beam geometries for the derivation of predicted dose. Du is the prescribed dose for a field of diameter f containing a shield of diameter s. D0 is the dose at the centre of the field in the absence of the shield, and Ds is the dose beneath the shield. (a) The simple case of a centrally located shield and (b) the extreme case of shielding at the field periphery.
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If p is small, the relative dose under the shield becomes simply:
If it is assumed that the primary photon fluence is uniform over the field dimension, the treatment dose received in the unshielded area, Du, relative to that in an open field, D0, will be given by:
where BSF' indicates that the point of interest does not lie on the centre of the unshielded field. This backscatter contribution to dose Du can be determined by sector integration [4].
Dividing 
Equation (1) by Equation (3) gives thedose under the shield, Ds, relative to the prescribed dose in the unshielded area, Du:
By a similar argument, for the geometry shown in Figure 1b the relative dose will be given by:
where BSF'' represents the backscatter contribution to a point on the field perimeter at the position of the shield.
If it is assumed that scatter contributions are independent of the position of the prescription point and the shield within the field, and that p is small, then Equations (4) and (5) may be replaced by a single expression:
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Measurements
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The geometry used to determine primary beam attenuation and to verify predicted dose under the eye shields, as expressed by Equation (1), is shown in Figure 2. Measurements were made using a PTW Markus chamber (collecting electrode diameter 0.53 cm) connected to a PTW Unidos E electrometer (PTW-Freiburg, Freiburg, Germany). For transmission measurements, a lead alloy collimator was used to reduce the fieldfrom a 2 cm diameter applicator to a 1 cm diameter field incident on a Perspex/polystyrene phantom, with the surface of the chamber at a depth of 5 mm. This geometry provides an estimate of the dose beneath the eye shield resulting from both primary photons and photons scattered from the eye shield, i.e. the geometry is between narrow and broad beam conditions. For measurements representing treatment conditions, the chamber was fitted with a waterproof Perspex cap, 0.86 mm thick, and placed in a water phantom positioned so that the top of the cap was level with the water surface. The 3 cm and 5 cm diameter applicators had a source-to-skin distance (SSD) of 30 cm, and the 10 x 10 cm2 and 20 x 20 cm2 applicators had a SSD of 50 cm. Stand-off was 4 cm and 7 cm for the 30 cm SSD and 50 cm SSD applicators, respectively. Two sizes of eye shield were used. The "small" eye shield had a diameter of 1.7 cm and the "medium" eye shield had a diameter of 2.2 cm.
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Figure 2. Measurement geometry (not to scale). The arrangement for determination of primary beam transmission is shown on the left. The set-up for measurement of dose behind the eye shields is shown on the right. The 3 cm and 5 cm diameter applicators had a source-to-skin distance (SSD) of 30 cm, and the 10 x 10 cm2 and 20 x 20 cm2 applicators had a SSD of 50 cm. Stand-off was 4 cm for the 30 cm SSD and 7 cm for the 50 cm SSD applicators.
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Results and discussion
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The dose due to primary photons beneath the eye shields was found to be less than 0.1% and 0.5 (±0.1)% for the 150 kV and 260 kV beams, respectively. This level of attenuation is more than adequate bearing in mind the much higher dose resulting from backscattered photons.
Predicted and measured dose beneath the shields placed on the beam central axis for fields sizes of 3 cm and 5 cm diameter, and 10 x 10 cm2 and 20 x 20 cm2 are shown in Table 1. Predicted dose was determined using Equation (1) for the field sizes projected from the end of the applicator to the water surface. For this simple geometry, agreement between predicted and measured dose was found to be within 1% in all cases. The higher doses obtained for the 150 kV beam are a result of the larger backscatter factors for this beam quality. For treatment fields larger than 5 cm in diameter, the dose under the shields is greater than 10% of the unshielded dose at 150 kV, rising to 25% or more for a 20 x 20 cm2 field.
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Table 1. Predicted (using Equation (1)) and measured percentage dose under small (1.7 cm diameter) and medium (2.2 cm diameter) eye shields relative to dose in the centre of the field in the absence of the shield. Stand-off was 4 cm for the 3 cm and 5 cm diameter applicators (30 cm source-to-skin distance (SSD)) and 7 cm for the 10 x 10 cm2 and 20 x 20 cm2 applicators (50 cm SSD)
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In practice, the relative dose given by Equations (4), (5) or (6) is required to give the fraction of dose prescribed to the treatment area that is received beneath the shield. The simple expression of Equation (6) is evaluated in Table 2 for the field sizes studied compared with predictions of Equations (4) and (5) where backscatter contributions were determined by sector integration for both geometries shown in Figure 1, assuming a uniform primary beam fluence. (BSF was averaged over 50 contributing beam segments between 0 and 
radians for both geometries in Figure 1.) Equation (6) is seen to provide dose estimates to within 2% of Equation (4) predictions (corresponding to the geometry of Figure 1a). As the shield is moved to the perimeter of the field, the scatter contribution is reduced, as reflected in the lower dose predictions of Equation (5). The simple expression of Equation (6) therefore provides a worst case estimate of dose for both geometries in Figure 1. Comparison of Table 1 predictions with Table 2 shows that only a small underestimate of dose, of the order of 2%, would be given by using the simpler expression of Equation (2).
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Conclusion
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Narrow beam transmission through shielding materials for kilovoltage photon energies predictably increases with photon energy. However, when predicting dose below shields partially obscuring the beam, the dose received is largely due to photons backscattered from the unshielded area of the beam, not from primary photons. An accurate estimate of dose beneath the shield can be made by a simple calculation using backscatter factors for shield and treatment field dimensions. The dose beneath shielded areas is largest where the backscatter contribution is greatest, which occurs for beam qualities of approximately 8 mm Al (150 kV), being dependent on field size as well as energy [2]. The eye shields investigated in this work, designed for use in megavoltage electron beams, have been found suitable for use in kilovoltage photon therapy.
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Footnotes
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Present address for C R Baker: Radiotherapy Division, Department of Allied Health Professions, The University of Liverpool, Liverpool L69 3GB, UK. 
Received for publication September 8, 2001.
Accepted for publication May 16, 2002.
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References
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- Shiu AS, Tung SS, Gastorf RJ, Hogstrom KR, Morrison WH, Peters LJ. Dosimetric evaluation of lead and tungsten eye shields in electron beam treatment. Int J Radiat Oncol Biol Phys 1996;35:599–604.[Medline]
- Grosswendt B. Dependence of the photon backscatter factor for water on source-to-phantom distance and irradiation field size. Phys Med Biol 1990;35:1233–45.
- Klevenhagen SC, Aukett RJ, Harrison RM, Moretti C, Nahum AE, Rosser KE. The IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al–4 mm Cu HVL; 10–300 kV generating potential). Phys Med Biol 1996;41:2605–25.[Medline]
- Redpath AT, Williams JR. Treatment planning for external beam therapy: principles and basic techniques. In: Williams JR, Thwaites DI, editors. Radiotherapy physics in practice (2nd edn). Oxford, UK: Oxford University Press, 2000.
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Abstract
Introduction
