Film dosimetry analyses on the effect of gold shielding for Iodine‐125 eye plaque therapy for choroidal melanoma

Wu, Andrew; Krasin, Frank

doi:10.1118/1.596556

Cited by 19 publications

(22 citation statements)

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“…Plaques are constructed of the gold alloy Modulay, which is 77% gold, 14% silver, 8% copper, and 1% palladium by weight. 11 Modulay, with a density of 15.8 g / cm 3 ͑obtained from http://www.jelenko.com/ENGLISH/alloyគspec/ MODULAY-BOOK.pdf, accessed 28 August, 2008͒, is less dense than pure gold ͑19.3 g / cm 3 ͒. The seed carrier insert is made of Silastic, which is 6.3% hydrogen, 24.9% carbon, 28.9% oxygen, 39.9% silicon, and 0.005% platinum by weight and has a density of 1.12 g / cm 3 .…”

Section: Methodsmentioning

confidence: 99%

“…The effect of the backing has been explored in the literature. [7][8][9][10][11][12] Though a small dose enhancement ͑relative to water͒ exists near the backing due to the emission of fluorescence photons by the gold alloy, the plaque's dominant effect is to decrease the backscatter of radiation, resulting in a significant dose reduction in regions further away from the plaque. With the half-value layer of 125 I radiation in pure gold of the order of 0.01 mm, there is very little transmission through the 0.5 mm backing.…”

Section: Introductionmentioning

confidence: 99%

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Monte Carlo dosimetry for and eye plaque brachytherapy

2008

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A Monte Carlo study of dosimetry for eye plaque brachytherapy is performed. BrachyDose, an EGSnrc user code which makes use of Yegin's multi-geometry package, is used to fully model 125I (model 6711) and 103Pd (model 200) brachytherapy seeds and the standardized plaques of the Collaborative Ocular Melanoma Study (COMS). Three-dimensional dose distributions in the eye region are obtained. In general, dose to water is scored; however, the implications of replacing water with eye tissues are explored. The effect of the gold alloy (Modulay) backing is investigated and the dose is found to be sensitive to the elemental composition of the backing. The presence of the silicone polymer (Silastic) seed carrier results in substantial dose decreases relative to water, particularly for 103Pd. For a 20 mm plaque with a Modulay backing and Silastic insert, fully loaded with 24 seeds, the dose decrease relative to water is of the order of 14% for 125I and 20% for 103Pd at a distance of 1 cm from the inner sclera along the plaque's central axis. For the configurations of seeds used in COMS plaques, interseed attenuation is a small effect within the eye region. The introduction of an air interface results in a dose reduction in its vicinity which depends on the plaque's position within the eye and the radionuclide. Introducing bone in the eye's vicinity also causes dose reductions. The dose distributions in the eye for the two different radionuclides are compared and, for the same prescription dose, 103Pd generally offers a lower dose to critical normal structures. BrachyDose is sufficiently fast to allow full Monte Carlo dose calculations for routine clinical treatment planning.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Monte Carlo dosimetry for and eye plaque brachytherapy

2008

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“…60 The film was positioned in a central plane of the eye plaque. They reported relative doses normalized to 5 mm depth (on central axis) from the inner sclera, as presented in isodose curve plots in a central plane.…”

Section: Iiia Literature Reviewmentioning

confidence: 99%

Dosimetry of ¹²⁵I and ¹⁰³Pd COMS eye plaques for intraocular tumors: Report of Task Group 129 by the AAPM and ABS

et al. 2012

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Dosimetry of eye plaques for ocular tumors presents unique challenges in brachytherapy. The challenges in accurate dosimetry are in part related to the steep dose gradient in the tumor and critical structures that are within millimeters of radioactive sources. In most clinical applications, calculations of dose distributions around eye plaques assume a homogenous water medium and full scatter conditions. Recent Monte Carlo (MC)-based eye-plaque dosimetry simulations have demonstrated that the perturbation effects of heterogeneous materials in eye plaques, including the gold-alloy backing and Silastic insert, can be calculated with reasonable accuracy. Even additional levels of complexity introduced through the use of gold foil "seed-guides" and custom-designed plaques can be calculated accurately using modern MC techniques. Simulations accounting for the aforementioned complexities indicate dose discrepancies exceeding a factor of ten to selected critical structures compared to conventional dose calculations. Task Group 129 was formed to review the literature; re-examine the current dosimetry calculation formalism; and make recommendations for eye-plaque dosimetry, including evaluation of brachytherapy source dosimetry parameters and heterogeneity correction factors. A literature review identified modern assessments of dose calculations for Collaborative Ocular Melanoma Study (COMS) design plaques, including MC analyses and an intercomparison of treatment planning systems (TPS) detailing differences between homogeneous and heterogeneous plaque calculations using the American Association of Physicists in Medicine (AAPM) TG-43U1 brachytherapy dosimetry formalism and MC techniques. This review identified that a commonly used prescription dose of 85 Gy at 5 mm depth in homogeneous medium delivers about 75 Gy and 69 Gy at the same 5 mm depth for specific (125)I and (103)Pd sources, respectively, when accounting for COMS plaque heterogeneities. Thus, the adoption of heterogeneous dose calculation methods in clinical practice would result in dose differences >10% and warrant a careful evaluation of the corresponding changes in prescription doses. Doses to normal ocular structures vary with choice of radionuclide, plaque location, and prescription depth, such that further dosimetric evaluations of the adoption of MC-based dosimetry methods are needed. The AAPM and American Brachytherapy Society (ABS) recommend that clinical medical physicists should make concurrent estimates of heterogeneity-corrected delivered dose using the information in this report's tables to prepare for brachytherapy TPS that can account for material heterogeneities and for a transition to heterogeneity-corrected prescriptive goals. It is recommended that brachytherapy TPS vendors include material heterogeneity corrections in their systems and take steps to integrate planned plaque localization and image guidance. In the interim, before the availability of commercial MC-based brachytherapy TPS, it is recommended that clinical medical physicists use...

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“…19 In the United States, the metal often used is a gold alloy (Modulay) consisting of 77% gold, 14% silver, 8% copper, and 1% palladium by weight. 20 Because the plaque is constructed in such a manner to minimize occupational exposures, there is little information in the literature regarding radiation doses to the surgeon's or assistant's hands. Indeed, the literature states that, for uveal melanomas, the extremity exposure would allow a surgeon to annually perform only 50 to 100 cases of plaque brachytherapy.…”

mentioning

confidence: 99%