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A New Inexpensive Customized Plaque for Choroidal Melanoma Iodine-125 Plaque Therapy
A New Inexpensive Customized Plaque for Choroidal Melanoma Iodine125 Plaque Therapy ANDREW K. VINE, MD,l RANDALL K. TENHAKEN, PhD,2 RICHARD F. DIAZ, MD,2 BARBARA B. MAXSON, DDS,3 ALLEN S. LICHTER, MD2
Abstract: The authors have developed a new inexpensive precious metal alloy plaque for use in customized iodine-125 plaque therapy. Each plaque is formed from two flat circular gold/palladium foils which are used in dental crown work. Using a simple manual mechanism, the two forms are stamped over a customized acrylic die shaped to the dimensions of the tumor base plus a 2-mm margin. Completed plaques consist of a back wall, a 2-mm side wall, and a 1.5-mm wide lip with holes for suture placement. Advantages include: simple construction from inexpensive components, customized shape, and iodine seeds that are readily visible on plane radiographs. Ophthalmology 96: 543-546, 1989
Small sealed sources of iodine-I 25 have radiation characteristics which make them attractive for use in treatment of intraocular tumors. The sealed sources emit low-energy photons which produce suitable dose deposition patterns for treatment of even large tumors but that are also easily shielded by thin layers of high atomic number metals for protection of operating personnel. Sealy et al,l Robertson et al,2 Packer and Rotman,3 and Packer4 have successfully used iodine-125 seeds mounted in lead or gold-backed plaques to treat choroidal malignant melanomas. The gold material on the posterior and lateral aspects of the plaque
Originally received: July 21, 1988. Revision accepted: December 28, 1988. Department of Ophthalmology, University of Michigan Medical Center, Ann Arbor. 2 Department of Radiation Oncology, University of Michigan Medical Center, Ann Arbor. 3 Department of Dentistry, University of Michigan Medical Center, Ann Arbor. 1
Presented as a poster at the American Academy of Ophthalmology Annual Meeting, Dallas, November 1987. Reprint requests to Andrew K. Vine, MD, Ocular Oncology Service, W. K. Kellogg Eye Center, University of Michigan, 1000 Wall St, Ann Arbor, MI 48105.
directs radiation to the tumor and reduces radiation exposure to the periocular tissues and surgeon's hands. The same basic principles are currently being used in the multicenter Collaborative Ocular Melanoma Study 5 sponsored by the National Eye Institute. Although the literature contains descriptions 3,4 for effective plaque design for iodine-I 25 therapy, there is very little available information concerning the actual plaque construction. We describe a simple process for constructing an inexpensive customized plaque for iodine-125 brachytherapy for choroidal malignant melanoma.
PLAQUE GEOMETRY The basic ocular geometry used to model the plaque is illustrated in Figure I. For a standard 24-mm diameter eye, the plaque lip with suture holes is designed to fit smoothly over the sclera with a 12-mm radius of curvature. For point sources, the dose drops off essentially as the inverse of the square ofthe distance from the source. To reduce scleral dose and also provide better depth-dose characteristics, the nearest surfaces of the iodine-I 25 seeds should be constrained to be at least 1 mm from the exterior scleral surface by filling the plaque with plastic. The seeds are 0.8 mm in diameter and 4.5 mm long. Therefore, a 543
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Fig 1. Basic geometry used to model the plaque for a standardized 24-mm diameter eye shows both dimensions (at right angles to each other) of the plaque and intraocular tumor. A drawing is made on graph paper for each plaque construction. The back wall of the plaque (8) has a radius of 14 mm. The seed centers (b) have a radius of 13.4 mm. The nearest seed surface to the sclera (c) has a radius of 13 mm. The external scleral surface (d) has a radi us of 12 mm and the internal scleral surface (e) has a radi us of II mm. The two dimensions of the melanoma are represented by the different shaded lines adjacent to the inner scleral surface.
FORMATION OF CUSTOMIZED ACRYLIC DIE
Fig 2. A 28.6-mm steel ball approximates the curvature of the back waU of the plaque. Autopolymerizing methylmethacrylate is used to make a negative impression of the steel ball. Denture wax is poured into the negative impression to make a wax impression which simulates the curvature of the steel ball.
Fig 3. The dimensions of the tumor base plus a 2-mm margin are drawn on the convex surface of the wax impression. The side walls are then carved vertically into the wax mold. After completion of the side waUs, a I.S-mm wide lip is carved into the wax mold. The indentation in the wax mold is designed to allow the plaque to be placed closer to the optic nerve.
plaque back wall with a 14-mm radius of curvature is required to both accommodate the dimensions of the seeds and hold the seeds I mm from the sclera. 544
A 28.6-mm steel ball is used to approximate the curvature of the back wall of the plaque. Autopolymerizing methylmethacrylate is used to make a negative impression of the steel ball. Denture baseplate wax is then poured into the negative impression to make a wax impression which simulates the curvature of the steel ball (Fig 2). This wax impression provides the curvature of the back wall of the plaque. The dimensions of the tumor base plus a 2-mm margin are drawn on the convex surface of the wax impression. The side walls (measured from diagrams such as in Figure 1 to provide a 2-mm central axis clearance) are then carved vertically into the wax mold. The height of the side wall is constantly checked by a small micrometer during this process. After completion of the side walls, a 1.5-mm wide lip is carved into the wax mold (Fig 3). A negative impression of the completed wax mold is made with vinyl polysiloxane impression material (Fig 4). The final plastic die is formed by pouring autopolymerizing methyl methacrylate into the vinyl polysiloxane impression material.
FORMATION OF PRECIOUS METAL ALLOY PLAQUE Each plaque is formed from two gold/palladium foils which are used in dental crown work (Fig 5). Using a simple manual mechanism, the two foils are stamped over the customized acrylic die to produce a thin metal alloy plaque (Fig 6). The formed plaque is heated over a butane burner to fuse the two forms. The elemental content of
VINE et aI
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CUSTOMIZED PLAQUE
Fig 4. A n~tive impression of the completed carved wax mold is made with vinyl polysiloxane impression material. The final plastic die is formed by pouring autopolymerizing methyl methacrylate into the negative impression.
Fig 6. The two gold/palladium foils readily conform to the curvatures of the plastic die.
Fig S. Two gold/palladium foils are placed over the plastic die which is held in a manual stamping mechanism.
the alloy plaque is approximately 60% gold diffused with 39% palladium. Small holes are drilled into the surrounding lip for suture placement and the lip is reformed to the correct curvature by pressing the plaque over a 25.4-mm diameter steel ball. The completed plaque is polished to remove any sharp edges (Fig 7). Individual iodine-125 seeds are arranged and bonded onto the inside back surface according to precalculated patterns. The seeds are then covered with clear autopolymerizing methyl methacrylate from the back wall to the lip. The curvature of the lip is maintained by the acrylic plastic by smoothing the plaque over a 25.4-mm steel ball as the acrylic hardens. The finished plaque may be sterilized in 2% glutaraldehyde or other commercially available cold sterilizing solutions.
Fig 7. Small holes are drilled into the surrounding lip for suture placement. The completed plaque is polished to remove any sharp edges.
DISCUSSION The described protocol requires approximately 2112 hours to construct a customized plaque ready for the insertion of iodine-125 radioactive seeds. The components of the acrylic die and plaque are inexpensive and readily available from dental suppliers. Each gold/palladium foil costs $18. The manual stamping mechanism can also be purchased at dental supply distributors. In contrast, a customized plaque from a dental supply house would cost approximately $500 to $1000. The two fused gold/palladium foils result in a finished thickness of 0.10 to 0.13 mm which corresponds to approximately ten half-value 545
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layers of attenuation for iodine-125 photons. Intraoperative measurements confirm that this precious metal alloy plaque almost completely attenuates posterior radiation from the iodine-125 seeds. The side walls are designed to limit lateral radiation and result in an anteriorly directed radiation field to the choroidal tumor. Variability of the height of the side walls in the completed plaque is less than 0.10 mm. Similarly, the variability of the side wall angles is minimal. The customized shape has allowed us to successfully treat peripapillary choroidal tumors by notching the plaque to allow approximation of the plaque to the optic nerve. The customized shape is also very useful in treating unusually shaped linear tumors. In these cases, a circular plaque results in a much larger mass that has to be sutured to the posterior sclera and subsequently covered with Tenon's capsule and conjunctiva. An additional advantage of this plaque is that the 3mm long silver wires within the sealed iodine-125 seeds are readily visible on plane radiographs (x-ray films) obtained postoperatively after plaque placement. These ra-
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diographs are used in confirming that the planned dose distribution has been achieved in practice. Other commercially available plaques attenuate x-radiation so significantly that individual seeds are difficult to visualize on x-ray films even with high kilovoltage.
REFERENCES I
1. Sealy R, Ie Roux PLM, Rapley F, et al. The treatment of ophthalmic tumors with low-energy sources. Br J Radio11976; 49:551-4. 2. Robertson OM, Earle J, Anderson JA. Preliminary observations regarding the use of lodine-125 in the management of choroidal melanoma. Trans Ophthalmol Soc UK 1983; 103:155- 60. 3. Packer S, Rotman M. Radiotherapy of choroidal melanoma with iodine125. Ophthalmology 1980; 87:582-90. 4. Packer S. lodine-125 radiation of posterior uveal melanoma. Ophthalmology 1987; 94:1621-6. 5. Earle J, Kline RW, Robertson OM. Selection of iodine 125 for the Collaborative Ocular Melanoma Study. Arch OphthalmoI1987; 105: 763-4.
Abstract: The authors have developed a new inexpensive precious metal alloy plaque for use in customized iodine-125 plaque therapy. Each plaque is formed from two flat circular gold/palladium foils which are used in dental crown work. Using a simple manual mechanism, the two forms are stamped over a customized acrylic die shaped to the dimensions of the tumor base plus a 2-mm margin. Completed plaques consist of a back wall, a 2-mm side wall, and a 1.5-mm wide lip with holes for suture placement. Advantages include: simple construction from inexpensive components, customized shape, and iodine seeds that are readily visible on plane radiographs. Ophthalmology 96: 543-546, 1989
Small sealed sources of iodine-I 25 have radiation characteristics which make them attractive for use in treatment of intraocular tumors. The sealed sources emit low-energy photons which produce suitable dose deposition patterns for treatment of even large tumors but that are also easily shielded by thin layers of high atomic number metals for protection of operating personnel. Sealy et al,l Robertson et al,2 Packer and Rotman,3 and Packer4 have successfully used iodine-125 seeds mounted in lead or gold-backed plaques to treat choroidal malignant melanomas. The gold material on the posterior and lateral aspects of the plaque
Originally received: July 21, 1988. Revision accepted: December 28, 1988. Department of Ophthalmology, University of Michigan Medical Center, Ann Arbor. 2 Department of Radiation Oncology, University of Michigan Medical Center, Ann Arbor. 3 Department of Dentistry, University of Michigan Medical Center, Ann Arbor. 1
Presented as a poster at the American Academy of Ophthalmology Annual Meeting, Dallas, November 1987. Reprint requests to Andrew K. Vine, MD, Ocular Oncology Service, W. K. Kellogg Eye Center, University of Michigan, 1000 Wall St, Ann Arbor, MI 48105.
directs radiation to the tumor and reduces radiation exposure to the periocular tissues and surgeon's hands. The same basic principles are currently being used in the multicenter Collaborative Ocular Melanoma Study 5 sponsored by the National Eye Institute. Although the literature contains descriptions 3,4 for effective plaque design for iodine-I 25 therapy, there is very little available information concerning the actual plaque construction. We describe a simple process for constructing an inexpensive customized plaque for iodine-125 brachytherapy for choroidal malignant melanoma.
PLAQUE GEOMETRY The basic ocular geometry used to model the plaque is illustrated in Figure I. For a standard 24-mm diameter eye, the plaque lip with suture holes is designed to fit smoothly over the sclera with a 12-mm radius of curvature. For point sources, the dose drops off essentially as the inverse of the square ofthe distance from the source. To reduce scleral dose and also provide better depth-dose characteristics, the nearest surfaces of the iodine-I 25 seeds should be constrained to be at least 1 mm from the exterior scleral surface by filling the plaque with plastic. The seeds are 0.8 mm in diameter and 4.5 mm long. Therefore, a 543
OPHTHALMOLOGY
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APRIL 1989
•
"1mm~
10:1 Scale
VOLUME 96
•
NUMBER 4
Fig 1. Basic geometry used to model the plaque for a standardized 24-mm diameter eye shows both dimensions (at right angles to each other) of the plaque and intraocular tumor. A drawing is made on graph paper for each plaque construction. The back wall of the plaque (8) has a radius of 14 mm. The seed centers (b) have a radius of 13.4 mm. The nearest seed surface to the sclera (c) has a radius of 13 mm. The external scleral surface (d) has a radi us of 12 mm and the internal scleral surface (e) has a radi us of II mm. The two dimensions of the melanoma are represented by the different shaded lines adjacent to the inner scleral surface.
FORMATION OF CUSTOMIZED ACRYLIC DIE
Fig 2. A 28.6-mm steel ball approximates the curvature of the back waU of the plaque. Autopolymerizing methylmethacrylate is used to make a negative impression of the steel ball. Denture wax is poured into the negative impression to make a wax impression which simulates the curvature of the steel ball.
Fig 3. The dimensions of the tumor base plus a 2-mm margin are drawn on the convex surface of the wax impression. The side walls are then carved vertically into the wax mold. After completion of the side waUs, a I.S-mm wide lip is carved into the wax mold. The indentation in the wax mold is designed to allow the plaque to be placed closer to the optic nerve.
plaque back wall with a 14-mm radius of curvature is required to both accommodate the dimensions of the seeds and hold the seeds I mm from the sclera. 544
A 28.6-mm steel ball is used to approximate the curvature of the back wall of the plaque. Autopolymerizing methylmethacrylate is used to make a negative impression of the steel ball. Denture baseplate wax is then poured into the negative impression to make a wax impression which simulates the curvature of the steel ball (Fig 2). This wax impression provides the curvature of the back wall of the plaque. The dimensions of the tumor base plus a 2-mm margin are drawn on the convex surface of the wax impression. The side walls (measured from diagrams such as in Figure 1 to provide a 2-mm central axis clearance) are then carved vertically into the wax mold. The height of the side wall is constantly checked by a small micrometer during this process. After completion of the side walls, a 1.5-mm wide lip is carved into the wax mold (Fig 3). A negative impression of the completed wax mold is made with vinyl polysiloxane impression material (Fig 4). The final plastic die is formed by pouring autopolymerizing methyl methacrylate into the vinyl polysiloxane impression material.
FORMATION OF PRECIOUS METAL ALLOY PLAQUE Each plaque is formed from two gold/palladium foils which are used in dental crown work (Fig 5). Using a simple manual mechanism, the two foils are stamped over the customized acrylic die to produce a thin metal alloy plaque (Fig 6). The formed plaque is heated over a butane burner to fuse the two forms. The elemental content of
VINE et aI
•
CUSTOMIZED PLAQUE
Fig 4. A n~tive impression of the completed carved wax mold is made with vinyl polysiloxane impression material. The final plastic die is formed by pouring autopolymerizing methyl methacrylate into the negative impression.
Fig 6. The two gold/palladium foils readily conform to the curvatures of the plastic die.
Fig S. Two gold/palladium foils are placed over the plastic die which is held in a manual stamping mechanism.
the alloy plaque is approximately 60% gold diffused with 39% palladium. Small holes are drilled into the surrounding lip for suture placement and the lip is reformed to the correct curvature by pressing the plaque over a 25.4-mm diameter steel ball. The completed plaque is polished to remove any sharp edges (Fig 7). Individual iodine-125 seeds are arranged and bonded onto the inside back surface according to precalculated patterns. The seeds are then covered with clear autopolymerizing methyl methacrylate from the back wall to the lip. The curvature of the lip is maintained by the acrylic plastic by smoothing the plaque over a 25.4-mm steel ball as the acrylic hardens. The finished plaque may be sterilized in 2% glutaraldehyde or other commercially available cold sterilizing solutions.
Fig 7. Small holes are drilled into the surrounding lip for suture placement. The completed plaque is polished to remove any sharp edges.
DISCUSSION The described protocol requires approximately 2112 hours to construct a customized plaque ready for the insertion of iodine-125 radioactive seeds. The components of the acrylic die and plaque are inexpensive and readily available from dental suppliers. Each gold/palladium foil costs $18. The manual stamping mechanism can also be purchased at dental supply distributors. In contrast, a customized plaque from a dental supply house would cost approximately $500 to $1000. The two fused gold/palladium foils result in a finished thickness of 0.10 to 0.13 mm which corresponds to approximately ten half-value 545
OPHTHALMOLOGY
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APRIL 1989
layers of attenuation for iodine-125 photons. Intraoperative measurements confirm that this precious metal alloy plaque almost completely attenuates posterior radiation from the iodine-125 seeds. The side walls are designed to limit lateral radiation and result in an anteriorly directed radiation field to the choroidal tumor. Variability of the height of the side walls in the completed plaque is less than 0.10 mm. Similarly, the variability of the side wall angles is minimal. The customized shape has allowed us to successfully treat peripapillary choroidal tumors by notching the plaque to allow approximation of the plaque to the optic nerve. The customized shape is also very useful in treating unusually shaped linear tumors. In these cases, a circular plaque results in a much larger mass that has to be sutured to the posterior sclera and subsequently covered with Tenon's capsule and conjunctiva. An additional advantage of this plaque is that the 3mm long silver wires within the sealed iodine-125 seeds are readily visible on plane radiographs (x-ray films) obtained postoperatively after plaque placement. These ra-
546
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NUMBER 4
diographs are used in confirming that the planned dose distribution has been achieved in practice. Other commercially available plaques attenuate x-radiation so significantly that individual seeds are difficult to visualize on x-ray films even with high kilovoltage.
REFERENCES I
1. Sealy R, Ie Roux PLM, Rapley F, et al. The treatment of ophthalmic tumors with low-energy sources. Br J Radio11976; 49:551-4. 2. Robertson OM, Earle J, Anderson JA. Preliminary observations regarding the use of lodine-125 in the management of choroidal melanoma. Trans Ophthalmol Soc UK 1983; 103:155- 60. 3. Packer S, Rotman M. Radiotherapy of choroidal melanoma with iodine125. Ophthalmology 1980; 87:582-90. 4. Packer S. lodine-125 radiation of posterior uveal melanoma. Ophthalmology 1987; 94:1621-6. 5. Earle J, Kline RW, Robertson OM. Selection of iodine 125 for the Collaborative Ocular Melanoma Study. Arch OphthalmoI1987; 105: 763-4.