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Plaque Radiotherapy of Retinoblastoma
Letters to the Editor
Plaque Radiotherapy of Retinoblastoma Dear Editor: The article by Shields et al entitled, "Plaque Radiotherapy in the Management of Retinoblastoma" (Ophthalmology 1993;100:216-24), presents the authors' experience with single brachytherapy treatments for primary and recurrent retinoblastomas. The authors lump their experience with four different types of radioactive plaque sources (cobalt 60, iridium 192, ruthenium 106, and iodine 125) and treat them as relative equivalents. They state that, "The isotope selected in each case was determined (solely?) by the radiation oncologists in an effort to attain optimal dose distribution depending on tumor size, vitreous seeding, and tumor location." The readership would have benefited more from the authors' data on doses to critical ocular structures, or at least some reference points outside the targeted zone. These data could be used to demonstrate how these choices were made. Then, in an apparent change in philosophy, the authors point out that they have come to use iodine 125 plaques exclusively, "because of its radiation safety advantages and ease for custom design." Were there any data to support this switch? There is ample literature supporting the use oflow-energy isotopes for the treatment of intraocular tumors.':" In the Discussion section, the authors speculate that the use of ophthalmic plaques will decrease irradiation of normal orbital structures, thereby decreasing the incidence of secondary, radiation-induced cancers. Their "measurements" oforbital irradiation range from "undetectable to 200 cGy." How were these doses determined? Did the authors consider that the dose near to certain radioactive plaques (in those small baby orbits) could be significantly higher than the alternative (35-40 Gy) delivered by external beam radiotherapy? Last, the authors have previously described patients treated with multiple "rotating plaques," at the Oncology Service at Wills Eye Hospital. What happened to them? PAUL T. FINGER, MD SAMUEL PACKER, MD New York, New York References I. Packer S. Low energy isotopes for the treatment ofposterior choroidal melanoma. In: Bornfeld N, Gragoudas ES, Hopping W, eds. Tumors ofthe Eye: Proceedings ofIntemational Symposiums in Geneva, 1987, and Essen, 1989. Amsterdam: Kugler, 1991;399-405. 2. Packer S, Stoller S, Lesser ML, et al. Long-term results of iodine 125 irradiation of uveal melanoma. Ophthalmology 1992;99:767-74.
3. Finger PT, Moshfeghi DM, Ho TK. Palladium 103 ophthalmic plaque radiotherapy. Arch Ophthalmol 1991;109:1610-13.
Authors'reply
Dear Editor: Drs. Finger and Packer point out interesting questions relative to the use of plaque brachytherapy, although some of these points have been previously published and were outside the intended scope of our study. I Our selection of specific isotopes for plaque brachytherapy has evolved over time as we have recognized their inherent advantages and disadvantages. We initially used cobalt 60 based on Stallard's' pioneer work and favorable results treating retinoblastoma. Because cobalt's high energy resulted in higher tissue doses and personnel exposure, it was jointly decided by the ophthalmology and radiation oncology teams to evaluate other isotopes to replace cobalt 60. Ruthenium 106 and iridium 192 were both used temporarily before it was decided that iodine 125 offered the most advantages. Subsequently, we used iodine 125 in most of our patients. We have previously analyzed the specific advantages and disadvantages of the different isotopes used in plaque brachytherapy.' We currently use iodine 125 because of its radiation safety advantages and ability for custom design. There is abundant information to support this. For example, due to the high energy ofcobalt 60 (1.25 MeV), a lead thickness of approximately 11 mm would be required to decrease the radiation dose to 50%. In contrast, iodine 125 is shielded to less than 0.1 % of the total dose using a sheet of gold only 0.5 mm in thickness (the half-value layer of gold is 0.025 mm). It is therefore evident that the dose of radiation outside of the plaque (which is covered by the gold shell) is negligible. This has been confirmed by other investigators including Packer and associates" who have stated about iodine 125 plaques: "the absorption properties of gold are known and therefore the dose posteriorly and laterally is known to be negligible." "Iodine 125 irradiation of choroidal melanoma results in a lower radiation dose than with Cobalt 60, Ruthenium 106, Iridium 192, . . . or proton beam adjacent to the tumor," and "the shielding and directional radiation minimizes radiation to the optic nerve and delivers less radiation to the eye compared to other radiation sources (Cobalt 60, Ruthenium 106, Iridium 192)" and "the flexibility ofI-125 seeds is realized in designing a plaque for the treatment of several retinoblastomas within the same eye.?" "Com-
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Plaque Radiotherapy of Retinoblastoma Dear Editor: The article by Shields et al entitled, "Plaque Radiotherapy in the Management of Retinoblastoma" (Ophthalmology 1993;100:216-24), presents the authors' experience with single brachytherapy treatments for primary and recurrent retinoblastomas. The authors lump their experience with four different types of radioactive plaque sources (cobalt 60, iridium 192, ruthenium 106, and iodine 125) and treat them as relative equivalents. They state that, "The isotope selected in each case was determined (solely?) by the radiation oncologists in an effort to attain optimal dose distribution depending on tumor size, vitreous seeding, and tumor location." The readership would have benefited more from the authors' data on doses to critical ocular structures, or at least some reference points outside the targeted zone. These data could be used to demonstrate how these choices were made. Then, in an apparent change in philosophy, the authors point out that they have come to use iodine 125 plaques exclusively, "because of its radiation safety advantages and ease for custom design." Were there any data to support this switch? There is ample literature supporting the use oflow-energy isotopes for the treatment of intraocular tumors.':" In the Discussion section, the authors speculate that the use of ophthalmic plaques will decrease irradiation of normal orbital structures, thereby decreasing the incidence of secondary, radiation-induced cancers. Their "measurements" oforbital irradiation range from "undetectable to 200 cGy." How were these doses determined? Did the authors consider that the dose near to certain radioactive plaques (in those small baby orbits) could be significantly higher than the alternative (35-40 Gy) delivered by external beam radiotherapy? Last, the authors have previously described patients treated with multiple "rotating plaques," at the Oncology Service at Wills Eye Hospital. What happened to them? PAUL T. FINGER, MD SAMUEL PACKER, MD New York, New York References I. Packer S. Low energy isotopes for the treatment ofposterior choroidal melanoma. In: Bornfeld N, Gragoudas ES, Hopping W, eds. Tumors ofthe Eye: Proceedings ofIntemational Symposiums in Geneva, 1987, and Essen, 1989. Amsterdam: Kugler, 1991;399-405. 2. Packer S, Stoller S, Lesser ML, et al. Long-term results of iodine 125 irradiation of uveal melanoma. Ophthalmology 1992;99:767-74.
3. Finger PT, Moshfeghi DM, Ho TK. Palladium 103 ophthalmic plaque radiotherapy. Arch Ophthalmol 1991;109:1610-13.
Authors'reply
Dear Editor: Drs. Finger and Packer point out interesting questions relative to the use of plaque brachytherapy, although some of these points have been previously published and were outside the intended scope of our study. I Our selection of specific isotopes for plaque brachytherapy has evolved over time as we have recognized their inherent advantages and disadvantages. We initially used cobalt 60 based on Stallard's' pioneer work and favorable results treating retinoblastoma. Because cobalt's high energy resulted in higher tissue doses and personnel exposure, it was jointly decided by the ophthalmology and radiation oncology teams to evaluate other isotopes to replace cobalt 60. Ruthenium 106 and iridium 192 were both used temporarily before it was decided that iodine 125 offered the most advantages. Subsequently, we used iodine 125 in most of our patients. We have previously analyzed the specific advantages and disadvantages of the different isotopes used in plaque brachytherapy.' We currently use iodine 125 because of its radiation safety advantages and ability for custom design. There is abundant information to support this. For example, due to the high energy ofcobalt 60 (1.25 MeV), a lead thickness of approximately 11 mm would be required to decrease the radiation dose to 50%. In contrast, iodine 125 is shielded to less than 0.1 % of the total dose using a sheet of gold only 0.5 mm in thickness (the half-value layer of gold is 0.025 mm). It is therefore evident that the dose of radiation outside of the plaque (which is covered by the gold shell) is negligible. This has been confirmed by other investigators including Packer and associates" who have stated about iodine 125 plaques: "the absorption properties of gold are known and therefore the dose posteriorly and laterally is known to be negligible." "Iodine 125 irradiation of choroidal melanoma results in a lower radiation dose than with Cobalt 60, Ruthenium 106, Iridium 192, . . . or proton beam adjacent to the tumor," and "the shielding and directional radiation minimizes radiation to the optic nerve and delivers less radiation to the eye compared to other radiation sources (Cobalt 60, Ruthenium 106, Iridium 192)" and "the flexibility ofI-125 seeds is realized in designing a plaque for the treatment of several retinoblastomas within the same eye.?" "Com-
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