Conformal Episcleral Plaque Therapy

Melvin A. Astrahan, Ph.D., Gary Luxton, Ph.D., Qiang Pu, B.S. and Zbigniew Petrovich M.D.
International Journal of Radiation Oncology, Biology, Physics,39: 505-519, 1997.
(the figures in this online version have been enhanced compared to those published in the journal)

Department of Radiation Oncology
University of Southern California School of Medicine
Los Angeles, CA 90033

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Melvin A. Astrahan, Ph.D.
Department of Radiation Oncology
University of Southern California School of Medicine
Kenneth Norris Cancer Center
1441 Eastlake Ave.
Los Angeles, CA 90033

Phone: (323) 865-3091
Fax: (323) 865-0000


Purpose: Episcleral plaque therapy (EPT) with sealed I-125 sources is widely used in the treatment of choroidal melanoma. In EPT, as elsewhere in radiotherapy, concern for normal tissue tolerance has frequently been a dose limiting factor. The concept of conformal therapy, which seeks to improve dose homogeneity within the tumor and greatly reduce the dose to uninvolved structures may provide a solution to this problem. Radioactive sources are typically distributed uniformly over the surface of an episcleral plaque and are sometimes offset slightly from the scleral surface in order to reduce the dose to the sclera relative to the apex and prescribed therapeutic margin at the tumor base. Nevertheless, it is not uncommon for scleral dose to exceed the dose to the apex of intermediate to tall tumors by a factor of 4 or more. The availability of low energy sealed sources such as I-125 prompted the development of gold-backed plaques to shield non-involved periocular tissues. The concept of shielding can be extended to include collimation of individual sources. The potential advantages of individual source collimation include reduced scleral dose, more homogeneous tumor dose, and superior shielding of adjacent normal structures such as the fovea as compared to previous plaque designs.

Methods and Materials: A three dimensional treatment planning system has been extended to design a plaque which incorporates individually collimated I-125 sources. Thermoluminescent dosimetry (TLD) and radiochromic film were used to compare calculated dose rate distributions with measured dose rates in an acrylic phantom.

Results: Calculations predict that source collimation in the form of a "slotted" gold plaque will achieve the purposes of the study. The collimating effect of the slots is demonstrated qualitatively using radiochromic film, and the accuracy of the calculation is demonstrated quantitatively with TLD.

Conclusions: The episcleral plaque described in this report is simpler to assemble than previous plaque designs. It produces a more homogeneous dose distribution in the tumor, reduces scleral dose by up to 50% as compared to conventional designs, and significantly reduces radiation dose to uninvolved structures adjacent to the plaque.

Key Words: Brachytherapy, Dosimetry, Conformal, Plaque,I-125, Choroidal melanoma.

Article: Part 1 (Introduction & Methods) | Part 2 (Results) | Part 3 (Discussion & References)

Plaque Simulator References | Guide Contents