Radiotherapy External and chapteEpimacular• 49

Figure 49.2  Neovista VIDION brachytherapy system dose profile. It was designed to have therapeutic radiation levels at the neovascular complex in eyes with exudative age-related macular degeneration while limiting the radiation exposure to nontarget tissues.

Radiation has been used in several forms to treat intraocular retinoblastoma. External beam radiotherapy has been extensively used since Verhoeff and Reese pioneered the method in the early 1900s. Because retinoblastoma is a radiosensitive tumor, its control and eye preservation rates are relatively high. Intensity-modulated radiation therapy is an alternate method of delivering radiotherapy that reduces the dose to normal tissues and has been reported effective to control retinoblastoma.24 Brachytherapy is the treatment of choice in certain types of retinoblastoma25 and its use has increased as the danger of external beam therapy in genetically predisposed patients with heritable retinoblastoma has been recognized.

Radiation has been used to treat serous retinal detachment secondary to circumscribed choroidal hemangiomas.26 The radiation has been delivered by external beam, proton bean, and brachytherapy. The resolution of subretinal fluid has been successfully achieved in most cases. Combined chemotherapy with radiotherapy is also recommended for patients with primary intraocular lymphoma.27

Radiotherapy has also been used in cases of retinal and choroidal metastasis for palliation or prevention of symptoms in patients with concomitant systemic therapy and for those untreated patients for whom effective systemic therapy is not available.28 Most retinal and choroidal metastasis responds to treatment and regression of tumor and associated retinal detachment can be assessed in most cases, avoiding the need for enucleation.

Although not common, ocular radiotherapy has been described in the treatment of cavernous hemangioma of the retina and retinal capillary hemangioma in patients with von Hippel–Lindau disease.29

OPERATIVE TECHNIQUES

PLAQUE PLACEMENT TECHNIQUE

Before placing an iodine-125 plaque it is important to obtain the best estimate of the tumor size, including base and height dimensions. These dimensions will determine the size of the plaque containing iodine-125 (Figure 49.3). The most important measurement is the base diameter of the tumor.

Figure 49.3  Radiation plaques used in the Collaborative Ocular Melanoma Study (COMS). These plaques consist of gold housing and seed carrier insert for iodine-125 isotopes.

The tumor size can be estimated by using retinal photographs or measuring grids. During photograph measurements, the horizontal diameter of the optic disc head can be used as a unit of measurement equivalent to 1.5 mm. An alternative is to use a measuring grid, as described by Hilton.30 The measuring grid is designed so that each grid square overlies an area equivalent to approximately 1 disc diameter (DD). The image of the tumor is viewed monocularly using indirect ophthalmoscopy with a measuring grid placed over a 20-D Nikon lens, and the number of squares (DDs) overlying the tumor is counted. Then, it is converted into millimeters.

The size of the iodine-125 plaque is determined using the estimate tumor diameter plus a tumor-free perimeter of 2 mm or more. For example, a tumor with a 10-mm base diameter should be treated with a plaque that is 14 mm in diameter. Standardized A-scan ultrasonography is used to determine the thickness of the tumor.

During the surgical placement of the plaque, a peritomy should be performed and the surface of the tumor area exposed. If extraocular extension of more than 2 mm is observed, enucleation should be considered.

At surgery, the tumor base can be localized using transillumination. There are several methods to outline the scleral borders of the tumor area. Frequently it is possible to visualize the shadow of the tumor while the globe is transilluminated through the cornea. The perimeter of the tumor base should be verified with indirect ophthalmoscopy. Another technique involves the use of the MIRA diathermy transillumination unit. The intense light of the angled fiberoptic transillu­ minator allows the operator to visualize the tumor with indirect ophthalmoscopy while simultaneously visualizing the transsclerochoroidal illumination from the fiberoptic light tip as it is moved to localize the perimeter of the tumor. When the fiberoptic light is visualized at the perimeter of the tumor, a diathermy mark can be made on the sclera. These marks should be made more visible using a surgical marking pencil. A caliper is then used to measure the tumor diameter. In order to simulate the placement of the radiation plaque, a transparent acrylic dummy plaque with a diameter equal to the planned therapeutic