A STEREOTACTIC RADIATION-THERAPY DEVICE FOR RETINOBLASTOMA USING A NONCIRCULAR COLLIMATOR AND INTENSITY FILTER

The proximity of the lens to the retina makes the treatment of retinob lastoma a challenge for external beam radiation therapy. The similar t o 1 mm separation between the posterior edge of the lens and the anter ior region of the retina causes a trade-off between coverage of the en tire retina and excessive dose to the lens. A stereotactic, LINAC base d, lens sparing technique for treating retinoblastoma is presented. Th e technique uses noncoplanar arcs with the lens at isocenter. A specia l noncircular collimator blocks the lens but it also causes the dose d istribution to vary across the retina. A fluence modulation filter is used to reduce the dose inhomogeneity across the target. The resulting dose distribution is roughly hemispheric, providing both anterior cov erage of the retina and lens blocking unlike conventional techniques. The method used to develop the collimator and filter assembly is prese nted. Dosimetry of the assembly was carried out using radiochromic fil m, and the results were entered in a treatment planning system. The do se distribution as measured in a phantom is provided and compared to c alculations. (C) 1998 American Association of Physicists in Medicine.