PD-103 VERSUS I-125 FOR OPHTHALMIC PLAQUE RADIOTHERAPY

Purpose: A dosimetry study comparing the use of I-125 vs. Pd-103 radio active seeds for ophthalmic plaque brachytherapy. Methods and Material s: Palladium-103 (Pd-103) seeds in ophthalmic plaques were used to tre at 15 patients with intraocular malignant melanoma. Computer-aided sim ulations were performed to evaluate the intraocular dose distribution of I-125 versus Pd-103 ophthalmic plaques (delivering equivalent apex doses). Seven target points were selected. Starting at the outer scler al surface, four were located along the central axis of the plaque: th e 1 mm point (the inner sclera), the 6 mm point, the tumors apex, and the opposite eye wall. We also evaluated the fovea, optic nerve, and t he lens because they were considered to be critical structures. Result s: These studies demonstrated that the lower energy photons generated by Pd-103 seeds (average 21 KeV) in ophthalmic plaques were more rapid ly absorbed in tissue than photons generated by I-125 (average 28 KeV) . Therefore, during ophthalmic plaque radiotherapy, Pd-103 photons wer e found to be more rapidly absorbed within the tumor and less likely t o reach most normal ocular structures. On average, the use of Pd-103 d ecreased the dose to the fovea by 5.7%, to the optic nerve by 8.4%, to the lens by 26%, and to the opposite eye wall by 38.4%. Conclusion: P alladium-103 ophthalmic plaque brachytherapy resulted in slightly more irradiation of the tumor and less radiation to most normal ocular str uctures.