Charged-particle Radiotherapy
Proton-beam therapy has the advantage of delivering a homogenous dose to the entire tumor, whereas in brachytherapy the tumor base receives several times the dose of the tumor apex. This is achieved by the physical properties of the accelerated proton particles, as once the calculated dose is delivered to the target tissue, there is a sharp fall of radiation dose, which is called the Bragg peak phenomenon. This allows sparing of normal surrounding tissues. Despite the Bragg peak, radiation doses to surface tissues are high, especially if the tumor is anterior. The relative biological effectiveness (RBE) is the ratio of reference photon dose and the proton dose which is necessary to cause the same of level of effect. The usual prescribed dose is 60–70 RBE given in 4 fractions. Although almost all uveal melanomas can be treated with this technique in a theoretical basis, most centers consider a 20-mm basal diameter and 12-mm thickness as the upper limits.
Two articles on proton-beam radiotherapy for iris melanoma indicate that it is well tolerated and that the main complication is radiation cataract, which is treatable, whereas glaucoma is often difficult to control. Lumbroso-Le Rouic et al. reported that the tumor response at 2 years was a flat lesion for 6.3% of cases and partial regression in 75%.
A study on 147 patients with medium or large melanomas treated with proton-beam therapy reported a 5-year eye-retention rate of 71.3% and a disease-specific survival rate of 87.7%. The 10-year metastasis rate varies between 25 and 30%. Approximately 70% of the radiation dose to the tumor is deposited in the anterior segment, and therefore more adnexal and anterior segment side effects, such as eyelash loss, dry eye, keratitis, corneal ulceration, iris neovascularization and neovascular glaucoma, iridocyclitis, cataract, retinal detachment, optic neuropathy, radiation maculopathy and vitreous hemorrhage have been reported. The elevated costs and very few centers available around the world limit the access to this effective modality.