Stereotactic radiosurgery for AMD: a Monte Carlo-based assessment of patient-specific tissue doses.

PURPOSE: To define the radiation doses to nontargeted ocular and adnexal tissues with Monte-Carlo simulation using a stereotactic low-voltage x-ray irradiation system for the treatment of wet age-related macular degeneration.
METHODS: Thirty-two right/left eye models were created from three-dimensional reconstructions of 1-mm computed tomography images of the head and orbital region. The resultant geometric models were voxelized and imported to the MCNPX 2.5.0 radiation transport code for Monte Carlo-based simulations of AMD treatment. Clinically, treatment is delivered noninvasively by three divergent 100-kVp photon beams entering through the sclera and overlapping on the macula cumulating in a therapeutic dose. Tissue-averaged doses, localized point doses, and color-coded dose contour maps are reported from Monte Carlo simulations of x-ray energy deposition for several tissues of interest, including the lens, optic nerve, macula, brain, and orbital bone.
RESULTS: For all eye models in this study (n = 32), tissues at risk did not receive tissue-averaged doses over the generally accepted thresholds for serious complication, specifically the formation of cataracts or radiation-induced optic neuropathy. Dose contour maps are included for three patients, each from separate groups defined by coherence to clinically realistic treatment setups. Doses to the brain and orbital bone were found to be insignificant.
CONCLUSIONS: The computational assessment performed indicates that a previously established therapeutic dose can be delivered effectively to the macula with the scheme described so that the potential for complications to nontargeted radiosensitive tissues might be reduced.