Dosimetry characterization of a multibeam radiotherapy treatment for age‐related macular degeneration
Authors: Lee Choonsik , Chell Erik , Gertner Michael , Hansen Steven , Howell Roger W. , Hanlon Justin , Bolch Wesley E. 
Published: November 2008

Age‐related macular degeneration (ARMD) is a major health problem worldwide. Advanced ARMD, which ultimately leads to profound vision loss, has dry and wet forms, which account for 20% and 80% of cases involving severe vision loss, respectively. A new device and approach for radiation treatment of ARMD has been recently developed by Oraya Therapeutics, Inc. (Newark, CA). The goal of the present study is to provide a initial dosimetry characterization of the proposed radiotherapy treatment via Monte Carlo radiation transport simulation. A 3D eye model including cornea, anterior chamber, lens, orbit, fat, sclera, choroid, retina, vitreous, macula, and optic nerve was carefully designed. The eye model was imported into the MCNPX2.5 Monte Carlo code and radiation transport simulations were undertaken to obtain absorbed doses and dose volume histograms (DVH) to targeted and nontargeted structures within the eye. Three different studies were undertaken to investigate (1) available beam angles that maximized the dose to the macula target tissue, simultaneously minimizing dose to normal tissues, (2) the energy dependency of the DVH for different x‐ray energies (80, 100, and ), and (3) the optimal focal spot size among options of 0.0, 0.4, 1.0, and . All results were scaled to give to the macula volume, which is the current treatment requirement. Eight beam treatment angles are currently under investigation. In all eight beam angles, the source‐to‐target distance is , and the polar angle of entry is 30° from the geometric axis of the eye. The azimuthal angle changes in eight increments of 45° in a clockwise fashion, such that an azimuthal angle of 0° corresponds to the 12 o'clock position when viewing the treated eye. Based on considerations of nontarget tissue avoidance, as well as facial‐anatomical restrictions on beam delivery, treatment azimuthal angles between 135° and 225° would be available for this treatment system (i.e., directly upward and entering the eye from below). At beam directions approaching 225° and higher, some dose contribution to the optic nerve would result under the assumption that the optic nerve is tilted cranially above the geometric axis in a given patient, a feature not typically seen in past studies. A total treatment dose of would be delivered in three treatments at these selected azimuthal angles. Dose coefficients, defined as the macula radiation absorbed dose per unit air kerma in units of Gy/Gy, were 16% higher for x‐ray beams in comparison to those at , thus requiring only 86% of the integrated tube current (mAs) for equivalent dose delivery. When 0.0, 0.4, and focal spot sizes were used, the dose profiles in the macula are very similar and relatively uniform, whereas a focal spot size produced a more nonuniform dose profile. The results of this study demonstrate the therapeutic promise of this device and provide important information for further design and clinical implementation for radiotherapy treatments for ARMD.