SU‐E‐T‐713: Kilovoltage Stereotactic Radiosurgery for AMD: A More Complete Evaluation of Effective Dose

Purpose: Age‐related Macular Degeneration is a leading cause of blindness for the elderly population in industrialized nations. Monthly injections of ranibizumab stabilize the progression of the disease, and a novel non‐invasive stereotactic radiosurgery is being explored as an adjunct to injections. An estimation of effective dose for a 3 beam × 8 Gy therapeutic dose to the macula had been reported previously by the authors of this paper using head and neck phantoms. Several improvements have been made: expansion of the geometric models to include tissues in the torso, an estimation of leakage contribution, and effect of lead vest usage.Methods: Male and female hybrid voxel torso phantoms were derived from CT slice segmentation and scaled to ICRP Publication 89 reference values. A MATLAB code was developed to “wrap” a single layer of voxels around the torsos, representative of a 0.5 mm lead vest equivalent. A three‐beam photon treatment (100 kVp) was modeled using the MCNPX radiation transport code to evaluate effective dose as per the ICRP Publication 103 schema. A similar calculation was made for leakage, but using a “hard” mono‐energetic (100 keV) point source as an approximation. Results: The calculations yielded an effective dose of 0.389 mSv for the control group, and 0.369 and 0.376 mSv for the use of a lead vest with and without a neck cover, respectively. Comparatively, 0.281 mSv was reported previously for the head and neck phantoms (19% versus 99% wTˈs). Leakage contribution was estimated to be approximately 0.009 mSv Conclusions: The expanded geometric models and inclusion of leakage contribution provide a more complete estimation of the effective dose than previously reported. The use of lead vests with and without a neck cover result in a 5% and 3% reduction of effective dose, respectively. This work was sponsored by Oraya Therapeutics.