SU‐E‐T‐684: Simple Integrated Tissue‐Air Ratio (SITAR)

Purpose: Performing direct depth‐dose measurements on the IRay ophthalmological SRSdevice (designed to treat wet age‐related macular degeneration), with its narrow (4mm), low energy (100kVp) photon beam is not feasible using currently available equipment. The constant SAD indicates use of Tissue‐Air Ratio (TAR). The imperfect water‐equivalency of available tissue‐equivalent plastics at these energies and the absence of small water‐proof, energy‐independent detectors make it impossible to measure TAR directly. The Simple Integrated Tissue‐Air Ratio (SITAR) is a novel method of determining depth‐dose for this novel device.SITAR is Simple in that it is measured simply using a tissue‐equivalent plastic, and applied simply as if to water. It is Integrated because it integrates all appreciable effects: inverse R‐squared divergence, attenuation in water and in tissue‐equivalent plastic, and scatter.SITAR is used to characterize the depth dose of individual IRay devices. Methods: The attenuation properties of three tissue‐equivalent plastics are compared to those of water by direct measurement. The scatter properties of these plastics are compared to scatter in water using Monte‐Carlo simulation. The most water‐like plastic in attenuation and scatter is selected for use in SITAR. Comparing the properties of plastic to water, determinations are made of the SITAR Correction: values that correct measurements taken through the plastic to measurements in water. The SITAR Correction enables a simple series of measurements through plastic to be corrected to a TAR in water. Results: Plastic Water LR (CIRS, Norfolk, VA) is found to be the most water‐like plastic for the IRay treatment beam. Kerma measurements made through varying thicknesses of Plastic Water LR and converted with the SITAR Correction yield a very good estimate of measurements in water. Conclusions: SITAR allows each IRay device to be characterized for depth dose in water simply by taking measurements through plastic. Research sponsored by Oraya Therapeutics. This work is funded by Oraya Therapeutics Inc., Newark, CA