Poster — Wed Eve—21: 2D CBCT Dosimetry Using XR‐QA Model GAFCHROMIC Film

Abstract

We describe a 2D reference dosimetry system for dose measurements during cone‐beam computed tomograpfy (CBCT) scans using on‐board imager (OBI) on Varian Clinac‐iX linear accelerator that employs XR‐QA radiochromic film model, specifically designed for measurements at low energy photons. We report on surface doses and percent depth dose (PDDs) measurements during clinical CBCT scans on a humanoid Rando phantom. Response of XR‐QA model radiochromic film reference dosimetry system was calibrated in terms of air kerma in air. Pieces of XR‐QA films were taped on surface of the Rando phantom for surface dose, and XR‐QA film strips placed between Rando slices for PDDs measurements, during CBCT scans for different sites and CBCT protocols. Spatial dosimetry was performed using an Epson Expression 10000XL document scanner. Change in optical reflectance of unexposed film was subtracted from the exposed one to obtain final netR, which was converted to dose using previously determined calibration curve. Our measurements show that skin dose ranges from 0.07 cGy in Low Dose Head to 4.64 cGy in Pelvis Spot Light CBCT protocol, with uncertainty of 2% at higher doses, rising to 4% at 0.5 cGy and 12% at the lowest measured dose. Film strip profile measurements show different dose distributions depending on the CBCT mode used and the anatomical site imaged. The main advantage of the described system for dose measurements during CBCT scans is that it does not require a priori knowledge of the backscatter factors while providing 2D reference dosimetry at low kVp photon beams.