SU‐E‐T‐156: Fast Cone‐Beam Optical CT Scanning of a Radiochromic Solid Dosimeter for Clinical 3‐D Dose Verification

Purpose: There is an urgent need for fast 3‐D dose verification for advanced radiotherapy techniques such as IMAT. This study is to evaluate the accuracy and clinical relevance utilizing a fast cone‐beam optical CT scanner with a radiochromic solid dosimeter, PRESAGE®. Methods: A first‐generation laser optical CT scanner, in conjunction with gels and radiochromic solid dosimeters, has been successfully used for 3‐D dose verification of complex treatments such as IMRT and radiosurgery. However, a major limitation with this type scanner is the long scanning time (8‐hours), which makes same‐day dose verification for treatment impossible. In this study three radiochromic dosimeters (PRESAGE®) were irradiated with 12Mev electron beam, 6MV photon beam, and 5‐field IMRT, respectively. Dose readout was performed using a Vista™ cone beam optical scanner (Modus Medical Devices Inc.). This scanner utilizes a LED diffuse light panel and a lens based CCD camera to capture a series of 2D optical projections through the region of interest while the object is being rotated 360°. Feldkamp filtered back projection was used to reconstruct the 3D dose map, with a spatial resolution of 1×1×1mm. Each dosimeter was scanned both before and after irradiation in order to eliminate artifacts caused by light scattering and dosimeter un‐uniformity. Dose distributions were compared with results from a first‐generation laser optical CT scanner and Eclipse planning system. Results: The PDDs measured with PRESAGE® and the beam data from Eclipse agree within 3% and 1mm. Comparison of the IMRT dose distributions between PRESAGE® measurements and Eclipse calculation shows 93% Gamma index passing rate (3%, 3mm criteria). The scanning time for a 3‐D dose map is less then 30 minutes. Conclusions: The Vista™ cone‐beam optical CT scanner and radiochromic dosimeter, PRESAGE®, has shown the potential for fast 3‐D dose verification.