SU‐E‐T‐99: Small Field Output Factor Measurement Using MAGIC Gel Dosimeter in 3T MRI

Purpose: Small field dosimetry is very important because of radiation therapy techniques that use small fields such as IMRT, gamma and body radiosurgery, cyberknife and tomotherapy. We investigated use of a MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) gel dosimeter to quantitatively measure small field output factors (OFs) for 6MV x rays. Methods: In this work, MAGIC (Gelatin 9%; Methacrylic acid 4%; CuSO4 0.1mM; Ascorbic ascid 2mM; Glucose 10%) gel phantoms were developed to measure the 6MV x‐ray output factors for 1×1 up to 10×10 cm2 square fields (Varian 2100C linear accelerator). For comparison, 3 ion chambers (PTW:TN30013, Exradin A12, Capintec PR‐05P), Gafchromic film (EBT2), and TLD (LiF‐100) were used to measure the small field OFs under identical experimental conditions: 6cm depth (solid water), SAD=100cm, SSD = 94cm, 6MV, 512 MUs per irradiation. Relative OFs were normalized to a reference field (10×10 cm2 @ SAD =100cm). MAGIC gel dosimeters were scanned in a 3T GE signa® EXCITETM clinical scanner using a Spin Echo pulse sequence for dose distribution readout (pixel size = 0.4mm, slice thickness 3mm, TR = 4000ms, TE = 10ms and 110ms, respectively). Gel dose distributions were then calculated using custom Matlab code. Results: 6MV x‐ray OFs versus field size for all detectors were graphically compared. The MAGIC polymer gel dosimeter OF for a 1×1 cm2 field is 0.612 (+/− 5%), approximately 2% different from the OFs measured using small volume dosimeters (TLD, EBT2 film and the Capintec PR‐05P ion chamber). Larger ion chamber (PTW:TN30013 and Exradin A12, both ∼ 0.6cc) OFs were low (OF = 0.26 for 1×1cm2) due to nonequilibrium and partial volume conditions. Conclusions: MAGIC gel dosimeter with 3T MRI scanner as a read‐out makes it an ideal tool for small field dosimetry.