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

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.

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.

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.

MAGIC gel dosimeter with 3T MRI scanner as a read-out makes it an ideal tool for small field dosimetry.