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Small Field Output Factor Measurement Using MAGIC Gel Dosimeter in 3T MRI

X Ding

X Ding*, J Bourland, W Dolesh, R Best, I Mcgowin, J Liu, Wake Forest Univ, WINSTON-SALEM, NC

SU-E-T-99 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

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 1x1 up to 10x10 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 (10x10 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 read-out (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 1x1 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 1x1cm²) 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.

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