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Chulalongkorn Medical Journal

Abstract

Objective : The objectives of this research are to study the characteristicsof GafchromicTM EBT film for 6 MV x-ray beams and to verifyisodose distribution of advanced radiation treatment techniques.Study design : Observational descriptive study.Setting : Division of Therapeutic Radiology and Oncology, Department ofRadiology, Faculty of Medicine, Chulalongkorn University.Materials and Methods : The characteristics of Epson perfection V700 flat-bed colorCCD were observed for the response, the repeatability,the reproducibility and color band. The characteristics ofGafchromicTM EBT film were studied for speed of film development,polarization, uniformity, field size and dose rate effects.The percentage depth dose and beam profile were measuredand compared with the data from 0.13 CC ionization chamber.The comparisons of fluence map in intensity modulatedradiotherapy (IMRT) plans measured by film and calculated byEclipse treatment planning were observed in two nasopharynxcancers and one brain tumor.Results : Responses of Epson perfection V700 flat-bed color CCD in termof pixel value and net optical density showed polynomial curve.The variations of scanner reproducibility and repeatability werewithin 1.0 and 1.1%, respectively. The GafchromicTM EBT filmwas best evaluated in the red channel of the Epson perfectionV700 scanner. The optical density growths with time seem tostabilize after 4 hours. The effect of EBT film polarization at 90Oand 270O, with 50 cGy delivered dose was up to 24% differenceof optical density when compared with orientation at 0O.The variation of EBT film uniformity was 7 % at 400 cGy. Slightlydeviation from the ionization chamber measurement for outputfactor for small field size and no dose rate dependence wereobserved for the range of dose rate study. The difference ofpercentage depth doses at 5 cm depth between EBT film andCC13 ionization chamber were less for all field sizes study.The deviation of beam profile between EBT film and CC13ionization chamber was presented at the region of off-axis andhigh dose gradient. The fluence map of IMRT plans measuredby GafchromicTM EBT and calculated by Eclipse planningsystem of two nasopharynx cancers and one brain tumor werecompared with the criteria of 3% dose difference and 3 mmdistance to agreement. The two plans showed over 90% passedbut one plan was below 90%.Conclusion : The characteristics of scanner should be observed beforeexamining the characteristics of GafchromicTM EBT film. Thecharacteristics of film such as the speed of film development,polarization effect and uniformity are very important because ofthe stability time of net optical density and the direction of nonpolarizationeffect should be selected. The uniformity of film isaffected to dose distribution verification, it should be correctedbefore scanning the film. Then, the GafchromicTM EBT films canbe used to verify isodose distribution of advanced radiationtreatment techniques.

Publisher

Faculty of Medicine, Chulalongkorn University

First Page

207

Last Page

219

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