Characteristics of GafchromicTM EBT film for dosimetric verification for high energy photon beams

Abstract

Objective : The objectives of this research are to study the characteristics of GafchromicTM EBT film for 6 MV x-ray beams and to verify isodose distribution of advanced radiation treatment techniques.

Study design : Observational descriptive study.

Setting : Division of Therapeutic Radiology and Oncology, Department of Radiology, Faculty of Medicine, Chulalongkorn University.

Materials and Methods : The characteristics of Epson perfection V700 flat-bed color CCD were observed for the response, the repeatability, the reproducibility and color band. The characteristics of GafchromicTM 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 measured and compared with the data from 0.13 CC ionization chamber. The comparisons of fluence map in intensity modulated radiotherapy (IMRT) plans measured by film and calculated by Eclipse treatment planning were observed in two nasopharynx cancers and one brain tumor.

Results : Responses of Epson perfection V700 flat-bed color CCD in term of pixel value and net optical density showed polynomial curve. The variations of scanner reproducibility and repeatability were within 1.0 and 1.1%, respectively. The GafchromicTM EBT film was best evaluated in the red channel of the Epson perfection V700 scanner. The optical density growths with time seem to stabilize after 4 hours. The effect of EBT film polarization at 90O and 270O, with 50 cGy delivered dose was up to 24% difference of optical density when compared with orientation at 0O. The variation of EBT film uniformity was 7 % at 400 cGy. Slightly deviation from the ionization chamber measurement for output factor for small field size and no dose rate dependence were observed for the range of dose rate study. The difference of percentage depth doses at 5 cm depth between EBT film and CC13 ionization chamber were less for all field sizes study. The deviation of beam profile between EBT film and CC13 ionization chamber was presented at the region of off-axis and high dose gradient. The fluence map of IMRT plans measured by GafchromicTM EBT and calculated by Eclipse planning system of two nasopharynx cancers and one brain tumor were compared with the criteria of 3% dose difference and 3 mm distance to agreement. The two plans showed over 90% passed but one plan was below 90%.

Conclusion : The characteristics of scanner should be observed before examining the characteristics of Gafchromic^TM EBT film. The characteristics of film such as the speed of film development, polarization effect and uniformity are very important because of the stability time of net optical density and the direction of nonpolarization effect should be selected. The uniformity of film is affected to dose distribution verification, it should be corrected before scanning the film. Then, the Gafchromic^TM EBT films can be used to verify isodose distribution of advanced radiation treatment techniques.