Dosimetric characteristics of an extended dose range radiographic film for verification of dynamic intensity modulated radiation therapy

Abstract

For two dimensional intensity modulated radiation therapy (IMRT) dose distribution verification, a new type of radiographic film, EDR2 (extended dose range) film, allowing measured doses above 600 cGy was currently used. The purpose of this study is to evaluate the properties of EDR2 film for dosimetric verification of dynamic IMRT. The Varian Clinac 23EX for 6 MV x-ray mode and Varian Clinac 21EX for 10 MV x-ray mode delivered at 300 MU/min were used for this study. The films were irradiated with the dose of 20-450 cGy at depth of 5 cm, 10 cm, and 15 cm with the field size of 2x2, 3x3, 10x10 and 15x15 cm2. The reproducibility and accuracy of film measurement which depend mainly on the film processor was within 1.9% for the dose up to 200 cGy and went up to 4.9% for the dose of 450 cGy for both energies. All the sensitometric curves showed linearity when the optical density ranged from 0.5 to 2.0 and the dose ranged from 80 to 330 cGy. When the field size was fixed and depth was varied, the sensitometric curves showed less effect of depth for 6 MV than 10 MV x-ray beam, the discrepancy occurred at the high dose of all field sizes. The dose discrepancy between 5 and 15 cm depth was within 1.6% for 6 MV and 3.4% for 10 MV x-ray beams at 2x2 cm2 field size. When the depth was fixed and the field size was varied, field size showed more discrepancy of sensitometric curves than depth, they were also more pronounce at the higher dose, the maximum dose discrepancy between the 2x2 cm2 and 15x15 cm2 field size was 4.2% for 6 MV and 2.8% for 10 MV x-ray beams at 5 cm depth. The result of energy response showed that EDR2 film had less energy dependence. The maximum dose discrepancy between 6 and 10 MV x-ray beams was only 0.3%. The percentage depth dose measured with EDR2 film compared to ionization chamber for 6 MV and 10 MV x-ray beams, showed good agreement in a shallow depth but more discrepancy at a deeper depth, especially for the small field size. Dose profile measured with EDR2 film were consistent with those measured with an ionization chamber except at the penumbra region. From the result the optimal field size of 3x3 cm2 and depth of 5 cm irradiated in the same film were chosen for the calibration curve that needed to be performed each time of IMRT plan verification for both x-ray energies. Verification of 2 IMRT plans for 6 MV and 2 IMRT plans for 10 MV x-ray beams showed a good agreement to the Eclipse treatment planning within 3% of dose difference and 3 mm of distance to agreement when using gamma value evaluation. The suitable field size, depth and dose range should be considered for EDR2 film to obtain the calibration curve which is used for accurate absolute dose distribution of IMRT field