Evaluation of EBT Gafchromic film for compensator based intensity modulated radiation therapy

Full text: The high spatial resolution and low energy dependency make the EBT GAFCHROMIC film suitable for quantitative evaluation of dose distributions especially high dose gradient regions. Application of Compensators to deliver IMRT, change the beam energy spectrum, therefore the reliability of film dosimetry in compensator-based IMRT might be questioned. Percentage depth doses and lateral beam profiles (at 5 cm depth), are measured with EBT films in solid phantom for various thickness (I, 2, 4 and 6 cm) of Cerrobend compensator at different field sizes and two energies (6-18 MV). To verify results, same measurement was implemented using PTW 0.125 ionization chamber in water phantom. There was good agreement between chamber and film measurements. Both film and chamber showed that Penumbral widths (at Depth = 5 cm) increased with increasing the energy, field size and compensator thickness. For film dosimetry at 6 MV, Penumbra widths (80%-20%) were found to be 5.7, 6.5, 7.3, 7.9 and 8.1 mm using 0, I, 2. 4 and 6 cm thickness for lOx10 cm2, respectively. For 18 MV. the same penumbra widths were found to be 8.5, 9.1, 9.8, 10.6 and 11.2 mm respectively. Beam hardening effect was resulted to decrease surface dose up to 12% and increase depth dose up to 3% for 6 MV, while for 18 MV, beam softening produced 8% dose increase in surface dose and 3.5% decrease at the same depth. Gafchromic EBT film provides an adequate measurement for penumbra, surface dose and depth dose variation for high energy x-ray beams.