Is EBT3 Energy Independent for Radiation-Therapy Photon & Electron Beams?
A Rubinstein, R Tailor*, Z Zhao, J Yang UT MD Anderson Cancer Center, Houston, TXSU-E-T-101 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: Evaluate the new GafChromic film EBT3 for energy dependence over clinical photon and electron beams down to75kVp
Methods: A single sheet of EBT3 (Lot A04041201) was cut into small pieces. Film pieces were irradiated at 6-9 known dose levels (0.4 - 12 Gy) with 9 beams: 75, 225, and 300 kVp x-rays, Cs-137, Co-60, 6 and 18 MV x-rays, and 6 and 20 MeV electrons. At each dose level, 3 pieces of film were irradiated to provide repeats. For KV x-ray beams including Cs-137, irradiations were performed in air with an appropriate build-up required for Cs-137; whereas for other beams irradiations were performed at dmax depth in water. The exposed films were read and analyzed with combination of flat-bed scanner (Epson 10000XL) and ImageJ software (red channel). Dose-consistency check was performed at 75 kVp by comparing film-response curve with exposure in-air versus that in Solid Water (depth 2.0 cm).
Results: In Ortho-voltage region, energy dependence is significant. Film sensitivity (OD) decreases with decreasing beam energy (kVp). At 75 kVp, film-sensitivity (OD) is 16% lower than that for 6 MV beam at 6 Gy dose level. For Mega-voltage beams including Cs-137, film EBT3 is found to be practically energy independent within the measurement uncertainty. Dose uncertainty arising from film uniformity and film-to-film variation is about the same (δ ± 1.1%) as reported1 for EBT2.
Conclusion: In Kilo-Voltage region, EBT3 shows significant energy dependence showing decrease in film-sensitivity with decreasing beam energy. However, the film is practically energy independent for Mega-voltage Photon & Electron beams including Cs-137. Film-sensitivity decrease for 75 kVp is consistent with the recently reported2 >11% reduction in OD for 50 kVp beam. In light of another recent publication3, energy dependence may arise from components < 25 KeV in the beams.