Practical use of Gafchromic^® EBT films in electron beams for in-phantom dose distribution measurements and monitor units verification

Abstract 

Purpose

The possibility of using the Gafchromic^® EBT films parallel to incident electron beams was assessed in order to facilitate quality assurance tests for electron dose calculation algorithms.

Materials and methods

Calibration curves were made for electron energies of 6, 9 and 12MeV. A set-up was suggested for EBT film irradiation parallel to the beam, and the dose measurements were compared to Ionization Chamber (IC) measurements in standard and small electrons beams. A more complex Quality Assurance (QA) set-up was performed with the cylindrical CARPET^® phantom in order to test our Treatment Planning System (TPS) (Eclipse, Varian Medical Systems, Palo Alto, California) for the clinical situation of a chest wall electron beam therapy. Two dimensional dose distribution and gamma index results were compared to the calculated distribution given by the TPS.

Results

The reproducibility was found to be better than 1.5%. We found that applying strong pressure and aligning carefully the film edge with the phantom surface, as recommended for radiographic films, did not completely eliminate the air gap effect. Adding an ultrasound transmission gel and 2 complementary EBT films on the surface gave satisfactory results. The absolute dose for the reference 10×10cm² field was always within 1% of IC measurements and for smaller elongated fields (5×10, 4×10 and 3×10cm²) the mean difference was −1.4% for the three energies. The mean difference with the IC measurements in R₁₀₀, R₉₀ and R₅₀ was 0.9mm for all fields and for the three energies. The mean difference in the width of the 90% and the 50% isodoses at R₁₀₀ was 0.6mm. With the CARPET^® phantom set-up very good agreement was found in the 2D dose distribution; 99% of the points satisfied the γ<1 criteria (3%–3mm).

Conclusion

EBT films parallel to the beam axis could be used for absolute measurements of 2D dose distribution if ultrasound gel and overlying perpendicular films are added on the phantom surface.

Keywords: Gafchromic EBT, Film dosimetry, Electron beams, Quality assurance