gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

Comparison of Gafchromic® EBT and EBT2 film in photon and carbon ion beams

Meeting Abstract

  • B. Hartmann - German Cancer Research Center, Heidelberg
  • M. Martišíková - German Cancer Research Center, Heidelberg
  • O. Jäkel - German Cancer Research Center, Heidelberg

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog082

DOI: 10.3205/09ptcog082, URN: urn:nbn:de:0183-09ptcog0824

Published: September 24, 2009

© 2009 Hartmann et al.
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Outline

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Background: Increasing accuracy in radiation delivery to a patient using scanning particle beams, leads to increased demands on quality assurance. At the Heidelberg ion beam therapy center (HIT) the utilization of radiochromic films for this purpose is considered instead of radiographic films which are commonly used.

The self-developing Gafchromic® EBT film (International Specialty Products 2006 Gafchromic® EBT, Self-developing film for radiotherapy dosimetry, Wayne, NJ, USA), offering high spatial resolution, is widely used for relative photon dosimetry. Recently the Gafchromic® EBT2 film has been released, having the same sensitive component but containing a yellow marker dye for enhanced protection against ambient light exposure (International Specialty Products 2006 Gafchromic® EBT2 Dosimetry Film, Wayne, NJ, USA).

The aim of this study was to investigate the performance of EBT2 film in photon and heavy ion beams and to compare it to EBT film.

Material and methods: The basic properties of EBT2 films have been investigated using photons from a 60Co source. All films were digitized one day after irradiation using the commercial flatbed scanner EPSON Expression 10000XL. Results for EBT films have been taken from previous studies (Martišíková M, et al. Phys Med Biol. 2008;53:7013.; Martišíková M, et al. Nucl Instrum Methods. 2008;A 591:171.).

In reassessing the size of the studied film area and characteristics of multiple scans, the validity of the EBT scan protocol (International Specialty Products 2006 Gafchromic® EBT, Self-developing film for radiotherapy dosimetry, Wayne, NJ, USA) for EBT2 films has been verified. The dose-response of EBT2 films has been measured between 0.1 and 15 Gy and the longtime development was measured. The film response homogeneity was examined regarding pixel values (PVs) of film sheets irradiated to 0 Gy, 0.3 Gy and 1 Gy.

Darkening in ion beams was studied using a 400 MeV/u carbon ion beam at HIT. The relative efficiency (RE) of the film was measured as a function of the dose between 0.1 and 20Gy.

Results: The darkening of the EBT2 film in a photon beam was compared to that of EBT films from three different lots. The dose-response curves of the EBT films coincide, whereas the sensitivity of EBT2 films is about 50% lower. While EBT films show intra-sheet inhomogeneities below 1% (International Specialty Products 2006 Gafchromic® EBT, Self-developing film for radiotherapy dosimetry, Wayne, NJ, USA), inhomogeneities within single EBT2 film sheets are much higher, with up to 7% differences in PVs. After irradiation the absolute discrepancy in PVs is enhanced while the relative discrepancy stays approximately the same for both investigated dose levels.

The RE of EBT films in a carbon ion beam was measured to be about 0.7, independent on dose. EBT2 films show a RE of about 0.8.

Conclusion: While the EBT film is well suited for relative dosimetry in conventional photon beams, the performance of EBT2 films has to be further investigated, especially concerning the response homogeneity. Differences in response for photons and carbon ions are similar for both EBT and EBT2, however they have to be taken into account.