Dosimetric properties of Gafchromic® EBT films in monoenergetic medical ion beams

For quality assurance measurements in radiotherapy with scanned ion beams, such as beam profile measurements and dosimetrical verification of patient plans, high spatial accuracy is required. We investigated the Gafchromic EBT film, which is the ancestor of the currently available EBT2 film, for this purpose. The excellent spatial resolution and improvements in sensitivity as compared to older types of radiochromic films make it a suitable candidate. In this contribution a study of the dose-response relation for monoenergetic beams (plateau), based on the absolute dose, is presented. We found the film response to be comparable for protons and photons. For carbon ions we observed a decrease in the darkening efficiency of about 30% with respect to photons. This effect is likely to be caused by the ion track structure. We did not find any dependence of the relative efficiency on the applied dose and energy over typical ranges used in ion beam therapy. Furthermore, it is shown that overlaps of ion tracks are not an issue for EBT film dosimetry in medical ion beams. The above findings enable the use of EBT films calibrated in a beam of a particular ion type for dose measurements at any other dose and energy. The uncertainty of the film darkening determination in ion beams is also discussed.