Directional dependence in film dosimetry: radiographic and radiochromic film

The trend towards conformal, intensity modulated radiotherapy treatments has established the need for a true integrating dosimeter. In traditional radiotherapy, radiographic film dosimetry is commonly used. The accuracy and reproducibility of film optical density as an indicator of dose is influenced by several variables, including the chemical processing conditions. As a result radiochromic film, with all the advantages of radiographic film but without the need for chemical processing, has increased in popularity, although the low-dose sensitivity of radiochromic film does remain a disadvantage for some experiments. Several studies have investigated the reproducibility of radiochromic film results, but none have specifically addressed the well-known directional dependence seen with traditional radiographic film. In this study, the directional dependence of radiographic (Kodak X-omat V) and radiochromic (Gafchromic (TM)) films were measured. It was found that both films over responded when exposed parallel to the central axis of the beam as opposed to perpendicular exposure. An attempt is made to explain the reason for the responses of both films in terms of spectral effects and the air gap between the phantom segments. Although radiographic film exposed parallel rather than perpendicular to the central axis of the beam exhibits a measured difference in film response at depth, this over response does not occur when the extent of the film is restricted to a small region at the centre of the phantom (this case an air gap is not introduced across the phantom). This suggests that it is the air gap rather than the orientation of the film that is the cause of the over response. Furthermore, when film occupies a slice through the entire phantom an over response occurs for both radiographic and radiochromic film, indicating that spectral effects are not the cause.