Directional dependence in film dosimetry: radiographic and radiochromic film

The trend towards conformal, intensity modulated radiotherapy treatments ha s established the need for a true integrating dosimeter. In traditional rad iotherapy, radiographic film dosimetry is commonly used. The accuracy and r eproducibility of film optical density as an indicator of dose is influence d 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, alth ough the low-dose sensitivity of radiochromic film does remain a disadvanta ge for some experiments. Several studies have investigated the reproducibil ity 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 b oth films over responded when exposed parallel to the central axis of the b eam 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 th e 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 n ot 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 p hantom). This suggests that it is the air gap rather than the orientation o f the film that is the cause of the over response. Furthermore, when film o ccupies a slice through the entire phantom an over response occurs for both radiographic and radiochromic film, indicating that spectral effects are n ot the cause.