Predicting energy response of radiographic film in a 6 MV x-ray beam using Monte Carlo calculated fluence spectra and absorbed dose.

The advantage of radiographic film is that it allows two-dimensional, high-resolution dose measurement. While there is concern over its photon energy dependence, these problems are considered acceptable within small fields, where the scatter component is small. The application of film dosimetry to intensity modulated radiotherapy (IMRT) raises additional concern since the primary fluence may vary significantly within the field. The varying primary fluence in combination with a large scatter fraction, present for large fields and large depths, causes the spectrum at various points within the IMRT field to differ from the spectrum in the uniform fields typically used for calibrating the film. As a result, significant artifacts are introduced in the measured dose distribution. The purpose of this work is to quantify and develop a method to correct for these artifacts. Two approaches based on Monte Carlo (MC) simulations are examined. In the first method, the film artifact, as quantified by film and ion chamber output measurements in uniform square fields, is derived from the MC calculated ratio of absorbed doses to film and to water. In the second method, the measured film artifact is correlated with MC calculated photon spectra, revealing a strong correlation between the measured artifact and the "scatter"-to-"primary" ratio, defined by the ratio of the number of photons below to the number of photons above 0.1 MeV, independent of field size and depth. These methods are evaluated in high- and low-dose regions of a large intensity-modulated field created with a central block. The spectral approach is also tested with a clinical IMRT field. The absorbed dose method accurately corrects the measured film dose in the open part of the field and in points under the block and outside the field. The dose error is reduced from as much as 16% of the open field dose to less than 1%, as verified with an ion chamber. The spectral method accurately corrects the measured film dose in the open region of the centrally blocked field, but does not fully correct for the film artifact for points under the block and outside the field, where the spectrum is substantially different. Applied to the clinical field, the corrected film measurement shows good agreement with data obtained with a two-dimensional diode array.