SU‐E‐T‐84: Characterization of a Three‐Dimensional Radiochromic Film Stack Dosimeter for Measurements of 6 MV Photon Beams

Purpose: To investigate the energy response, orientation dependence, and water equivalence of a novel three‐dimensional radiochromic film stack dosimeter (FSD) and verify its accuracy for measurements of 6 MV photon beams. Methods: The FSD consists of 22 films, 3.8 cm in diameter, separated by 1 mm‐thick water‐equivalent‐plastic spacers. A model of the FSD was created using MCNP5. The photon energy spectrum through each film in the FSD from a 6 MV photon source was simulated to ascertain any changes that would produce an energy‐dependent response. The absorbed dose within the FSD was simulated for incident beam angles of 0° to 90° relative to the axis normal to the film plane to investigate the orientation dependence of the dosimeter. To determine the water equivalence of the FSD, the percent‐depth‐dose (PDD) profile within a cylindrical water phantom was simulated and compared with PDD measurements with the FSD within a cylindrical, water‐equivalent‐plastic phantom. Separate exposures of the FSD were performed with a 6 MV slit field incident normal and parallel to the film plane. The results of these exposures were compared with TLD microcube measurements to verify the accuracy of the FSD. Results: Variations in the photon energy spectrum throughout the FSD are minimal, producing an absorbed‐dose energy response of less than 0.1%. The absorbed dose within the FSD varies less than 1.5% as a function of incident beam angle. PDD measurements with the FSD agree with Monte Carlo simulations within 2%. Differences between FSD and TLD measurements within the slit field are less than the respective expanded overall measurement uncertainties of 2.5% and 5.2%. The FSD and TLD measurements agree within 0.5 mm in the field penumbra. Conclusion: For 6 MV photons, the FSD is energy independent, orientation independent, and water equivalent within 2%, and has been verified for three‐dimensional dosimetry.