Improving the energy response of external beam therapy (EBT) GAFCHROMIC Dosimetry films at low energies (s100keV)

Many studies have been conducted to investigate the energy dependence of External Beam Therapy (EBT) GAFCHROMICTM dosimetry films and showed that the different film models are water equivalent down to about 100 keV and their response is energy independent down to about 100 keV. The purpose of our study is to investigate the energy dependence of commercially available films to understand the physics behind the energy variation of the films response as well as the intrinsic energy dependence and to develop a new prototype with more uniform energy response at low energies (≤100 keV). The study is comprised of two components. The first component is evaluating the energy dependence of three different commercial GAFCHROMICTM dosimetry film models that represent the three different film configurations, EBT, EBT2 and EBT3 that have been manufactured up to date. The energy dependence evaluation can be further divided into two parts. The first part is experimentally evaluating the energy dependence of the response by irradiating the films to a fixed dose of 2Gy to water at different beam qualities (ranging from 50kVp to 60Co) and then measuring the corresponding response of the film, i.e. net optical density change, using a flat bed document scanner. The second part of the energy dependence evaluation is using the DOSRZnrc Monte Carlo code, which is one of the user codes of the EGSnrcMP (Electron-Gamma-Shower) Monte Carlo code, to calculate the corresponding absorbed dose to water energy dependence of the films, i.e. the ratio of dose to water to dose to the active volume of the film, using the experimental beams spectra, which were calculated using SpekCalc, an x-ray spectrum generating software, by matching the measured HVL, tube potential and added filtration of the experimental beams. The first component of our study would help us 1) understand the physics behind the energy variation of the film response, 2) evaluate the intrinsic energy dependence of the film, i.e. dose to film sensitive layer per unit film response as a function of beam quality, using the measured energy response and the corresponding absorbed dose to water energy dependence, 3) guide us to what can be done to improve the energy dependence at low energies. The second component of the study is adjusting the film sensitive layer composition with the guidance of Monte Carlo simulations of the absorbed dose to water energy dependence of the film to reduce the energy dependence of the film response at low energies, and making new film prototypes based on the new possible elemental compositions, then experimentally evaluating their energy response. Improving the energy dependence of the EBT GAFCHROMICTM dosimetry film at low energies will make it more suitable for high quality clinical dosimetry. It would be very convenient for clinical dosimetry to have one film dosimetry calibration curve that works at kilovoltage and megavoltage beam qualities. This would allow us to create the film dosimetry calibration curve at, for example, 60Co and then use it for film dosimetry at kilovoltage and other megavoltage beam qualities. On the other hand, since the x-ray spectrum changes in quality and quantity with depth as it penetrates into patient tissue, it is very important that the response of the detector as a function of beam quality be as flat as possible to minimize the uncertainty and error in dose determination. Thus, improving the energy dependence at low energy will result in improving the accuracy of film dosimetry.

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