title: Improving the energy response of external beam therapy (EBT) GafChromicTM dosimetry films at low energies (≤ 100 keV).
abstract: OBJECTIVE
  Purpose of this work is to investigate the effects of varying the active layer composition of external beam therapy (EBT) GafChromic(TM) films on the energy dependence of the film, as well as try to develop a new prototype with more uniform energy response at low photon energies (⩽ 100 keV).
  
  
  METHODS
  First, the overall energy response (S(AD, W)(Q)) of different commercial EBT type film models that represent the three different generations produced to date, i.e., EBT, EBT2, and EBT3, was investigated. Pieces of each film model were irradiated to a fixed dose of 2 Gy to water for a wide range of beam qualities and the corresponding S(AD, W)(Q) was measured using a flatbed document scanner. Furthermore, the DOSRZnrc Monte Carlo code was used to determine the absorbed dose to water energy dependence of the film, f(Q). Moreover, the intrinsic energy dependence, kbq(Q), for each film model was evaluated using the corresponding S(AD, W)(Q) and f(Q). In the second part of this study, the authors investigated the effects of changing the chemical composition of the active layer on SAD, W(Q). Finally, based on these results, the film manufacturer fabricated several film prototypes and the authors evaluated their S(AD, W)(Q).
  
  
  RESULTS
  The commercial EBT film model shows an under response at all energies below 100 keV reaching 39% ± 4% at about 20 keV. The commercial EBT2 and EBT3 film models show an under response of about 27% ± 4% at 20 keV and an over response of about 16% ± 4% at 40 keV.S(AD, W)(Q) of the three commercial film models at low energies show strong correlation with the corresponding f(-) (1)(Q) curves. The commercial EBT3 model with 4% Cl in the active layer shows under response of 22% ± 4% at 20 keV and 6% ± 4% at about 40 keV. However, increasing the mass percent of chlorine makes the film more hygroscopic which may affect the stability of the film's readout. The EBT3 film prototype with 7.5% Si shows a significant improvement in the energy response at very low energies compared to the commercial EBT3 films with 4% Cl. It shows under response of 15% ± 5% at about 20 keV to 2% ± 5% at about 40 keV. However, according to the manufacturer, the addition of 7.5% Si as SiO2 adversely affected the viscosity of the active fluid and therefore affected the potential use in commercial machine coating. The latest commercial EBT3 film model with 7% Al as Al2O3 shows an overall improvement in SAD, W(Q) compared to previous commercial EBT3 films. It shows under response at all energies <100 keV, varying from 20% ± 4% at 20 keV to 6% ± 4% at 40 keV.
  
  
  CONCLUSIONS
  The energy response of films in the energy range <100 keV can be improved by adjusting the active layer chemical composition. Removing bromine eliminated the over response at about 40 keV. The under response at energies ≤ 30 keV is improved by adding 7% Al to the active layer in the latest commercial EBT3 film models.
authors: Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada and Department of Radiation Oncology, Jewish General Hospital, Montréal, Québec H3T 1E2, Canada.
authors: H
authors: H
authors: Bekerat
authors: Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada and Department of Radiation Oncology, Jewish General Hospital, Montréal, Québec H3T 1E2, Canada.
authors: S
authors: S
authors: Devic
authors: Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada and Department of Radiation Oncology, Jewish General Hospital, Montréal, Québec H3T 1E2, Canada.
authors: F
authors: F
authors: DeBlois
authors: Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada.
authors: K
authors: K
authors: Singh
authors: Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada.
authors: A
authors: A
authors: Sarfehnia
authors: Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada.
authors: J
authors: J
authors: Seuntjens
authors: Ashland Specialty Ingredients, 1361 Alps Road, Wayne, New Jersey 07470.
authors: Shelley
authors: S
authors: Shih
authors: Ashland Specialty Ingredients, 1361 Alps Road, Wayne, New Jersey 07470.
authors: Xiang
authors: X
authors: Yu
authors: Ashland Specialty Ingredients, 1361 Alps Road, Wayne, New Jersey 07470.
authors: D
authors: D
authors: Lewis
mesh_terms: D000535:Aluminum / Q000737:chemistry; D002713:Chlorine / Q000737:chemistry; D005373:Film Dosimetry / Q000379:methods* / Q000592:standards; D009010:Monte Carlo Method; D011188:Potassium / Q000737:chemistry; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D012015:Reference Standards; D012825:Silicon / Q000737:chemistry; D014867:Water
chemical_list: D014867:Water; D002713:Chlorine; D000535:Aluminum; D011188:Potassium; D012825:Silicon