title: Dependency of EBT2 film calibration curve on postirradiation time.
abstract: OBJECTIVE
  The Ashland Inc. product EBT2 film model is a widely used quality assurance tool, especially for verification of 2-dimensional dose distributions. In general, the calibration film and the dose measurement film are irradiated, scanned, and calibrated at the same postirradiation time (PIT), 1-2 days after the films are irradiated. However, for a busy clinic or in some special situations, the PIT for the dose measurement film may be different from that of the calibration film. In this case, the measured dose will be incorrect. This paper proposed a film calibration method that includes the effect of PIT.
  
  
  METHODS
  The dose versus film optical density was fitted to a power function with three parameters. One of these parameters was PIT dependent, while the other two were found to be almost constant with a standard deviation of the mean less than 4%. The PIT-dependent parameter was fitted to another power function of PIT. The EBT2 film model was calibrated using the PDD method with 14 different PITs ranging from 1 h to 2 months. Ten of the fourteen PITs were used for finding the fitting parameters, and the other four were used for testing the model.
  
  
  RESULTS
  The verification test shows that the differences between the delivered doses and the film doses calculated with this modeling were mainly within 2% for delivered doses above 60 cGy, and the total uncertainties were generally under 5%. The errors and total uncertainties of film dose calculation were independent of the PIT using the proposed calibration procedure. However, the fitting uncertainty increased with decreasing dose or PIT, but stayed below 1.3% for this study.
  
  
  CONCLUSIONS
  The EBT2 film dose can be modeled as a function of PIT. For the ease of routine calibration, five PITs were suggested to be used. It is recommended that two PITs be located in the fast developing period (1 ∼ 6 h), one in 1 ∼ 2 days, one around a week, and one around a month.
authors: Department of Medical Imaging and Radiological Sciences, I-Shou University, Kaohsiung 82445, Taiwan.
authors: Liyun
authors: L
authors: Chang
authors: Department of Radiation Oncology, Chi Mei Medical Center, Liouying, Tainan 73657, Taiwan.
authors: Sheng-Yow
authors: SY
authors: Ho
authors: Department of Medical Imaging and Radiological Sciences, I-Shou University, Kaohsiung 82445, Taiwan.
authors: Hueisch-Jy
authors: HJ
authors: Ding
authors: Medical Physics and Informatics Laboratory, Department of Electronics Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan.
authors: Tsair-Fwu
authors: TF
authors: Lee
authors: Department of Radiation Oncology, Sinlau Christian Hospital, Tainan 70142, Taiwan.
authors: Pang-Yu
authors: PY
authors: Chen
mesh_terms: D002138:Calibration; D005373:Film Dosimetry / Q000379:methods*; D011879:Radiotherapy Dosage; D013997:Time Factors