Dose measurement based on spectral chi-square minimization using GAFchromic MD-55 film

Abstract

A new method of measuring dose using GAFchromic MD-55-2 type film is proposed. It involves 2D fitting of optical density, as a function of both wavelength and dose, to a set of measured calibration data. An unknown dose to a film can then be determined by minimizing the chi-square between the measured optical densities and the fitted calibration data. The methodology was tested, using 60Co calibration, by measuring the dose deposited in films irradiated by a 32p source in a semi-infinite geometry. The results are compared with the expected dose obtained From beta counting, taking into account the dose loss correction due to various attenuating materials, including the film substrate itself, which was determined to be 20%. It was found that the root mean relative deviation between the measured and expected doses was approximately 3%. The excellent agreement also demonstrated the possibility of calibration between gamma and beta particles.

Dosimetric properties of the new 125I BrachySeed™ model LS-1 source

“…10 Despite the shortcomings of the GafChromic film, which can limit the precision as in early film dosimetry for 125 I seed, 11 careful film and equipment control can greatly improve precision and accuracy. 10,12,13 In this work, we performed an experiment involving two Model 6711 seeds as controls. The measured dose parameters were compared with published values and used as a benchmark for the validation of our experimental methodology.…”

“…The calibration procedure has been described in an earlier work. 12 The exposed films from the long exposure were kept together in a light tight environment at room temperature for approximately one week before readout. It was important to keep the exposed films in identical environmental and temporal conditions so as to reduce the effects from these external factors and to establish a proper control with the Model 6711.…”

“…The net optical density spectral curves were smoothed over wavelengths to reduce the noise due to the thin-film interference and were then converted to dose distribution via an interpolating spline fitting and spectral chi-square minimization method. 12 This method increases the sensitivity of the film response at small doses and avoids the nonlinear effects at large doses. The films had been precalibrated using a 60 Co source.…”