High‐sensitivity GafChromic film dosimetry for 125I seed

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Abstract
The dose response of high‐sensitivity GafChromic film to photons from 125I seeds for doses up to 200 Gy was established. The optical densities were measured using two types of densitometers: (a) a Macbeth spot densitometer with broadband light spectrum, and (b) an LKB He‐Ne laser scanning microdensitometer with red light of wavelength 632.8 nm. The net optical density was found to be a power function of dose with exponents of 0.858 and 0.997, for the Macbeth and LKB densitometers, respectively. Film sensitivity with the LKB densitometer was about double of that with the Macbeth densitometer. The dose measurements were performed using the high‐sensitivity GafChromic films for 125I model 6702 seed in solid water phantom. Each film was positioned parallel to the seed's long axis and centered at the seed's transverse axis. Films were exposed at various distances, ranging from contact to 3 cm from the seed center. The radiation dose delivered to the film center varied from 7 to 50 Gy, depending on the distance. The optical density at the film center was measured using both types of densitometers. Dose conversion was achieved with the established dose response curves for the respective densitometers. The dose values, along the seed's transverse axis obtained using both densitometers, were compared with each other, and also compared with published thermoluminescent dosimeter (TLD) data and Monte Carlo results. General agreement was found. It was concluded that the high‐sensitivity GafChromic film measurement is a feasible method for 125I seed dosimetry in solid water phantom. Furthermore, in order to investigate the energy dependence of the dose response of the high‐sensitivity GafChromic film, the dose responses of such film for photon energies, 660 and 1250 keV, emitted by 137Cs and 60Co, respectively, for doses up to 200 Gy were studied. The dose response curves, obtained with the LKB laser densitometer, were essentially linear for both radionuclides. However, those curves obtained with the Macbeth densitometer were fitted to power functions of dose, with an exponent of 0.839 and 0.849 for 137Cs and 60Co, respectively. The relative sensitivities for 125I and 137Cs compared to that for 60Co were found to be 0.56 and 0.93, respectively.

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