Dosimetry of brachytherapy by the Oncoseed 6711 using glass rod dosimeters and EGS5 in a geometry lacking radiation equilibrium scatter conditions

Full text: The aim of this study was to develop a dose calculation method which is applicable to the inter-seed attenuation and the geometry lacking the equilibrium radiation scatter conditions in brachytherapy. The dose obtained from measurement with a radiophotoluminescent glass rod dosimeter (GRD) was compared to the dose calculated with the Monte Carlo (MC) code 'EGS5,' using an 1251 source structure. The GRDs were irradiated with 1251 on coseed 6711 in a human head phantom. The phantom was a cylinder made of 2 mm thick PMMA with a diameter of 18 cm and length of 16 cm. Some of the GRD positions were so close to the phantom surface that the backscatter margin was less than 5 cm, insufficient for photons. The EGS5 simulations were found to reproduce the relative dose distributions as measured with the GRDs to within 25% uncertainty in the geometry lacking the equilibrium radiation scatter conditions. The absolute value of the GRD measurement agreed with the American Association of Physicist in Medicine Task Group No 43 Updated Protocol (AAPM-TG43U1) formalism to within 3% of the reference point. The results of this investigation support the validity of both the MC calculation method and GRD measurement in this study as well as the TG-43U1 formalism. Also, this calculation is applicable to inter-seed attenuation and the geometry lacking the equilibrium radiation scatter.