Investigation of dose distribution 252Cf Isotron brachytherapy source based on TG-43U1 protocol by Monte Carlo method.

Introduction:
The commercial 252Cf sources are too large in size and clinical applications of neutron brachytherapy (NBT) are limited to a small number of intracavitary treatments of cervical cancers. Recently, under the Cooperative Research and Development Agreement (CRADA) with Isotron Inc., the Oak Ridge National Laboratory (ORNL) encapsulated a new medical 252Cf sources, called Isotron sources that used to intracavitary and interstitial brachytherapy. On the other hand, prior to the clinical application of brachytherapy sources, their dose distributions should be investigated. In this study, Monte Carlo calculations of dose distribution the 252Cf Isotron source is done in the water phantom.
 
Materials and Methods:
Dose distributions parameters based on TG-43U1 protocol are radial dose function and anisotropy function. Physical and geometrical parameters of the 252Cf source were simulated by MCNPX (2.6.0) code based on TG-43U1 protocol in the water medium. To estimate the dose rate distribution in water, the source was situated in the center of a spherical 20 cm radius water phantom. The Radial dose function was determined in water, in a cylindrical annulus 0.2 mm × 0.2mm deep positioned along the transverse axis at distances ranging from 0.5 to 10 cm from the source center. The neutron absorbed doses in water were calculated using F6 tallies. Anisotropy functions were calculated at intervals of 1, 2, 3, 4, 5, 10 cm and at different polar angles from θ=0º to 90º with respect to the source long axis in the water phantom.
Results:
Statistical uncertainty for neutron absorbed dose rates at r ≤ 3.5 cm are lower than 0.3%, and at 4-10cm are lower than 0.6%. For radial dose function results show that the radial dose function decreased more slowly. Uncertainty calculated is lower 0.4%. The Anisotropy function exhibited little anisotropy about the capsule. Thus Anisotropy function can be considered unity for practical purposes with no significant loss in accuracy due to the thin walls, low cross-sections, and High-Z materials comprising the encapsulation. The uncertainty in calculated data is lower 1 %.
 
Conclusion:
The calculated dosimetry parameters of Isotron source indicate that Anisotropy function can be considered unity for practical purposes. The radial dose function with increased distances decreased more slowly. Then neutrons can leave their energy at greater distances from the source. Hence, neutrons can be used to treat tumors that are large, due to their greater penetration depth.