ABSTRACT
As in other branches of radiation therapy, Monte Carlo (MC) simulation has become an essential dosimetry tool in modern brachytherapy, playing key roles in both clinical practice and research. The most established application of MC methods in brachytherapy is the determination of dose rate distributions around individual radiation sources. Modern sources generally contain low energy radionuclides such as 103Pd, 125I, or 131Cs (mean energies <0.05 MeV, referred to henceforth as low-energy sources) or higher energy radionuclides such as 192Ir and 137Cs (mean energies 0.355 and 0.662 MeV). There are also miniature x-ray sources generating at 50 kVp Bremsstrahlung spectrum. Both source geometries and clinical applications are quite variable. In low dose rate (LDR) brachytherapy, radioactive material and radio-opaque markers are encapsulated to form permanently implantable seeds. In high dose rate (HDR) brachytherapy using a remote afterloader, an iridium pellet is welded to the tip of a drive cable. Miniature x-ray sources with tungsten anodes also fall in the HDR category, even though they emit photons in the low-energy range. While inverse square law dependence is the dominating feature of brachytherapy dose distributions, photon attenuation and scatter build-up in the surrounding medium as well as radiation interactions within the source structure give rise to anisotropic dose distributions. The significant modulation of dose distributions must be properly modeled to attain
clinically acceptable dosimetric accuracy, and these features are not readily derived from analytical methods such as the Sievert integral (Williamson, 1996). The complexities of experimentally measuring single-source dose distributions caused by the sharp dose gradients, low photon energies, and dose rates associated with brachytherapy make computational dosimetry techniques such as MC simulations an essential tool in brachytherapy (Williamson and Rivard, 2005). Furthermore, properly accounting for radiobiological effects in low-energy brachytherapy requires calculation of photon and electron spectra at various distances from sources, which is most accurately done by MC modeling.
