Dosimetric Characteristics of the Brachytherapy Sources Based on Monte Carlo Method

The term of brachytherapy, also known as “internal radiotherapy, sealed source radiotherapy, curietherapy or endocurietherapy” is from Greek work brachy means short distance and therapy (treatment) and also known as internal radiotherapy or sealed source radiotherapy. Brachytherapy is a special form of radiotherapy where a radioactive source is carefully placed on or inside the area to be treated. Brachytherapy sources are usually encapsulated; they can be used within the body cavities close to the tumor, placed in a lumen of organs, implanted in to the tumor or placed over the tissue to be treated. The main purpose in radiation therapy is controlling disease and reducing side effects. For a good clinical result one must assure the dose concentrate in the tumor mass and surrounding volume which is at risk of the tumor micro-extensions, while minimizing radiation received by the normal tissue. This can be verified by experimental measurement which is the base of Brachytherapy dosimetry. Due to the high dose gradient near the source and low signal to noise ratio at great distances, experimental dosimetry in Brachytherapy is very complicated or even in small distances is impossible. Also the dose variation with angle in 4┨ geometry of the source must be considered, since routine experimental measurement does not represent this. One of the widely used techniques for solving this problem is Monte Carlo simulation of radiation transport. The calculation of dose distributions at small distances and also validation of experimental measurement can be done by one of the powerful codes such as MCNP, BEAM, EGSnrc, PENELOPE, GEANT4, and ETRAN/ITS. One of the important parameters in the calculation process is validation of the Monte Carlo calculations with measurement results. This chapter starts with an introduction lecture about brachytherapy history and a short review of the different models for sealed Brachytherapy sources. A brief discuss of brachytherapy dosimetry with computer and measurement is provided. In 1995 the American Association of Physicists in Medicine (AAPM) Task Group No. 43 published a protocol including new formalism for brachytherapy dose calculation and updated in 2004 as TG-43U1. The concept of AAPM recommendation and TG 43 formalism are introduced. Application of Monte Carlo in simulation, guideline for Monte Carlo dosimetry, calculation methodology, requirement for simulation and validation of calculation are then outlined. A