Herrera, Ramses

High dose rate HDR brachytherapy dose distribution is highly localized and has a very
sharp dose fall-off. Thus one of the most important part of the treatment is the localization and
immobilization of the applicator from the implantation to the setup verification to the treatment
delivery. The smallest motions of the patient can induce a small rotation, tilt, or translational
movement of the applicator that can convert into miss of a significant part of the tumor or to over
irradiating a nearby critical organ.
The purpose of this study is to revise most of the HDR types of treatments with their applicators
and their localization challenges. Since every millimeter of misplacement counts the study will
look into the necessity of increasing the immobilization for several types of applicators.
This study data indicates that an improvement of the immobilization devices for HDR is
absolutely necessary. Better applicator fixation devices are required too. Developing new
immobilization devices for all the applicators is recommended.

A Monte Carlo model has been developed using MCNP5 to simulate the Nucletron Ir-192 HDR source in order to investigate the influence of tissue heterogeneities on dose calculations compared to the dose in homogeneous water media, as it is typically calculated by brachytherapy Treatment Planning Systems (TPS). Validity of the simulation was verified in water medium in comparison with peer reviewed results using the dosimetric parameters recommended by AAPM, Task Group-43. The dose-rates in simulated prostate, bladder and rectum were compared to those obtained in the homogeneous water phantom. Based on the resulting dose differences, it is inferred that TPS algorithms for brachytherapy dose calculations overestimate the dose to tissues like prostate and bladder by up to 49%. A clinically relevant dose underestimation of 5.5% to the rectum was also found. We recommend that further investigation using actual patient CT data as input to the Monte Carlo simulation be performed.