Chemical dosimetry in the near zone of brachytherapy sources

New treatment methods and technical advances in the field of radiation oncology have challenged Medical Physicists to design sophisticated dosimeters to measure the dose in the near-zone of brachytherapy sources. To accurately map steep dose-gradients in the near-zone the detector should not perturb the radiation field and should be tissue equivalent, energy independent, and dose rate independent. The treatment of neointimal hyperplasia, prostate cancer with permanent brachytherapy seed implants, and transition zone effects in 3-D conformal therapy has spurred the development of new dosimeters and apparatuses for evaluating steep dose-gradient regions. This project has developed two dosimetry systems to accurately measure the radial dose in the near-zone of clinically used 137Cs and 192Ir sources. A polymer gel dosimeter was designed and implemented for this project. This study represents the first attempt to use a custom-made polymer gel dosimeter to exclusively investigate the radial dose in the near-zone of brachytherapy sources. The custom-made polymer gel dosimeter in conjunction with a high-field strength small-bore MR scanner was determined to provide accurate and reproducible data of the dose in the near-zone. A second dosimetry system consisted of gaf-chromic film-"the gold standard"-and a custom-built precision scanner. The scanner was designed with a 670nm wave length diode-laser and precision-built micrometers for scanning. The scanner provided optimal scanning conditions for the gaf-chromic film. The gaf-chromic film data for the radial dose in the near-zone of the clinically used 137Cs and 192Ir sources agreed well with the data from the polymer gel dosimeter. This project has confirmed the radial dose results in the near-zone of the often-cited Monte-Carlo data of Williamson. The polymer gel and the gaf-chromic film radial dose data agree well with the Monte-Carlo data in the near-zone of a single 137Cs source and a single 192Ir seed. This project has extended the reliable data in the near-zone from 3mm down to 1mm. Both detectors were shown to be excellent candidates for near-zone dosimetry, and have created an empirical benchmark for the Monte-Carlo theorist.