Intracavitary offline "in vivo" dosimetry for high dose-rate prostate brachytherapy with 192Ir: development of technology and first results of its application

The first results of development and clinical testing the technology “in vivo” intracavitary dosimetry using offline mini- and microdetectors in the form of crystals of millimeter sizes and powders of microcrystals (Al2O3, alanine) in application to high dose rate brachytherapy of prostate cancer with 192Ir are presented in the paper. The detectors were hermetically packed inside flexible tissue-equivalent tubes in the conditions of electrons equilibrium, and then were placed inside the medical catheters. The measurements of radiation-induced signals in the detectors were conducted using two independent methods - by the method of thermally stimulated luminescence (TL) and by the method of electron paramagnetic resonance (EPR). Two different methods were used in order to be sure in the results of dose measurements. Absorbed doses were estimated using calibration dependencies constructed for each detector after irradiation of detectors by standard sources of ionizing radiation. This technology allow to measure of absorbed doses in many parts of the body and in a case of intracavitary locations of minidetectors, and in the absence of cable connections with registration systems. The testing of this technology was begun in A. Tsyb MRRC starting from 2016. The areas of interest for instrumental dose assessments defined by radiation oncologist are the following: bladder (catheter with microdosimeters was inserted through the urethra), and rectal area (catheter with microdosimeters was inserted through rectum). The usage of medical catheters is a part of high dose rate brachytherapy medical technology. Presence of detectors inside the catheter does not interfere with the carrying out of all necessary medical procedures - because of the miniature sizes of detectors. All procedures with inserting of microdetectors were carried out by a radiation oncologist under ultrasound control. Comparison of results of instrumental dosimetry and calculated data shows that in the region of the bladder the measured dose values agree quite well with calculated doses (the differences do not exceed 5% of calculated dose). Meanwhile, the calculated dose in the rectal region significantly exceeds the data obtained by instrumental method - from two to five times. It was concluded that existing system of therapeutic dose planning by calculation needs essential improvement in order to be more precise with accounting for real geometry of irradiated organs and tissues, as well as differences in densities of the irradiated tissues, especially between bone and soft tissues.