In Vivo Dosimetry With a Linear MOSFET Array to Evaluate the Urethra Dose During Permanent Implant Brachytherapy Using Iodine-125

Purpose To develop a technique to monitor the dose rate in the urethra during permanent implant brachytherapy using a linear MOSFET array, with sufficient accuracy and without significantly extending the implantation time. Methods and Materials Phantom measurements were performed to determine the optimal conditions for clinical measurements. In vivo measurements were performed in 5 patients during the 125 I brachytherapy implant procedure. To evaluate if the urethra dose obtained in the operating room with the ultrasound transducer in the rectum and the patient in treatment position is a reference for the total accumulated dose; additional measurements were performed after the implantation procedure, in the recovery room. Results In vivo measurements during and after the implantation procedure agree very well, illustrating that the ultrasound transducer in the rectum and patient positioning do not influence the measured dose in the urethra. In vivo dose values obtained during the implantation are therefore representative for the total accumulated dose in the urethra. In 5 patients, the dose rates during and after the implantation were below the maximum dose rate of the urethra, using the planned seed distribution. Conclusion In vivo dosimetry during the implantation, using a MOSFET array, is a feasible technique to evaluate the dose in the urethra during the implantation of 125 I seeds for prostate brachytherapy.