In vitro evaluation of an iodine radionuclide dosimeter (IRD) for continuous patient monitoring

In vivo dosimetry of the patients treated by I-131 is important from patient dosimetry and radiation protection points of view. Knowledge of delivered dose to the target volume and adjacent normal tissues can improve the effectiveness of radioiodine treatment. Herein, design, fabrication, and assessment processes of an iodine radionuclide dosimeter (IRD) are explained. Two CsI(Tl) scintillator crystals coupled to photodiodes were used in IRD fabrication with specifications derived from Monte Carlo (MC) simulation. Linearity, sensitivity, and long-term performance of the system were tested. Delivered dose due to a known administered activity of I-131 was calculated by MC simulation which was validated based on the Medical Internal Radiation Dose (MIRD) Committee formalism, and the calibration factor was provided. Using the current mode signal acquisition method, the system showed a linear response up to 8.2 GBq radioiodine activity to prohibit the pile-up error without a need for correction factor. On the other hand, IRD was sensitive down to the rarely detectable activity of 7.4 MBq. A prototype version of the IRD system has been developed to guide the hospital staff for the safe release of iodine – administered patients and to provide an insight for physicians about the delivered dose to the thyroid and nearby organs.