A minimally invasive method to evaluate 131I kinetics in blood

The dose limiting factor for 131I therapy in patients with thyroid cancer is myelotoxicity, thus an accurate determination of radioiodine activity in the red marrow is of paramount importance. The reference method for red marrow dosimetry in radiometabolic therapy is based on the measurement of radioiodine kinetics, particularly the activity/time curve in blood. Such a measurement requires withdrawal of blood samples at various times after 131I administration. This procedure involves some potential risk from the radiation protection point of view, such as possible contamination of personnel with blood and disposal of the radioactive blood samples (and syringes). We present here a minimally invasive method to evaluate radioiodine kinetics in the blood, which only requires one blood sample and a set of measurements on the patient's thigh made with a collimated NaI(Tl) probe. The method has been validated in four patients treated with 131I for thyroid cancer.