Revision 4 for 'Iodine-131'

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I131

131I is a radioisotope used in the diagnosis and treatment of thyroid lesions. It is one of the oldest radiotracers used in nuclear medicine, spanning over 50 years. It is predominately used in ablation therapy for patients post thyroidectomy for thyroid cancer. 

131I is taken up by the thyroid cells and undergoes trapping and organification. It decays by beta emission (which damages the thyroid tissue) and gamma emission (with a 364 keV photopeak used in imaging), with a half life of approximately 8 days. The high energy gamma emission leads to poor resolution images. However, the long half-life is beneficial for detecting occult metastatic disease because imaging can be done over a few days after oral administration of the radiopharmaceutical.

It is predominately used in ablation therapy for patients post thyroidectomy for thyroid cancer. Generally, the surgeon performs a near-total thyroidectomy (in order to preserve parathyroid function and due to the inherent difficulty in locating thyroid tissue deep within the neck). Therefore, an 131I  scan is first performed after the patient has had surgery to look for functioning remnant thyroid tissue. 

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