Abstract
<title>Abstract</title> <p>Purpose Metastatic differentiated follicular thyroid cancer with functioning metastases is an uncommon and challenging clinical presentation. Radioactive iodine (RAI) is an effective therapy for treatment of metastatic disease and for management of hyperthyroidism resulting from functioning disease. Determining the optimal iodine-131 (131I) administered activity is challenging, as there may be unpredictable and prolonged whole-body retention of activity, due to 131I metabolism and recirculation. Most RAI therapy is performed using a fixed or empiric approach. However, there are advantages to adopting dosimetry approaches and determining the maximum tolerable activity (MTA). Iodine-124 (124I) positron emission tomography/computed tomography (PET/CT) dosimetry prior to 131I RAI therapy aims to ensure the maximum radiation absorbed dose to tumour lesions is safely delivered while minimising off-target effects and toxicity. Methods A 74-year-old female with functioning follicular thyroid carcinoma (FTC) metastases underwent 124I PET/CT dosimetry prior to 131I RAI administration. The 124I PET/CT estimated that the MTA was 3100 MBq of 131I which would deliver an estimated 217 Gy to a right lung metastasis index lesion. Based on this estimation, an activity of 3000 MBq 131I was subsequently administered to safely achieve a target lesion dose of 200 Gy. Results The patient’s serum thyroglobulin decreased from 21,148 µg/L pre-therapy to 4,610 µg/L six months post-therapy, indicating a good response. Her regular carbimazole was discontinued one month post 131I RAI treatment, and she was commenced on thyroxine with a thyroid-stimulating hormone (TSH) target of less than 0.1. Apart from mild left hip pain, she is now asymptomatic. Conclusion Using 124I PET/CT for dosimetry calculations, as in this case, is a useful approach in RAI therapy for metastatic differentiated thyroid cancer (DTC), with the aim to ensure the maximum radiation absorbed dose to tumour lesions is safely delivered while minimising off-target effects and toxicity.</p>