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Sodium Iodide (123I). The major indications for thyroid imaging with sodium iodide (123I) are for evaluation of thyroid function (uptake) and/or morphology (imaging). The patient fasts before receiving the oral dose of 0.1 to 0.4 mCi (3.7—14.8 MBq) of sodium iodide (123I) typically in an encapsulated dosage form. Images are obtained of the thyroid and surrounding area 4 to 6 hours after ingestion.

Sodium Iodide (131I). Diagnostic use of sodium iodide (131I) with microcurie doses for evaluating thyroid function and/or morphology is no longer preferred because of the higher radiation dose associated with particle decay from this isotope. Doses from 1 to 10 mCi (37-370 MBq) are used for whole-body imaging to detect metastatic thyroid cancer and/or residual thyroid tissue following thy-roidectomy. Whole-body images are obtained 48 to 72 hours later.

Therapeutic use of sodium iodide (131I) occurs in hyperthyroidism or differentiated types of thyroid cancers. A patient with hyperthyroidism will receive an oral dose of 5 to 10 mCi (185-370 MBq) of sodium iodide (131I). The dose could be higher for patients with toxic nodular goiter or other special situations. Treatment doses for differentiated thyroid carcinoma with or without metastases range from 30 to 200 mCi (1,110-7,400 MBq). Seven days after the therapeutic dose, a whole-body scan is performed in an effort to detect metastases not seen on the pretherapy diagnostic images. Thyroid hormone medication will be discontinued for several weeks prior to radiotherapy and thyroid-stimulating hormone (TSH) levels must be evaluated to ensure maximal uptake of the radiotracer for therapy.15

Iodine (131I) Tositumomab (Bexxar). Tositumomab is a murine IgG monoclonal antibody directed against the CD20 antigen found on the surface of normal and malignant B lymphocytes. Iodine tositumomab (131I) is a radioiodi-nated derivative of the antibody indicated for the treatment of patients with CD20 antigen-expressing relapsed or refractory, non-Hodgkin lymphoma. A schematic representing an IgG antibody labeled with covalently linked I-131 is shown. The exact number and position of the covalently linked I-131 is not described.

The treatment regimen occurs in two discrete steps: the dosimetric (diagnostic distribution) and therapeutic steps. Each step consists of a sequential infusion of unlabeled Tositumomab followed by iodine (131I) tositumomab. The patient is premedicated with a thyroid protective agent (either potassium iodide or Lugol solution) for at least 24 hours, and will also receive acetaminophen and diphenhydramine shortly before the infusions begin. The dosimetric step administers 450-mg unlabeled tositumomab followed by 5 mCi (185 MBq) of iodine (131I) tositumomab.

The patient is imaged three times over the next 7 days to assess the biodistribution of the radiolabeled antibody. Imaging occurs within 1 hour following the radiolabeled antibody infusion, 2 to 4 days after the infusion, and also 6 to 7 days after the infusion. If the dosimetric dose's biodistribution is altered, no further drug will be administered. If the dosimetric dose's biodistribution is as expected, the thera peutic dose can be administered 7 to 14 days after the dosi-metric infusion.

The therapeutic dose is calculated based on the assessment of the patient's biodistribution and elimination of the dosimetric infusion as well as the patient's platelet count. Clinical studies have reported calculated therapeutic doses of 450-mg unlabeled tositumomab followed by 50 to 200 mCi (1,300-5,900 MBq) of iodine (131I) tositumomab. Thyroid protective agents are administered daily until 2 weeks after the last radioiodine dose.16

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