Radioactive Iodine Total Body Scan

Indication

For patients with differentiated thyroid cancer, radioactive iodine total body scan is indicated for

1) Evaluating and measuring locoregional uptake before radioactive iodine ablation following thyroidectomy

2) Imaging after ablation or treatment with radioactive iodine

3) Follow-up of patients with moderate or high risk of recurrent thyroid cancer, when Tg measurements are unreliable.

What is Radioactive Iodine Total Body Scan? How Does It Show the Tumor Focus?

Theranostics is a recently developing field of the medicine. This approach takes body images using a tumor-specific agent to locate the tumor and its metastasis and their potential future locations and it also uses a specific agent with pre-determined therapeutic efficiency for the diseased tissue. This approach enables switching from traditional medicine to contemporary personalized medical procedures.

Administration of low-dose radioactive iodine (I-131 or I-123) to gain images with high sensitivity and specificity of tumor tissues for diagnostic purposes combined with high-dose radioactive iodine (I-131) for specific and targeted treatment of these tumors is the oldest known example of theranostic practices in differentiated thyroid cancer.

Thyroid tissue has a unique ability to collect all iodine from the bloodstream and store it in the gland. Similar to iodine, radioactive iodine is taken up to thyroid gland via sodium-iodine symporter (NIS) membrane proteins and stored in follicular cells of the thyroid gland. NIS membrane proteins are also found in papillary- and follicular-type differentiated thyroid cancers, although their levels are lower compared to healthy thyroid tissue. I-131 is a radionuclide with physical half-life of 8.1 days; it emits 364 KeV energy gamma radiation used in imaging studies and an average of 0.192 MeV energy beta particle used in treatment. Physical half-life of I-123, which is another radio-isotope used for imaging, is approximately 13 hours and 159 KeV gamma radiation is used for imaging.

Anaplastic (undifferentiated) and medullary thyroid cancers as well as other rarer malignant diseases of thyroid gland show no iodine uptake; thus, radioactive iodine is not used for those patients.

Who is Eligible For Radioactive Iodine Total Body Scan?

Radioactive iodine total body scan is indicated for patients with differentiated thyroid cancer, for evaluating and measuring locoregional uptake after thyroidectomy and prior to radioactive iodine ablation, for imaging after ablation or treatment with radioactive iodine and for follow-up of patients with moderate or high risk of recurrent thyroid cancer, when Tg measurements are unreliable.

Is Radioactive Iodine Total Body Scan Safe?

Radioactive iodine total body scan is a safe imaging modality which is performed by administering radioactive iodine isotopes (I-131 or I-123)  at low diagnostic dosage before the treatment.

Radioactive iodine total body scan is contraindicated for pregnant patients. It might be necessary to stop breastfeeding in nursing mothers.     

Preparation before Radioactive Iodine Total Body Scan

Prior to radioactive iodine total body scan, medical history regarding the thyroid gland, surgery/pathology reports and past imaging studies as well as doses of past radioactive iodine therapy are reviewed and below listed blood tests are analyzed, when necessary:

• Thyroid function tests (e.g. TSH, fT3, fT4)
• Tg, antiTg
• Beta-hCG, which should performed to rule out pregnancy for all female patients of child-bearing age.

Patients preparation is crucial as thyroid hormone supplements (LT4, LT3) and iodine-containing drugs and foods block radioactive iodine uptake in residual thyroid tissue and extra-thyroidal metastatic lesions.

High TSH level (TSH > 30 mIU/L) is also very important regarding the success of radioactive iodine total body scan. TSH increases radioactive iodine uptake both in healthy thyroid tissue and thyroid cancer cells. There are certain ways to increase the TSH level before the radioactive iodine total body scan:

Cessation of Thyroid Hormone Replacement

If it is planned to stop thyroid hormone replacement before radioactive iodine total body scan, LT4 hormone preparations (Levotiron, Tefor, Euthyrox, Bitiron) should be stopped for minimum 4 weeks and LT3 hormone supplements (Tiromel) should be stopped for at least 2 weeks. Patients who take LT4 can use LT3 for the first two weeks. Targeted TSH is > 30 mIU/L and TSH should be analyzed before administration of radioactive iodine.

Use of Recombinant TSH (rhTSH)

rhTSH (Thyrogen) is administered to hip muscle in 2 doses at 24-hour interval as an alternative to cessation of thyroid hormone replacement. Radioactive iodine is administered by mouth 24 hours after the last dose of rhTSH. Diagnostic serum thyroglobulin (Tg) analysis should be done 72 hours after the final rhTSH injection when recombinant TSH is used. Low-Iodine Diet Low-iodine diet started 2 weeks before a radioactive iodine total body scan is an important part of preparation before the procedure. This diet increases the success rate of radioactive iodine total body scan. Medicines Which Prevent Administration of Radioactive Iodine

  Food That Are Not Allowed Due to High Iodine Concentration

Allowed Foods

How is Radioactive Iodine Total Body Scan Performed?

Oral food and water intake should be stopped for 4-6 hours before radioactive iodine is administered and this restriction should be maintained for approximately 2 hours after the treatment.

When use of prophylactic oral antiemetic drugs is considered for patients complaining of nausea, it should be given before administration of the radioactive iodine.

Radioactive iodine (I-131 or I-123: 185 Mbq (5 mCi)) is in liquid or capsule form; it is administered by mouth at outpatient settings in the Nuclear Medicine Clinic.  

Images are obtained in the Nuclear Medicine Clinic 48 to 72 hours after administration of radioactive iodine.

Patients with normal renal functions are recommended to drink 2.5-3 liters of liquid (excluding milk) and void frequently for 3 days following the radioactive iodine treatment. Thus, radiation exposure for the urinary bladder and salivary glands can be minimized.

Defecation at least once a day is important for minimizing the radiation dose in bowels and increasing the quality of images. Mild laxatives can be used in case of constipation.

It is necessary for patients, who received I-131 radio-isotope, to comply with certain rules to protect their relatives from unnecessary radiation exposure. These restrictions, which are recommended by the Turkish Atomic Energy Authority (TAEK), are listed below, although they depend on the dosage:

Restrictions Recommended by TAEK for Patients Who Received I-131