I 131 therapy

Iodine 131 - beta emitter for treatment and gamma emission for imaging, 8 day half life

Primary Therapeutic Uses

• Hyperthyroidism caused by diffuse or nodular goiter
• Post-surgical ablation of thyroid gland remnants
• Treatment of functioning thyroid metastases

Factors in Estimating Dose:

• Degree of uptake, bulk of tissue, length of residence of Iodine within the tissue (decrease dose for renal failure), radiosensitivity of organ.
• Get Uptake and Scan 2-4 weeks before treatment to help decide dose. Use I123 becase no beta, no stunning of gland

Primary Hyperthyroidism

Treatment options include medication (PTU), surgery and I131 ablation. Graves often treated medically first, but often medication compliance is poor due to side effects.

Determination of I-131 Dose:

• Diffuse Toxic Goiter (Graves Disease)
  • Can use fixed dose between 10-15 mCi or try to adjust based on size of gland, presence of nodularity and uptake
  • Goal is to make patient hypothyroid
  • Can be repeated in 3-6 month intervals
  • Orbitopathy may worsen after treatment

• Toxic Nodular Goiter (Plummer's Disease)
  • Large multinodular glands require larger doses in the 30 mCi range
  • Single nodule can be treated with 15-25 mCi
  • Even with larger doses, often do not cause hypothyroidisms because rest of gland is suppressed.

Patient Preparation

• NPO after midnight
• Pregnancy Test
• Cessation of breat feeding indefinately
• Most stop oral medications 5-7 days pretreatment including multivitamins, *Don't stop beta blockers, don't affect treatment
• Expect sore throat
• Drink pleanty of fluids and void frequently
• May have initial spike in hyperthyroid symptoms in the first few days, then effects should be apparent in 3-4 weeks
• Can't have IV contrast within 3-4 weeks
• Can't have amiodarone within 3-6 months
• No allergy exists, even if patients have IV contrast allergy

Thyroid Carcinoma

• Whole body I 131 scan is usually done 4-6 weeks after surgery (gives time for wounds to heal)
• Treat papillary and follicular not medullary or anaplastic types
• Give rTSH (thyrogen) IM daily, 2 days before treatment
• Post treatment scan day 7-10
• Monitor thyroglobulin after treatment

• Low Risk Disease: resected 1.5 cm nodule confined to gland - 30 mCi
• High Risk Disease:
  • Contained to Thyroid - 100 mCi
  • Positive Lymph Nodes - 150 mCi
  • Lung Metastasis - 200 mCi
  • Bone Metastasis - 250 mCi

• Pulmonary fibrosis has been described when treated lung metastasis with cumulative doses greater than 600 mCi
• Lemon drops helps with damage to salivary glands
• Bone marrow depression seen at cumulative doses greater than 800 mCi

Patient Precautions

NRC Regulations

• Radiation safety officer involved for any dose greater than 33 mCi
• Can treat as outpatient upto 250 mCi

See Also

Thyroid imaging