Iodine-131 - UW-L Brachy CourseIodine-131 - UW-L Brachy Course
Figure 1. Illustrates I-131 scan prior to I-131 therapy. Figure 2. Illustrates I -131 scan post I-131 therapy.
Courtesy of Thyrolink
Iodine-131 - UW-L Brachy Course
Figure 3 Illustrates Iodine 131 in capsule and liquid form.
Courtesy of Covidien
Relevant historical data: Iodine was first discovered in 1811 by Benard Cortois. Courtois discovered the iodine was found in water were parts of seaweed ash were dissolved. Iodine is typically found in seaweed, sponges and other materials. The stable form of iodine is called Iodine 127. Iodine is a solid with a bluish black color. In 1938, Iodine 131 was discovered at the University of California- Berkeley by Glenn T. Seaborg and John J. Livingood. Iodine -127 is placed in a nuclear reactor and fission is done with uranium atoms to produce Iodine-131.
Chemical/Radioactive Composition: Chemical Symbol: I
Atomic Number (Z, # of protons) = 53
Mass Number (A, # of protons + # of neutrons)=131
I-131 is produced in a nuclear fission. It is iodine 127 before being placed in nuclear reactor.
Energy Characteristics: Iodine 131 emits both gamma rays and beta particles. The average energy delivered from gamma ray is 364 keV. The maximum energy delivered by beta particles is 600 keV and the average energy from Beta particles is 190 keV. 2mm in tissue is the maximum range from the emitted β particles. The exposure rate constant for Iodine 131 is 2.27 R-cm2/mCi-hr.
Exposure Rate Constant: Exposure rate constant for I-131 is 2.27 R-cm ^2/mci-hr
Half-life Properties: Half- Life for Iodine 131 is 8.02 days, and decays about 8% per day.
Forms available for use: Iodine 131 is available as aqueous solution or as sodium iodine capsules.
HVL in lead: Iodine 131 has a HVL in lead of 2.5 mm and the TVL (Tenth Value Layer) in lead of 10 mm.
Measurement/Calibrations/QA: Prior to the administration of Iodine 131, the patient’s room needs special preparation to avoid contamination of I 131. First, anything that the patient touches should be covered with absorbent pads or plastic such as the telephone, floor, faucets, etc. Second, any body excretions (urine, stool or vomitus) should be flushed down toilet more than one time. Finally, after patient’s discharge room should be surveyed and contaminated material disposed properly. When the patient is discharged the dose measurement must be lower than 5 mSv as mandated by NRC.
Used in formula/calculation: Formula to calculate the effective half-life (T eff):
T eff=T phy x Tbio
T phy + T bio
Calculation to determine fixed activity per gram of tissue:
Administered Activity (μCi) =
μCi /g selected x gland weight (G) x 100
%uptake @ 24hr
Delivered dose method:
Administered Activity (μCi) =
Gy selected x gland weight (g) x 100
% uptake @ 24 hr x 90
Uses in Radiation Oncology: I-131 is used for treatment of hyperthyroidism and thyroid cancer.I-131 has a been used in conjuction with monoclonal antibodies to treat certain types of Non-Hodgkins lymphoma.
Treatment Planning: The treatment dose is all dependent upon the disease and also dose is dependent of Radioiodine Uptake Test (RAIU).
Hyperthyroidism: The typical dose given for hyperthyroidism is 2.96-7.4 MBq (80-200 μCi) per gram of thyroid tissue.
Thyroid Cancer: A) Thyroidectomy: 2.75–5.5 GBq (75–150 mCi) is the typical dose given and the amount of the residual functioning tissue present. B) Thyroid Cancer present neck or mediastinal lymph nodes: 5.55–7.4 GBq (150–200 mCi) is the typical dose given C) Distant metastases: >7.4 GBq (200 mCi) is the dose given.
Prior to administration of Iodine 131 all medications containing iodine should be discontinued. Post-treatment a whole body scintigraphy should be performed 3 to 14 day after administration of Iodine 131 for the purposes of staging.
One other interesting fact: In the Chernobyl accident it was learned that childeren are more to sensitive to get thyroid cancer from exposure to Iodine 131. Adults are quite resistant to get thyroid cancer when exposure to Iodine 131, but are likely to develop leukemia.
Key Terms:
μCi= microCurie (10-6 Ci)
GBq= gigabecquerel (109 Bq)
MBq= megabecquerel (106 Bq
Illustrating room preparation prior to administration of I 131:

Perez, Carlos A, Luther W. Brady, Edward C. Halperin, and Rumpert K. Schmidt-Ullrich.
Principles and Practice of Radiation Oncology. 4th Edition. Philadelphia: Lippcincott Wlliams& Wilkens,
2004 Society of Nuclear Medicine :Procedure Guideline for Therapy of Thyroid Disease with Iodine-131 (Sodium Iodine) Version 2.0
Thyrolink:Thyroid scintigraphy is done both in the case of benign and malignant thyroid disease (Figure 1&2) Accessed at on March 13,2008
Covidien:Sodium Iodide I 131 Capsules Therapeutic (Figure 3) Accessed at on March 13,2008

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