Iodine-125 - UW-L Brachy CourseIodine-125 - UW-L Brachy CourseIodine-125 - UW-L Brachy Course

Relevant historical data: 1946 - Allen Reid and Albert Keston discovered Iodine-125.
1960 - Dr. William G. Myers (1908-1988) introduced I-125 into biomedicine as a radionuclide for cancer therapy, diagnostic and investigative purposes.
1965-1967 - I-125 seeds conceived and patented by Don Lawrence, health physicist in California.
1965 - First I-125 seed implant brachytherapy procedure at Sloan-Kettering Cancer Center.
Chemical/Radioactive Composition: Chemical symbol: I; Atomic number (Z=53);
Mass number (A=125); Number of neutrons: 72.
I-125 is produced by bombarding Xe-124 with neutrons in a nuclear reactor.
Types of radiation: X-ray & Gamma.
Energy Characteristics: Decay mode: I-125 decays by electron capture to Te-125 with emission of characteristic photons and electrons.
Energy Levels:
X-ray/Gamma Photon: 27.4 keV
X-ray: 31.4 keV
Gamma: 35.5 keV
Fluorescent X-rays: 22.1 keV
Exposure Rate Constant: Exposure Rate Constant = 1.45 R-cm^2/mCi-hr.
Air kerma strength = 1.27 cGy-cm^2/mCi-hr (1.27U)
Half-life Properties: Half-life = 59.4 days; 1.2% in one day.
Forms available for use: Loose seeds or strands in different designs in an encapsulated welded titanium capsule. The titanium encapsulation ensures good tissue compatibility. Used as temporary implants (eye) or permanent implants (prostate).
HVL in lead: HVL=.025 mm (lead); TVL=.08 mm (lead). A .25 mm lead sheet will provide >99.9% reduction in exposure.
Measurement/Calibrations/QA: The National Institute of Standards & Technology (NIST) has developed a wide-angle free-air ionization chamber that is capable of measuring an
I-125 source of about the size of 2 pi area. The source is calibrated by the vendor before shipment and bears a calibration certificate within limits of 10%. The calibration declared on the bill of lading must be verified by the radiation safety team. Agreement within +/- 5% is acceptable.
Source activities typically range from .1 mCi-5 mCi;
Higher activities can be obtained from temporary interstitial implants (1cGy-10cGy/hr).
The minimum peripheral dose (MPD) is the isodose surface just covering the prostate target volume and typical seed strengths required are .3 mCi (MPD=144cGy).
Used in formula/calculation: Mean Life (T avg) = 1.44 x 59.4 day = 85.5 days.
Specific Activity: 1739 Ci/g;
The f-factor (water/air): .910 (cGy/R);
Inhalation Dose Conversion Factor: 24.16 Days (mrem/mCi);
Ingestion Dose Conversion Factor: 38.48 mrem/mCi;
Absorbed Dose Constants: .005 g-rad/mCi-hr.
Uses in Radiation Oncology:
  • I-125 seed implantation is effective in treatment of prostate cancer by transperineal permanent brachytherapy;
  • I-125 is used for treatment of brain stem tumors by interstitial irradiation;
  • CT and image fusion-guided I-125 stereotactic brachytherapy is performed for diagnostic biopsies;
  • I-125 is used for the treatment of recurrent or progressive glioblastoma multiforme by permanent brachytherapy;
  • I-125 seeds are used for the temporary implant for the treatment of ocular melonoma;
  • I-125 is used in the medical and nuclear industry as a tool for measurement;
  • I-125 is used for efficiency calibration in gamma ray spectroscopy due to the benefit of data points at the emission energy of I-125.
Treatment Planning: The anisotrophy (differences in dose distribution around the seed) is higher due to differential attenuation of the low-enery x-ray emissions caused by the seed encapsulation. The process of determing the dose distribution obrtained from I-125 seeds is complex and uncertain. The Monte Carlo method of computational dosimetry and TLD detectors have helped to understand I-125's dosimetry.
One other interesting fact:
  • Solutions containing iodide ions should not be made acidic nor stored frozen as this will lead to the formation of volatile elemental iodine.
  • Care must be taken to avoid nicking, cutting, slicing or damaging a seed as this will release I-125 into the environment.
  • If < 1mCi of I-125 is spilled: considered minor spill.
  • If > 1mCi of I-125 is spilled: considered major spill.
  • Leak test requirements:
Unused, < 6 months in inventory: no leak test required.
Unused, > 6 months in inventory: leak test is required.
Temporary Implants: leak test required BEFORE each use.


Devlin, Phillip M. Brachytherapy Applications & Techniques. (2007). pp42.
Khan, Faiz. The Physics of Radition Therapy, 3rd ed. (2003). pp531, 540-2. (page 12),f1000m,isrctn's_patents_&_publications.htm#early%20history%20of%20I-125%20Seed%20development.

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