|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.|
X-ray/Gamma Photon: 27.4 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:|| |
|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:|| |
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.
https://uwlax.courses.wisconsin.edu/d2l/orgTools/ouHome/ouHome.asp?ou=666666 (page 12)
http://www.bumc.bu.edu/Dept/Content.aspx?DepartmentID=100&PageID=8746 http://www.ncbi.nlm.nih.gov/pubmed/15134626?dopt=Abstract&holding=f1000,f1000m,isrctn http://www.chem.uidaho.edu/seminars/m_watrous%20100907%20web2.pdf