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|Relevant historical data:|| Iridium: is a hard and brittle silvery white metal with low ductility. Iridium is dense, about twice as dense as lead and occurs in nature as two stable isotopes. Iridium-193 is the most prevalent form with about 63% of natural iridium, with iridium-191 accounting for the rest.|
|Chemical/Radioactive Composition:|| Name: Iridium|
Atomic Number: 77
Atomic Weight: 192.217
Classification: Transition Metal
|Energy Characteristics:||Energy: 0.136MeV-1.06MeV (average is 0.38 MeV)|
|Exposure Rate Constant:||Exposure Rate: 4.69 Rcm squared / mCi-h|
|Half-life Properties:||Half-Life: 73.83days and decays to Platinum-194 by emitting a beta particle and several gamma rays|
|Forms available for use:||Seeds or Wires|
|HVL in lead:||HVL: 2.5mm lead|
|Measurement/Calibrations/QA:|| Calibrated by the vendor before shipment and bear a calibration certificate with stated limits of uncertainty, usually 10%.|
QA- should be within +/- 5% with the vendor calibration
A dummy wire is guided pre-treatment to verify length and clearance.
|Used in formula/calculation:|| Dose Computation:|
Monte Carlo calculations:
The general TG-43 equations for the calibration of a dose at a point which includes anisotropy effects is:
Dose rate at a point in a medium = dose rate constant times air kerma strength of the source times by the geometry factor times the radial dose function time the anisotropy effects (See page 543 in Khan Physics)
|Uses in Radiation Oncology:||Iridium is used medically in brachytherapy techniques to treat various forms of cancer. Iridium-192 implants are used especially in the head & neck and breast. They are produced in a wire form and are introduced through a catheter to the target area. After the dose is delivered the implant is removed|
|Treatment Planning:|| Temporary prostate implants: |
Almost always iridium-192 (according to Khan pg 542). The treatment is given mainly as a boost to external beam therapy. The needles are implanted intraoperatively with the guidance of a transperineal template. Iridium seeds in plastic ribbons are afterloaded into the needles and left into place for about 3 days. Usually 10-15 needles are required for the implant. The patient is hospitalized and bed-bound for the duration of the implant, requiring analgesia.
Ir-192 is best suited for HDR treatments because of its higher specific activity and lower photon energy.
The sources are made of thin flexible wires which can be cut to desired lengths. Nylon ribbons containing iridium seeds of 3mm long and 0.5mm in diameter are spaced with their centers 1cm apart. Both the wires and seed ribbons are suitable for afterloading techniques.
The source for HDR is a small line source welded to the end of a flexible cable drive, also known as the source wire. The demensions of the source varies from 0.3-0.6mm in diameter and 3.5-10mm in length depending on the HDR model. The source when not in use is stored in a shielded safe of the HDR unit. With the leakage not to exceed 1mR/h at a distance of 10cm.
The HDR unit is made of several channels and an indexer system to direct the source to each channel which are on a rotating turret. Applicators of catheters implanted in the patient are connected to the channel by catheters called transfer tubes of transfer guides. Before the actual source is used for treatment, a dummy source wire is extended to varify that the path is clear.
|One other interesting fact:|| The name iridium is appropriate, for its salts are highly coloured. Iridium is white, similar to platinum, but with a slight yellowish cast. It is very hard and brittle, making it very hard to machine, form, or work. It is the most corrosion resistant metal known, and was used in making the standard metre bar of Paris, which is a 90% platinum-10% iridium alloy. This metre bar has since been replaced as a fundamental unit of length.|
Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN.
Faiz M. Khan. The physics of radiation therapy 3rd edition. Chapters 15, 22-23
Web elements peridodic table Professional Edition. The University of Sheffield. http://www.webelements.com/webelements/elements/text/Ir/key.html
Argonne National Laboratory/EVS. Human Health Fact Sheet, March 2005. Iridium
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