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| Version | User | Scope of changes |
|---|---|---|
| Apr 5 2009, 2:48 PM EDT (current) | LaRosa.Fran | 1 word added, 1 word deleted |
| Apr 5 2009, 2:44 PM EDT | LaRosa.Fran | 1 photo added, 1 photo deleted |
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| Photos: |  Iridium powder |  Iridium Source |
 Ribbons containing Iridium-192 |  |
| Relevant Historical Data: | Iridium was discovered in 1803 by Smithson Tennant among insoluble impurities in natural platinum from South America. It is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes. However, iridium does find a number of specialized industrial and scientific applications. Iridium is employed when high corrosion resistance and high temperatures are needed, as in spark plugs, crucibles for recrystallization of semiconductors at high temperatures, electrodes for the production of chlorine in the chloralkali process, and radioisotope thermoelectric generators used in unmanned spacecraft. Iridium compounds also find applications as catalysts for the production of acetic acid.(2) Iridium has been linked with the extinction of the dinosaurs and many other species 65 million years ago. The unusually high abundance of iridium in the clays of the K–T geologic boundary was a crucial clue that led to the theory that the extinction was caused by the impact of a massive extraterrestrial object with Earth—the so-called Alvarez hypothesis. Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. It is thought that due to the high density and siderophilic ("iron-loving") character of iridium, most of the iridium on Earth is found in the inner core of the planet.(2) | ||||||||||||
| Chemical/Radioactive Composition: | Name: Iridium-192 Symbol: Ir-192 Atomic Number: 77 Atomic Mass: 192.217 Classification: Transition Metal. (1) Iridium-192 undergoes beta decay(3) One of the lesser-known members of the platinum group metals, iridium is white, resembling platinum, but with a slight yellowish cast. It possesses quite remarkable chemical and physical properties. Due to its hardness, brittleness, and very high melting point (the tenth highest of all elements), solid iridium is difficult to machine, form, or work, and thus powder metallurgy is commonly employed instead. It is the only metal to maintain good mechanical properties in air at temperatures above 1600 °C. Iridium has a very high boiling point (11th among all elements) and becomes a superconductor under 0.14 K.(2) | ||||||||||||
| Energy Characteristics: | Iridium-192 undergoes beta decay and has an energy of 370 keV.(3). Ir -192 decays via negative beta emission(95.6%) to Pt-192 and via electron capture (4.4%) to Os-192. (6) They emit a low-energy gamma ray and x-rays with energies below .0355 MeV. The low energy allows local shielding with metal foils only a few tenths of a millimeter thick and allows numerous applications not available with other radionuclides. ( | ||||||||||||
| Exposure Rate Constant: | 4.69 Rcm squared / mCi-h | ||||||||||||
| Half-life Properties: | Iridium-192 has a half life of 74 days, and it decays to stable platinum-192 and osmium-192 by emitting a beta particle and by electron capture; most of these decays (95%) are by beta emission.(1) | ||||||||||||
| Forms available for use: | Ir-192 used in medicine is in the form of tiny seeds, each about the size of a grain of rice. Industrial gauges hold pencil-like metal sticks of solid Ir-192 or small pencil-like tubes that contain pellets of Ir-192. (4) | ||||||||||||
| HVL in lead: | ~ 3 mm lead. (6) | ||||||||||||
| Measurement/Calibrations/QA: | A well-type chamber is most commonly used to calibrate Ir-192 seeds. The energy-response stability as well as the linearity of the well-type chamber are checked with the Ir-192 HDR source itself. This type of check is performed at least once at the end of the normal use of the source before its replacement by a new source.(10) A type of calibration that can be performed is with a free-air ionization chamber. This type of chamber is also used for measuring soft x-rays. You need an array of four to six seeds to enhance the ionization-current-to-background-current ration. Placement is very important because of the ionization that takes place to obtain a correct reading. Important factors in the use of this type of chamber are the defined air volume of the chamber and the mean background current. For the readings to be correct there needs to be no more than a 10% background correction and a source-to-chamber distance of no less than .25m. (8) Another study that was reported on calibration on Iodine-125 is using GAFCHROMIC film. A single seed source and a train of six seeds spaced 1mm apart enclosed by a nylon ribbon was evaluated. Each source was placed in a homogeneous solid water phantom directly below a stack of the films. The density on the films was then scanned five days after being exposed to the Iridium-192 with a microdensitometer. Isodose curve plots were then used to evealuate the results.(9) | ||||||||||||
| Used in formula/calculation: | Basically the dose rate in water is calculated by the inverse square law with smalll corrections for absorption, scattering, and anisotrophy. There is no correction for the inhomogeneity of the patients' tissues.(10) The formula uses: air kerma strength dose rate constant geometry factor radial dose function anisotrophy function The accepted tolerance is 3% from the calibration source vs. the sources to be used in treatment.(10) | ||||||||||||
| Uses in Radiation Oncology: | Iridium-192 is used medically in brachytherapy to treat various types of cancer. (Brachytherapy is a method of radiation treatment in which sealed sources are used to deliver a radiation dose at a distance of up to a few centimeters by surface, intracavitary, or interstitial application.) Iridium-192 implants are used especially in the head and breast. They are produced in wire form and are introduced through a catheter to the target area. After being left in place for the time required to deliver the desired dose, the implant wire is removed. | ||||||||||||
| Treatment Planning: | A 60-year-old woman presented to the dermatologist with multicentric, purple–red nodules spreading widely over the parietal scalp. A bleeding ulcer was found in the center of the conglomerated nodule. A bulky skip lesion formed an exophytic mass, 5 cm in diameter, located in the left temporal region. The patient recalled hitting her head on an iron pole 3 months before the visit, resulting in a head injury. Histological examination of an excision biopsy specimen made the diagnosis of malignant hemangioendothelioma. No lymph node or distant organ metastasis was found in image studies including chest radiography, ultrasonography of the neck, gallium scintigram and computed tomography (CT) of the head and neck. Platelet counts were less than 30 000/mm3. Blood coagulation tests showed prolongation of prothrombin time (PT), partial prothrombin time (PTT) and increased fibrin/fibrinogen degeneration products (FDP) and D-dimer. Frequent blood platelet transfusion resulted in only a small temporary recovery. As immunotherapy by systemic injection of interleukin 2 failed to improve coagulopathy, radiotherapy was added using a 6 MeV electron beam by a single field, 20 x 20 cm in size, covering the entire scalp skin from the top of the head. The prescription dose per fraction was 3 Gy, four fractions per week. It was discontinued when a total of 39 Gy had been delivered over 20 days because the left temporal tumor progressed in size and coagulopathy still remained unchanged. The isodose distribution calculated by a three-dimensional treatment-planning device (Cadplan version 3.3, Varian) revealed a significant underdosage in the growing tumor (Fig. 1). We then decided to adopt the iridium-192 surface mold technique to avoid the underdosage. To make the surface mold close to the entire tumor, the thickness of the lesion should be as even as possible. We then delivered further electron beam irradiation on the parietal and left temporal areas separately with each portal(s), 10 cm x 10 cm in size, at 2 Gy/fraction, five fractions/week, a total of 20 Gy over 13 days. Although the left temporal tumor ceased to grow, the prepared therapy flattened the targeted tumor far less than expected. We made a helmet with a heat-moldable plastic facemask instead of the plaster casting tape described originally (8). The left temporal part of the mask was cut to prevent its separation from the skin surface due to the exophytic tumor. Fourteen flexible polyethylene tubes of 5 Fr. gauge were fixed in parallel along the sagittal plane on the inner surface of the mask. For delivery on the left temporal tumor exposed beyond the mask, four other guide tubes were separately arranged on the mask (Fig. 2). Treatment planning was performed on a device (PLATO-BPS version 13.1, Nucletron) using the CT images obtained as the mold put on her head. Active source dwell positions were determined to cover the entire tumor at 5 mm intervals in each applicator. The reference points were set at 20 mm beneath the surface of the tumor. For the left temporal tumor, the depths were increased to 30 mm. Geometric optimization in volume was calculated to obtain a uniform dose distribution (Fig. 3). From the next day to the end of electron radiotherapy, a total of 12 fractions, three fractions per week, a total of 36 Gy was delivered over 33 days. The total dose used in brachytherapy was limited because it had already reached the threshold dose for skin at the end of the electron beam radiotherapy and the brain was also considerably irradiated in this method. The tumor showed an apparent decrease in volume during the treatment, especially in the left temporal portion, which became almost flat.
The patient complained of pain in the irradiated area of the head and needed analgesics for several months after the completion of the treatment. About 2 weeks after the end of therapy, the blood platelet count began to increase and it reached a normal value in 2 months. At the same time, FDP and D-dimer decreased. The tumor had almost disappeared 3 months after the brachytherapy (Fig. 4). Alopecia occurred and was irreversible in the irradiated area.
One year later, a metastatic nodule, 4 x 3 cm in size, developed at the lower pole of the parotid gland. It was treated by 4 MV photon irradiation with anterior–posterior opposed pair portals of 3 Gy/fraction, four fractions/week, a total dose of 60 Gy over 46 days. The tumor showed almost complete regression at the end of therapy. Two years later, several dusky, purple–red plaques appeared on the skin of the right temporal area, bordering the bold parietal skin previously irradiated; 4 MV electron beam irradiation of 3 Gy/fraction, four fractions/week and a total dose of 60 Gy over 29 days, was delivered by a 15 x 15 cm single portal with 5 mm bolus. At the end of this treatment the plaques became flat and they disappeared in 3 months. The patient is alive and free from disease now, 6 months after the last radiotherapy and 3 years after initial treatment. Recent plain radiography and MRI of the head did not indicate any damage to the skull or brain.(5) | ||||||||||||
| One other interesting fact: | Source : Neutron bombardment of 99.9% iridium metal > 450 Ci/g for standard specific activity material > 1000 Ci/g for high specific activity material Iridium can be taken into the body by eating food, drinking water, or breathing air. Gastrointestinal absorption from food or water is the likely source of internally deposited iridium in the general population. After ingestion or inhalation, most iridium is excreted from the body and never enters the bloodstream; only about 1% of the amount taken into the body by ingestion is absorbed into the blood. Twenty percent of the iridium that reaches the blood is excreted right away, 20% deposits in the liver, 4% deposits in the kidney, 2% deposits in the spleen, and the remaining 54% is evenly distributed among other organs and tissues of the body. Of the iridium that deposits in any organ or tissue, 20% leaves the body with a biological half-life of 8 days and 80% clears with a biological half-life of 200 days. On the basis of animal studies, retention of iridium was determined to be the same for all age groups. Most inhaled iridium compounds appear to clear the lungs quite rapidly.(1) |
Links:
CDC Radioisotope Brief
Calibration of I-192Ir-192
Lost Iridium-192 Source
References:
1. Human Health fact sheet: http://www.ead.anl.gov/pub/doc/Iridium.pdf
2. Iridium-192. Available at: http://en.wikipedia.org/wiki/Iridium. Accessed March 17, 2009.
3. Washington, C Leaver, D. Principles and Practice of Radiation Therapy. 2nd ed. St. Louis,MO: Mosby Inc;2004: p-324-325
4. CDC. Radioisotope Brief:Iridium-192 (IR-192). Available at:http://www.bt.cdc.gov/radiation/isotopes/iridium.asp. Accessed March 21 2009.
5. Oxford Jounals. Iridium-192 Brachytherapy for Hemorrhagic Angiosarcoma of the Scalp. Available at:http://jjco.oxfordjournals.org/cgi/content/full/33/4/198. Accessed March 21, 2009.
6. Shirish K. Jani, Shirish Jani, K. Handbook of Dosimetery Data of Radiotherapy, illustrated. CRC press1993. pg. 150
7. VanDyk, Jacob. The Modern Technology of Radiation Oncology. Medical Physics Publishing:1999: pg 704.
8. Journal of Research of the National Institute of Standards and Technology. Available at www.findarticles.com/p/articles. Accessed on April 3, 2009.
9. PubMed. Available at www.ncbi.nlm.nih.gov/pubmed.com. Accessed on April 2, 2009.
10.Dosimetry and Quality Assurance in High Dose Rate Brachytherapy with Iridium - 192. Available at www.sgsmp.ch/r13hdr-e.pdf. Accessed on April 2, 2009.
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