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Gold - 198
Photos: Photo #1 |  Photo #2: Gold radioactive recovery | |
 Photo #3: Gold seeds | ||
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| Relevant Historical Data: | An attractive and highly valued metal, gold has been known for at least 5500 years.³ Gold seeds replaced radon seeds for many years, until iodine 125 gained more widespread acceptance.² |
| Chemical/Radioactive Composition: | Chemical Symbol : Au Atomic Number : 79 Atomic Mass : 196.96655amu.⁴ Au-197 is bombarded with neutrons, when a neutron is absorbed, it becomes Au-198, then decays to Hg-198, a stable form of mercury. 8 |
| Energy Characteristics: | Monoenergetic energy of 412 kev.¹ Beta rays of maximum energy 0.96MeV are also emitted but are absorbed by the platinum wall.² |
| Exposure Rate Constant: | Exposure Rate Constant : 2.35 R-cm^2 / mCi-hr Air-Kerma Strength : 2.06 cGy-cm^2 / mCi-hr (2.06 U)² |
| Half-life Properties: | Half-Life: 2.69 days¹ This short half-life is a limiting factor for source transportation to reach the site and/or patient within a useful time frame.² |
| Forms available for use: | Cylindrical grains or seeds encapsulated in 0.1 mm thick platinum wall.¹ A gold seed is typically 2.5mm long with an outer diameter of 0.8mm.² |
| HVL in lead: | 2.5 mm² |
| Measurement/Calibrations/QA: | Gold seeds are shipped with very high activities, and by the time ready to be used they have an activity range of 5mCi/seed.¹ Ten percent of the seeds to be used for an implant should be drawn at random and assayed before use to verify the manufacturer's calibration. The results should agree with the manufacturer within +/-3 %. It is also important to maintain a seed count during the procedure at all times. An inventory of the seeds must be done post implantation to ensure all seeds are accounted for. A complete radiation survey of the patient, floor, linen, and personnel should be performed as well and documented in the patient chart. 7 |
| Used in formula/calculation: | 106 mCi of Au-198 is implanted into a pelvic mass. Determine the emitted radiation. Mean life of Au-198 = 1.44 times 2.7days=3.888days Emitted radiation= 106mCi x 3.888days= 412.128mCi-days |
| Uses in Radiation Oncology: | High dose rate therapy. Prostate seed implant.¹ Interstitial permanent implants.² Useful for applications requiring a high dose in a short amount of time, since initial dose rates can be up to 100cGy/hr. 7 |
| Treatment Planning: | In 1972, an open-surgical, retro-pubic brachytherapy method with permanent implants used for the radiation source was introduced by Carlton at Memorial Sloan-Kettering using gold-198 in combination with external beam radiation therapy. Gold-198 has been used as a permanent implant either alone or as adjunct to another. The theoretical advantage of a gold-198 implant is rapid delivery of radiation at a very high dose rate, thus potentially avoiding some of the radio-biologic problems associated with iodine. The higher energy of the source, however, results in less sparing of adjacent normal tissue, thereby limiting the dose that can be prescribed without complications occurring. An additional disadvantage of this isotope is the risk of radiation exposure to staff performing the implantation. Because of this radiation protection problem, gold-198 implantation has fallen out of favor at many centers.⁵ Brachytherapy using Au-198 grains to treat choroidal malignant melanoma study. Radioactive sources were placed into surgically constructed scleral pockets, see photo 4. The radiation dose was 120Gy at the apex of the tumor, which slowly became smaller and completely disappeared at one year and 10 months after treatment.This technique permits the tumor to be treated without the loss of the eye.⁶ |
| One other interesting fact: | There is roughly 1 milligram of gold dissolved in every ton of seawater, although extracting it currently costs more than the gold is worth. It has been estimated that all of the gold that has currently been refined could be placed in a cube measuring 20 meters on a side.³ |
Links:1. http://www.3rd1000.com/nuclear/halflife.htm
References:
1. Washington CM, Leaver DT. Aspects of brachytherapy. In: Principles and Practice of Radiation Therapy Physics, Simulation, and Treatment Planning. St.Louis, MO: Mosby_year Book, Inc.; 1996:39.
2. Khan Faiz. The Physics of Radiation Therapy. Third Edition. Philadelphia, PA: Lippincott Williams & Wilkins; 2003:p. 358,361.
3. http://education.jlab.org/itselemental/ele079.html Accessed 3/20/09
4. http://chemistry.about.com/od/elementfacts/a/gold.htm Accessed 4/2/09
5. http://caonline.amcancersoc.org/cgi/reprint/45/3/165.pdf Accessed 4/2/09
6. Kawamura T, Koga S, Nomura T et al. Brachytherapy of a choroidal melanoma using radioactive gold grains: a long term follow up study. In Radiation Medicine. 1999:17(3),243-246.
7. http://www.dosimetrytrainingtool.com
8. http://mysite.du.edu/~jcalvert/phys/scat.htm
Photo references:
#1cornerstonegoldrefining.com/History.html.
#3http://www.orau.org/ptp/collection/brachytherapy/seeds.htm Accessed 4/2/09
#4 Kawamura T, Koga S, Nomura T et al. Brachytherapy of a choroidal melanoma using radioactive gold grains: a long term follow up study. In Radiation Medicine. 1999:17(3),243-246.
Authors:
Denise,Michelle, Dana
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, Apr 12 2009, 11:55 AM EDT
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