Gold-198 - UW-L Brachy CourseGold-198 - UW-L Brachy Course
metallic yellow


Relevant historical data: Gold is one of the oldest and most widely used metals known to mankind. An exact date or time-frame to determine when gold was discovered is close to impossible according to several sources due to the long history associated with it. Dr. William Myers of Ohio State University is credited with developing several radioisotopes, including Gold-198 in 1905 for clinical brachytherapy.The element's chemical symbol, Au, is derived from the ancient Latin word for the element, aurum.
Chemical/Radioactive Composition: Chemical Symbol : Au
Atomic Number : 79
Atomic Mass : 196.96655amu
Energy Characteristics: Gold-198 emits a mono-energetic gamma ray of energy 0.412 MeV. (It also emits beta rays of maximum energy 0.96 MeV, but these rays are absorbed by the 0.1mm thick platinum wall surrounding the seed.) Gold-198 has a non-toxic solid decay product which does not pose a hazard.
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)
Half-life Properties: There are 18 known isotopes of gold. Gold-198 half-life: 2.7 days
Forms available for use: Au-198 is available as cylindrical grains or in a platinum encapsulated seed.
HVL in lead: HVL in Lead : 6mm
TVL in Lead : 9mm
Measurement/Calibrations/QA: Due to the short half-life, gold seeds are shipped with very high activities and by the time they are ready to be used, they have an activity in the range of 5 mCi/seed. Thus, gold gives the tissue a very high dose in a short time, a method called high-dose-rate therapy.
Used in formula/calculation:
  • Consider a 4.0 cm diameter sphere, volume 33.5 cm^3, which is designed to deliver 65 Gy.
  • The Manchester table for 33.5 cm^3 yields 376 mg-h/10Gy.
  • 65/10 * 376 = 2444 mg-h.
  • This must now be converted to the appropriate Au-198 activity.
  • 1 mCi (Au-198) * 2.38/8.25 - 0.288 mg Ra-226
  • 1 mCi decayed (Au-198) = 1.445 * 20698 days * 24 hours/day * 0.288 mg= 26.9 mg-h (Ra-226).
  • 2444 mg-hr/26.9 mg-h/mCi = 90.9 mCi (Au-198)
  • 90.9 mCi (Au-198)
  • The classic Manchester Rules: 6 parts of the activity (68mCi) are in the rind and 2 parts (23mCi) are in the core.
  • 12 seeds, 5.7 mCi/seed, should be placed on the surgace of the 4 cm diameter sphree and 6 seeds, 3.8 mCi each, should be evenly distributed throughout the volume of the sphere.
Uses in Radiation Oncology: Seeds or "grains" are used for permanent interstitial or intracavity implants. In some forms of cancer, the body develops large amounts of liquid in the space around the stomach and intestines (the peritoneum). One way to treat this collection of liquid is with colloidal gold. (Colloidal gold is a purplish suspension of minute particles of metallic gold. The radioactive form, made by exposure to neutrons, has been used by intracaviatary or interstitial injection.) The colloidal gold is injected into the peritoneum. It is unable to leave the peritoneum and go into the stomach and intestines. While in the peritoneum, the colloidal gold gives off radiation. The radiation kills cancer cells that cause the accumulation of fluid.
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.
One other interesting fact: Gold has many different uses throughout time, from manufacturing, coins, medals, jewelry, and art. One of the most famous uses of gold is the Olympic gold medal. Athletes from around the world aspire to winning gold and being recognized as the best. But the gold medal we know today is really gold. It's actually made of silver, with a thin layer of gold covering the medal. The last time gold was used in the Olympics was 1912.


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