Cesium-137

Cesium-137 - UW-L Brachy Course
Cesium-137 - UW-L Brachy Course



Cs-137

Radiation Check Source

dose distribution

Relevant historical data: In 1860, Gustav Kirchhoff and Robert Bunsen discovered nonradioactive cesium in mineral water in Germany. Radioactive cesium 137, and many other radionuclides that are used in nuclear medicine, was discovered in the early 1940's by Glenn T. Seaborg and his coworker Margaret Melhase. Seaborg noticed that all the fission products identified by 1940, a year after the discovery of fission by Hahn, et al., there were no long lived isotopes. Seaborg put Melhase on a research project to find a group I element among the fission products. Melhase was given 100 grams of a neutron-irradiated uranium to work with in March 1941. She discovered Cesium 137 over the next few months by studying the fission products. Cesium 137's yield from uranium fission is 6%, which makes it surprising to many that it was not discovered earlier.
Chemical/Radioactive Composition: chemical symbol: Cs Atomic number: 55 Mass number: 137. Cesium is a soft, silvery white-gray metal that occurs in nature as Cesium 133. The natural source yielding the greatest quantity of cesium is the rare mineral pollucite. American ores of pollucite, found in Maine and South Dakota, contain about 13% cesium oxide. Cesium is a liquid at room temperature, 28 degrees Celsius or 82 degrees Fahrenheit
Energy Characteristics: Cesium decays by a Beta emission to a metastable state of Ba137m. Barium-137m undergoes isomeric transition and stabilizes itself by emitting an energetic gamma ray with a half-life of about 2.6 minutes and x rays. It is this decay that makes cesium an external health hazard.
Beta ray energies= 0.514, 1.17MeV
Exposure Rate Constant: 3.28 R-cm squared/mCi-hr. Air kerma strength= 2.873 cGy-cm squared/mCi-hr (2.873 U)
Half-life Properties: 1/2 life= 30years, therefore decaying ~2% annually
Forms available for use: A pellet of cesium ceramic housed in a welded stainless steel capsule. Small amounts of Cs-137 are incorporated into Lucite disks, rods, and seeds. Larger sources are enclosed in lead containers (such as long tubes that are closed at each end) or small round metal containers. If the lead containers are opened the substance inside looks like a white powder and may glow.
Early Cs tubes had diameters of 2.65mm with physical lengths of 21mm and active lengths of 14-20mm.
HVL in lead: HVL=6.5mm TVL=18.5mm
Measurement/Calibrations/QA: Cesium is used in small amounts for calibration of radiation-detection equipment, such as Geiger-Mueller counters. In the past it has been used for equipment sterilization and food irradiation.
The source strength is commonly expressed in mgRaeq. Typical storage strengths for cesium range from 10-30mgRaeq. The dose rate from these sources at 1cm ranges from 60 cGy/hr to 180cGy/hr.
Used in formula/calculation: How many mCi of Cs-137 are present in 105mg equivalent Ra? (8.25/3.26)= 2.53 2.53 X 105mg=265.65mg
A vaginal cylinder is loaded with 20-15-20 mgRaeq sources of Cs-137. What is the total number of mCi of Cs-137? 8.25/3.26=2.53 55X2.53=139.2mCi
Uses in Radiation Oncology: Treatment of gynecologic cancers. LDR gynecological brachytherapy is still preformed at some facilities from a permanent inventory of Cs 137 tubes. Implanting Cs 137 needles directly into tissue, "hot loading", has been abandoned.
Prior to linear accelerators Cs 137 was used to treat gynecomastia (enlarged and painful mammary tissue caused by hormonal use in prostate patients) and other superficial lesions. This was done using a cesium unit, usually wall or ceiling mounted, non-isocentric.
Treatment Planning: Cesium 137 is used in LDR treatment of gynecologic cancers. Tandem and ovoids are used to house the radioactive source. Optimal placement of the uterine tandem and vaginal ovoids produces a pear-shaped distribution that delivers a high dose to the cervix and paracervical tissues and reduces the dose to the rectum and bladder. To optimize paracervical tissue dose, the tandem should be as long as anatomy permits, up to ~8cm. The ovoid should be the largest that will fit comfortably. The flange of the tandem should be flush against the cervix and the ovoids should surround it. Packing should be used around the ovoids to prevent a hot spot in the rectum and bladder. An AP and Lat film should be taken to check the position of the applicators. 35-40 mgRaEq is usually distributed in the tandem using 3 linear sources, with spacers if needed, or an equivalent number of Selectron pellets. Cesium 137 linear sources range between 17 and 22cm. Vaginal ovoids are usually loaded with 10 or 15 mg for 2.0cm, 15 or 20 for 2.5cm, 20mg for 3cm, and 5 to 10mg for mini ovoids. The loading and duration are determined by the vaginal surface dose, the size of the tumor, quality of the ovoid position, and total mg-hrs. Using the radiographs, point A and point B are located and the dose to them are calculated. Point A is 2cm superior and 2 cm lateral, following the tilt of the tandem, to the external os, where the uterine artery and ureter meet. Point B is 2 cm superior and 5 cm lateral to the external os and represents the obtuator nodes.
One other interesting fact: Cesium 137 has been used to sterilize screw worm flies in Central America. The flies are raised at the Tuxtla Gutierrez sterile fly facility, while in the pupal stage, are dosed with 5000-5500 rads of cesium 137 for approximately 1 minute and 50 seconds. The radiation dose leaves the flies normal yet sterile.
Margaret Melhase discovered cesium as an undergraduate student!! and she got credit for it!


Links:
www.bt.cdc.gov/radiation/isotopes/cesium.asp
www.en.wikipedia.org/wiki/Cs-137
www.ans.org/pubs/magazines/nn/pdfs/1999-7-3.pdf
www.npp.hw/mukodes/beta-e.htm
www.books.nap.edu/openbook.pho?record_id=10684
www.dosimetrytrainingtool.com/dtt

References:

Argonne National Laboratory, EVS Cesium PDF
The journal of Nuclear Medicine Vol. 40 No. 4 April 1999 "How Cesium-137 was Discovered by an Undergraduate Student"
Radiation Oncology: Management Decisions Chao et al.

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