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| Version | User | Scope of changes |
|---|---|---|
| Apr 6 2009, 10:18 PM EDT (current) | danacole | 77 words added |
| Apr 6 2009, 8:17 PM EDT | danacole | 43 words added, 1 word deleted |
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| Photos: |  Image 1 | |
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| 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 late 1930s by Glenn T. Seaborg and his coworker, Margaret Melhase.¹ Cs-137 in our environment came mostly from nuclear weapons testing in the 1950's and 1960's.8 |
| Chemical/Radioactive Composition: | Cesium is produced in nuclear reactor fuel as a natural by-product of nuclear fission. It can be chemically separated from spent nuclear fuel and is widely available.2. Radioactive Cesium is distributed as an insoluble powder or ceramic microsphere matrix enclosed in stainless steel needles or tubes.4 |
| Energy Characteristics: | Photon energy of 662 keV which is comparable to the average photon energy of radium(830keV).² Gamma Factor for Cs-137:³ The gamma rays from Cesium have almost the same penetrating power as radium in tissue.7 |
| Exposure Rate Constant: | The exposure rate constant of Cs-137 is 3.28 R-cm2/mCi-hr.4 |
| Half-life Properties: | The half-life of cesium-137 is 30.22 years.¹ |
| Forms available for use: | Wide variety of needle or tube configurations.² Insoluble powder or ceramic microsphere matrix enclosed in stainless steel needles or tubes.4 |
| HVL in lead: | The HVL in lead for Cesium-137 is 6.5 mm.³ |
| Measurement/Calibrations/QA: | They decay about 2% per year.2. Cs-137 decays by β-emission to a metastable state of Ba -137m which subsequently undergoes isomeric transition from Ba-137m to Ba-137 resulting in the emission of a single 0.662 MeV β-rays and a small number of 35 keV characteristic x-rays resulting from internal conversion.4 A Cesium source can be used clinically for about 7 years due to the long half life.7 A decay chart should be available to the treatment planner so that the correct source activity is used for the time of the procedure. The source loading, number of sources used, along with the total source strength should be documented. A 'Radioactive Materials' sign must be posted on the patient's door, and visitor time and distance from the patient restricted. After removal of the sources, the patient must be surveyed to ensure they were indeed removed completely.4 |
| Used in formula/calculation: | A vaginal cylinder is loaded with 20-15-20 mg Raeq sources of Cs-137. What is the total number of mCi of Cs-137? (Exposure rate constant Cs-137=3.26) 20+15+20=55mgRaeq total 55mgRaeq x 8.25Rcm2/mCihr = 139.2 3.26Rcm2mCihr |
| Uses in Radiation Oncology: | Cs-137 sources are extensively used for intracavitary implants for the treatment of gynecological cancers.4 A system consisting of a tandem and two ovoids is the most frequently used afterloading apparatus in the treatment of carcinoma of the uterine cervix.5 Cs-137 is only used for low dose rate brachytherapy due to its low specific activity.4 |
| Treatment Planning: | In a typical tandem and ovoid application, only the segment of the tandem lying within the uterine canal should be loaded. The keel on the tandem will indicate where the external cervical os is located and on a lateral radiograph one can measure the length of the tandem lying within the uterus. The distal source should extend to but not beyond the keel, and if this requires more than three sources, nylon spacers of appropriate size can be added either at the proximal end of tandem or between the sources. The desired dose distribution is approximately pear shaped in the anterior view, with the widest part near the cervix. Typically, the tandem is loaded with a 15-mg source at the proximal end, a 12-mg source in the center, and a 10-mg source in the distal end, which is in the cervix. Selection of source strength in the ovoids depends on their diameter and the seperation between them. Ovoid diameter size depends on the patient's vaginal vault size. Selection of source strength also depends on rectal and bladder dose. Typical source strengths are 10-mg in miniovoids and 15, 20, and 25 mg in small, medium, and large ovoids respectively.5 Currently the standard unit is the centigray to a specific anatomical point or isodose line. The anatomical points used for cervical and uterine treatment are points A and B. Point A is located 2 cm superior and 2cm lateral to the center of the cervical canal ( at the cervical os) in the plane of the uterus. Point B was originally 3 cm lateral to point A, but is currently noted as being 1cm lateral to the medial aspect of the pelvic side wall. The dose at point B is typically about one third that at point A.2 |
| One other interesting fact: | Advantages: · Cesium has a half-life of 30 years thus causing only a 2% change in dose-rate over one year. Hence a Cs-137 source can be used in a clinical setting for up to 8-10 years. · Cs-137 has dosimetric properties similar to Ra-226 and has been adopted as a "radium substitute" for clinical brachytherapy applications · -ray as compared to 0.830 MeV for Ra-226gCs-137 sources emit a 0.662 MeV thus leading to lower shielding costs associated with its clinical use. · Disposal of Cs-137 sources is relatively easier as compared to Ra-226.4 Disadvantages: · Low specific activity for Cs-137 precludes the use of Cs-137 for high dose rate applications in clinical brachytherapy.4 It is a soft, silvery-gold alkali metal with a melting point of 28 degrees C, making it one of only five metals that are liquid at room temperatures.6 This fact makes it capable of becoming a water contaminant. |
Links:
References:
1. http://www.epa.gov/rpdweb00/radionuclides/cesium.html#whodiscovered Accessed 3/10/09
2. 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.; 1996:39,44.
3. http://www.rpdinc.com/html/cesium-137.html Accessed 3/1709
4. http://www.dosimetrytrainingtool.com
5. Bentel GC. Dose calculations in brachytherapy. In: 2nd ed. Radiation Therapy Planning. New York, NY: McGraw-Hill; 1996:566-569.
6. http://en.wikipedia.org/wiki/Caesium
7. Khan, FM. The Physics of Radiation Therapy. 3rd ed. Philadelphia, PA: Lippincott Williams and Wilkins; 2003: 360-397.
8. http://www.epa.gov/radiation/radionuclides/cesium.html
Images:
Image 1. http://www.npp.hu/mukodes/beta-e.htm
Image 2 & 3 Bentel GC. Dose calculation in brachytherapy. In: 2nd ed. Radiation Therapy Planning. New York, NY: McGraw-Hill; 569,572.
Authors:
Denise,Michelle, Dana,
