BRACHYTHERAPY

Radium : is the 6th member in the Uranium Series

• Half Life is 1622 years
• Decays by alpha emmision (into Radon)
• Radon has a half life of 3.8 days
• When the process is done we are left with stable lead

Radium Needles:

• Made from radium chloride which is a paste or radium sulfate
• The radium is encapsulated in a metal cell about 1cm long and 1mm in diameter
• The cells are made of gold foil and sealed to prevent leakage of the radon gas
• The cells are loaded into platinum needles or tubes of a variety of lengths and activities
• *the needle plus the cells give 2 layers of protection against radium leakage

Test are performed on the radium needles to look for faulty metal casings:

• needles are placed in a test tube stuffed with cotton wool and left for 24 hours
• then the wool is tested for radioactivity
• test is done 2 times per year

Construction tests for radium needles:

• diagnostic x-rays are taken of the needle
• tells the position of the cells in the needle and the activity of the cells
• test is only done when the needles are bought

Types of Radium Sources:

Uniform linear activity: are available in full intensity (.66 mg/cm) or half intensity (.33 mg/cm)

• intensity is uniform throughout the needle

Indian Club : these is a higher activity at on end of the needle

DumbBell : there is high activity at both ends

 

Exposure Rate Constant (Gamma Ray Constant) : this is the exposure normally measured at 1 meter from the source

• for brachytherapy the constant is calculated to give exposure rate in R/h at 1cm from a 1 mCi POINT source (exposure is measured in air by Roentgens)
• the strength of the source is given as 1 mg of radium
• exposure is measurewd for an area of 1 square centimeter

Without Platinum Cap

(Exposure rate constant for 1mg of unfiltered radium source: 9.09 R cm2/h mg)

With Platinum Cap:

Exposure rate constant for 1mg of filtered (.5 mm pt) radium point source = 8.25 R cm2/h mg

• for every addition .1mm of platinum added to the source a 2% reduction in output occurs and must be taken into account

• The activity for a radium needle will not reach full activity till 30 days (once encased in platinum the radon gas will buildup and reach equilibrium after 30 days)

  The needles are often encapsulated in .5mm of platinum

• readings are then done again with the .5mm platinum cap

Secular Equilibrium :

• the half life of the parent is much greater then the half life of the daughter (with radium the radium is the parent and radon is the daughter)
• at equilibrium the rate at which the daughter is produced equals the rate at which it is decaying (the cause of this is the radon gas (daughter) is being replaced by the radium (parent)

Transient Equilibrium :

• if the parents half life is NOT much greater than the half life of the daughter, then a condition of transient equilibrium may be established
• after the time required to attain transient equilibrium has elapsed, the activity of the daughter decreases with the “apperent half-life” equal to half of the parent (example of this is Molybdenum into Technitium)

Other Radioisotopes:

Cessium 137:

• used as a substitute for radium in interstitial and intracavitary brachytherapy
• supplied in double encapsulated stainless steel needles and tubes
• half life = 30 years
• energy of gamma ray = .662 MeV (662 KeV)
• Exposure Rate Constant = 3.26 R cm2/ mCi- h (unfiltered)
• Conversion factor (radium to cesium) = 2.53 mCi of Cs137 / mg of Ra226
• Dose rate must be adjusted by 2% per year
• Advantages of Cessium over Radium are: requires less shielding and less hazardous

Cobalt 60 :

• Rarely used for brachytherapy
• Supplied in a wire encapsulated in a sheath of platinum or stainless steel
• Half life = 5.26 years
• Energy of gamma rays are = 1.17 and 1.33 MeV
• Exposure Rate Constant = 13.07 R cm2/ mCi- h (unfiltered)
• Conversion factor (cobalt to cesium) =.63 mCi of Co60 / mg of Ra226
• dose rate must be adjusted by 1% per month
• advantage is Co60 has a high specific activity
• disadvantages are: expensive, short half life, complex inventory system

Iridium 192:

• supplied as wires or ribbons containing seeds
• half life = 74.2 days
• average energy of the gamma ray = .38 MeV (380 KeV)
• Exposure Rate Constant = 4.69 R cm2/ mCi- h (unfiltered)
• Dose rate only varies by a few percent each treatment
• Low energy thus requires less sheilding