Cobalt 60 & LINAC unit Flashcards
Who invented the Cobalt-60 teletherapy machine in Canada in 1950s?
Harold E. Johns
It is manufactured in the nuclear reactor by irradiation of Cobalt 59 with slow neutrons for many months.
Cobalt 60 source
Half-life of Cobalt 60
5.27 years
HVL of Cobalt 60
12 mm lead
Smaller source is preferred becos of
smaller penumbra
It is the useful beam for treatment?
y-ray beam
The y emission of 1.17 MeV and 1.33 MeV gives an average energy of?
1.25 MeV
Contaminants of y-ray beam:
Scattered X-ray
Bremsstrahlung X-ray
Electrons
Results from the interaction of y-ray with Cobalt-60 source, capsule, housing and collimator.
Scattered X-ray
Results from the interaction of B-particle with the source and the steel capsule.
Bremsstrahlung X-ray
Results from the interaction of y-ray with plastic tray that holds the block.
Electrons
Plastic tray that holds the block
Cerrobend Block (Lead)
It is the number of transformations per second for each gram of radionuclide decaying at a fixed rate.
Specific Activity
The specific activity of Cobalt-60 source ranges from about?
75 to 200 curies per gram
2.8 to 7.4 TBq per gram
The high specific activity makes it possible to fabricate a small source with high y-ray output which results in:
- Large dose rate
- Long SSD
- Smaller penumbra
- Improved depth dose distribution
A typical installation has an output of..
750 to 9,000 Ci
Output is usually measured not in Ci but in?
rhm (Roentgen per hour at one meter)
The output determines the?
Treatment time
The housing used for shielding and containing the device for positioning the source?
Source Head
The source head is a steel shell filled with?
Lead, Tungsten, Depleted Uranium
The max. exposure rate in off position at 1m distance should not exceed..
2 mR/hr
The collimator system should not transmit..
5% of primary beam
The source head has a diameter of BLANK and wieghs BLANK
diameter of 1 meter,
weighs 1 ton or more
Methods of bringing the source from OFF to ON position:
Rotating Wheel, Sliding Drawer, Mercury Shutter, Shutter
Rotation is done by a stall type motor
Rotating Wheel
Pulled forward by a motor with spring coming under tension
Sliding Drawer
Mercury is allowed to flow into space immediately below the source
Mercury Shutter
This is fixed in front of the aperture and consisting of heavy metal alloy
Shutter
- Developed by H.E. Johns
- Consist of a series of movable interleaved bars.
- Made from lead-tungsten/tungsten
Collimator
This provides rectangular field size. Equpped with a light localizer (that serves both as a field illuminator and range finder for SSD set-up)
Collimator
A device which uses high frequency electromagnetic waves to accelerate charged particles such as electron to high energies through a linear tube.
Linear Accelerator
Used to produce high energy electron beam for treating superficial tumors or for treating deep seated tumors.
Linear Accelerator
Acceleration of electron is achieved either through:
traveling electromagnetic waves and stationary electromagnetic waves
2 types of linear accelerator
direct linear accelerator and helical linear accelerator
Major Components:
- Drive Stand
- Gantry
- Treatment Couch
- Console Electronic Cabinet
Klystron, Waveguide, Circular and Cooling System
Drive Stand
Electron, Accelerator Structure and Treatment Head
Gantry
A special type of electronic tube that generates microwave that is introduced to accelerator structure via waveguide system.
Magnetron/Klystron
This produces higher energy radiation and has longer life than magnetron
Klystron
A copper tube filled with an insulating gas that conducts the microwave from the klystron or magnetron into the accelerator structure
Waveguide
Directs the RF energy into the waveguide & prevents any reflected microwaves from returning to the klystron
Circulator
Allows many components in the gantry and drive stand to operate at constant temperature
Cooling system
Drive stand:
A. Magnetron/Klystron
B. Waveguide
C. Circulator
D. Cooling system
Gantry:
A. Electron Gun
B. Accelerator Structure
C. Treatment Head
Responsible for the production of electrons
Electron Gun
Consist of a concave tungsten cathode backed by a tungsten heating coil
Electron Gun
Electrons are the injected into the accelerator guide with an initial energy of 50keV
Electron Gun
An evacuated cylindrical chamber 1 to 2 m long, divided into compartment called Resonance Cavities
Accelerator structure
It has a special inner structure so that microwave power accelerates electrons in a straight path to the correct energy
Accelerator Structure
Accelerator guide mounted in the gantry horizontally
In a High Energy Machine (18-25 MV rays)
Accelerator guide mounted in the gantry vertically
In a Low Energy Machine (6 MV rays)
Provides sufficient shieding against leakage radiation in accordance with radiation protection guidelines
Treatment Head
Consist of a thick shell of high density shielding material such as lead, tungsten or lead-tungsten alloy
Treatment Head
Treatment Head components:
Bending magnet X-ray target Beam flattening filter Dose monitoring chamber Collimator assembly Shadow try Electron scattering
It functions to bend the electron group through a net angle of approx. 90 to 270 degrees and onto the x-ray target
Bending magnet
To produce x-ray, electrons are made to strike a target made of metal such as tungsten, copper or aluminum
X-ray target
It equalizes the intensity of the beam over its cross section. (Made from lead, tungsten, uranium, steel and Al)
Beam flattening filter
Used for electron beam therapy, the electrons are made to strike a scattering foil to scatter and flatten the beam.
Electron scattering foil
- Consist of several ion chambers or a single chamber with multiple plates
- Functions to monitor dose rate and field sysmmetry (flatness of beam)
Dose monitoring chamber
Plastic transparent tray that supports field shaping blocks for x-ray therapy
Shadow Tray
- Limits the beam and provides a continous range of field sizes from 0x0 to 40x40cm^2
- The field size is defined by the localizing system
Collimator
Treatment head consists of 2 sets of collimator:
- fixed primary collimator
- movable x-ray collimator
