External Beam Radiation Therapy Units Flashcards
Gorillas Can Swing Over Small Mountains and Can Leap past Very Big Creeks
Grenz-Ray, Contact Therapy, Superficial Therapy, Orthovoltage, Supervoltage, Megavoltage, Co-60, Linear Accelerator, Van De graff, Betatron, Cyclotron
Grenz-Ray Therapy
Less than 20 kVp
Contact Therapy
40-50 kVp
What another name for contact therapy?
Endocavitary
Superficial Therapy
50-150 kVp
Orthovoltage
150-500 kVp
Supervoltage
500-1000 kVp
Megavoltage
Less than 1 MV
Cobalt-60
1.25 MeV
Linear Accelerator
4-25+ MV
Van De Graff
2-10 MV
Betatron
Up to 5 MeV
Cyclotron
up to 250 MeV
Grenz Rays Facts
10 cm SSD, Skin Lesion, Low depth penetration, HVL: .4mm Al
Contact Therapy Facts
Tx using endocavitary applicators at short distances (2cm or less), HVL: 1.5cm Al, beneficial for tumors 1-2mm, completely absorbed in 2cm of tissue
Superficial Therapy Facts
HVL: 1-8mm Al, 15-20 cm SSD, skin cancers no deeper than 5 mm, Advantageous that we can use lead to block out structures
Orthovoltage Therapy Facts
Quality adjusted using Thoraeus/thin cooper filters, HVL: 1-4mm Cu, lesion 2-3cm deep,
Thoraeus Filter Components
Tin, copper and Aluminium
What is the arrangement of the Thoraeus Filter from source to patient?
Tin, Copper and Aluminum
What is the primary radiological filter of the thoraeus filter?
Tin
What is the secondary filter of the thoraeus filter and its responsibility?
Copper and to absorb the characteristic x-rays of the tin
What is the tertiary filter of the thoraeus filter and its purpose?
Aluminum and absorb the characteristic radiation of the copper
How effective is the overall thoraeus filter and what does it specifically contribute to the energy spectrum?
Two fold and the filter shifts the energy towards the higher end of the energy spectrum and filters out the possibilities of any characteristic radiation
Supervoltage Therapy Facts
Resonant transformers are used due to the potential restrictions of conventional transformers
Cobalt-60 Teletherapy Facts
Gamma Rays, Replaced every 5.26 years, 3-5k Curie, 1.25 MeV gamma rays, 360 rotation, no low energy radiation
Co-60 Capsule
30mm Cylinder Discs, 1-2cm Diameter capsule, Capsule absorbs beta particles allowing on photons to be emitted
Explain the mechanism that occurs in a Co-60 machine during treatment
Once the source is turned on, the sources moves into the collimator opening (Unshielded) and once treatment has concluded then the source is reverted back into the shielded position
Explain the purpose of the collimators and their structure for Co-60 and Linacs
Collimators are two pair of metal blocks that are utilized to manipulate the vary size and shape of the beam by moving
Explain the purpose of MLC and their structure for Co-60 and Linacs
Multi-leaf collimators are small tungsten leaves that move independently to shape a beam and/or field. These MLC vary from size of 0.5-2cm for linacs
What is penumbra?
Blurring edges of a structure on a radiograph
What are the types of penumbra? (3)
Transmission Penumbra, Geometric Penumbra and Scattering Penumbra
What is Transmission Penumbra and how can you resolve it?
Transmission penumbra is when radiation passes through the edges of the collimator, and the solution is utilizing collimators that are angled with the beam divergence
What is the geometric penumbra and how can you resolve it?
Occurs due to source size,
What is the penumbra equation?
P(d) = S(SSD+depth- SDD)/ SDD
Geometric Factors of penumbra
Focal Point, decreases the source size, decreases the penumbra, Increase SDD, decreases the penumbra,and increase Source to film distance increases the Penumbra
What is scatter penumbra?
Penumbra that is created when interactions within a medium create scatter radiation
Beam Flatness
Flatness = (Maximum prof. - Minimum Prof/ Max Prof - Minimum Prof ) x 100%
Co-60 uses flattening filter. True or False?
False
Linac uses flattening filter. True or False?
True
Timer Error Facts
Also called the Shuttle error, known as the time in which the timer is on and dose is not being delivered and also when the dose that is being delivered as the timer is off
Explain why Co-60 Teletherapy doesn’t require daily constancy checks
Co-60 doesn’t require daily constancy checks as a source is being utilized which a decay constant, in comparison Linacs utilize electrical impulses that can vary.
In regards to the source, how is a Co-60 teletherapy machine deliver radiation?
Moving the source into the “on” position, essentially exposing the source
What energy of photons can Linac’s produce?
4-25 MV
What energy of electrons can Linacs produce?
4-25 MeV
What is a Linac and how does it deliver radiation?
Linac’s are radiation therapy machines, that utilize microwave frequencies that accelerate particles such as electrons to high energies (3 GHz)
What is the microwave frequency utilized for Linacs?
3 GHz
General Pathway from Power Source to irradiation patient
Power Supply > Modulator > Magnetron/ElectroN Gun > Klystron > Wave-Guide > Accelerator Structure > Bending Magnetic > Target > Primary Collimator > Flattening Filter > Ion Chamber > Secondary Collimator > Tertiary Collimators
Explain the process of the way X-rays are produced from Power Source to Target Transmission.
The Power Supply sends power to modulator, the modulator sends a pulse to the Magnetron AND Electron gun via Pulse forming network and hydrogen thyraton. The Magnetron then creates a microwave pulse that is pushed through the Klystron to amplify the wave. The wave is then guide via the wave guide to the accelerator structure. The Electron gun then injects the electrons into the accelerator structure that has the microwaves. The Electrons ride the occulating waves in the vacuumed accelerator. The speeding electrons then interact with the 270 bending magnet (Achromatic) to transmit through the target.
Accelerator Facts
Highly Vacuumed Copper tube, can be traveling or standing wave
Explain Traveling wave in the accelerator structure
Microwaves travel through the structure towards the high energy end and absorbed at the end to limit the reflection of the wave
Explain Standing Wave in the accelerator
Each end of the accelerators have conductors that produce max reflecting waves at both ends of the structure, combine forwards and reverse create a standing wave.
What must an accelerator structure that operates with standing waves include?
Circulator or isolator to prevent between the the linac and microwave source to prevent back reflection from moving into the microwave source
What three orientations can an accelerator structure be in?
Vertical, Horizontal and Oblique
Magnetron Facts
Microwave source, high-power oscillator, Less costly but less stable, Power low energy linacs, 2-5 MW peak power
Klystron Facts
Microwave amplifier, driven by low power oscillator, Buncher and Catcher
Explain the Buncher and Catcher in the Klystron
Electrons accelerated into Buncher (Energized by low microwave energy), Alternating E field lead to bunching. Theses bunched reach the catcher and electrons decelerate and KE is converted into high power microwaves 3-7 MW, high energy linacs
Waveguide Facts
Copper tube that proved path for microwaves from source to accelerator structure, pressurized to prevent electrical breakdown from high power microwaves
Electron Transport System Facts
Pencil beams are ejected out of the acceleratore structures and depending on the energy of the linacs, you will have electron transport system to bend the beam
Explain the difference between low-energy and high-energy linac accelerator structures and a electron transport system
Low-energies typically have shorter accelerator tubes, which can directly shot the beam towards the target. However, high-energy accelerators have long tubes, which causes them to be oriented oblique or horizontal and an electron transport system to be used.
Explain whats the purpose of the electron beam system
The electron beam system is utilized to redirect beams from high energy linacs towards a target via bending magnet and coils
Bending Magnet facts.
Deflect electron beam towards target. Chromatic (90) and Achromatic (270), Low-energy do not need bending magnets
Chromatic Magnet
Bend the magnet to spread the beam (spatial dispersion)
Achromatic Magnet
Beam is refocused
Transmission Target Facts.
Intercepts the beam to generate x-rays, made of tungsten, focal spot is 2-3mm, target moves out of the way for electront treatment
How does energy and scattering distrubtion of x-rays relate?
Low energy has more backscatter and High Energy has more forward backscatter
Treatment Head Layout (X-ray)
Target > Primary Collimator > Flattening Filter > Ion Chamber > Secondary Collimator > Tertiary Collimator
Treatment Head Layout (Electrons)
Beam > Primary Collimator > Scattering Foil > Ion Chamber > Secondary Collimator > Accessory mount > Electron Applicator
Flattening Filter Facts.
Makes the intensity uniform in the cross section, made of High Z material such as lead, conical shape
Explain the shape of the distrubtion with a flattening filter
Hill shaped, climbs, levels out and then drops. Two high points known as the horns, beam is flattened
Flattening Filter Free (FFF) facts.
Developed to deliver increase dose rates while reducing scatter, leakage and out of field doses
Flattening Filter Pro’s and Con’s
Pro’s: Creates Dose Homogeneity across beam profile
Con’s: Reduced dose rate, increased scatter, increase leakage, beam hardening and neutron fluence
Scattering Foil Facts.
Thin foil (less than 1 mm) of high atomic number material, Spread out the pencil beam uniformly into a broad beam of E- (coulomb scattering),
What is the purpose of the dual foil system?
First foil is uniform in thickness, and second is thicker in the middle to combine for a more uniformly spread
Explain the shape of the distrubtion with a Scattering Foil
Similar to the flattening filter, without the horns
First beam collimator
Primary collimators that minimize the beam to its maximum diameter
What structure is after the flattening filter/scatteirng foil?
Monitoring Ion Chambers
What type of chambers are ion chamber and what purpose does the ion chamber serve?
Transmission chambers and monitors the dose and also the beam flatness
What are monitor chambers made of?
Low Z material, thin (Al), no effect on beam
What is the advantage of dual monitory chambers?
Redundancy and safety of dose delivery
What structure follows after passing through the monitor chamber?
Secondary Collimator or otherwise known as the two pairs of Asymmetric jaws
What size can the jaw sizing range from and also the thickness of the lead-tungsten of the jaws?
0x0 to 40x40 FS and to transmit less than 1% of primary beam
What is the structure that follows the jaws?
Multileaf Collimators
What is the purpose of MLC’s?
Provide irregular shaped field blocking and Intensity Modulation for IMRT
What is Sc account for when the field is blocked?
Sc accounts fo the increase photon scattering from collimator increases from the tunnel exposed to radiation as the colli opens
Where dose thE scatter radiation occur for Sc?
In the treatment head, secondary collimator (Jaws)
What does Sp account for in field blocking?
Accounts for photon scattering due to tissue
Where dose the scatter radiation occur for Sp?
Within the Patient
In the scenario you are given a field size and blocked field equivalent, which do you use for Sc?
the given field as that is the scatter from the field size that is not blocked
In the scenario you are given a field size and blocked field equivalent, which do you use for Sp?
the blocked field equivalent as the Sp is generated in the patient, where the FS is the blocked equivalent
What are some addition componets with the head of the linac?
Light localization system and ODI
What is the purpose of the light localization system?
To project the field at isocenter
What is the purpose of the Optical Distnace Indicator?
Known as the rangefinder and allows for correct set up to SSD
How photon energies and electron energies do most linacs have?
2 Photons and 5 Electrons
What treatments can be given with MLC?
3DCRT and IMRT
Record and Verification?
System database that is utilized to store machine delivery parameters
Electronic Portal Imager (EPID)
Imaging system that is utilized for target setup or portal verification
What energy does EPID utilize? (MV or KV)
MV
Imaging System/On-Board Imaging (OBI)
Target verification in IGRT
What energy does OBI utilize?
KV
The image is typically clearer in the MV than KV. True or False
False
Van de Graaf Accelerator facts.
Electrons sprayed onto belt and collect at the dome, high potential difference between the sphere and the ground, the potential is applied across the xray tube
Betatron Facts.
Hollow Vacuum donut, placed between two AC mag poles, electrons accelerate in changing magnetic field, electrons may spiral out of orbit and hit a target producing an x-ray, 45MV
Microtron
