First Year Exam: Linear Accelerator Flashcards
1
Q
How long does a pulse typically last?
A
A few microseconds
2
Q
What is the time interval between pulses typically?
A
A few milliseconds
3
Q
Where do the DC pulses originate from? Where do they get injected to?
A
Originate from modulator
Injected to electron gun and the klystron (or magnetron) simultaneously
4
Q
What is a Duty cycle?
A
Fraction of time that a beam is actually on
5
Q
What is the duty cycle of a Linac? What about a Co-60 machine?
A
1 for Co-60
1/1000 for Linac (microseconds/milliseconds)
6
Q
What do the monitor unit chambers measure? (3 things)
A
Dose rate
Integrated dose
Field symmetry
7
Q
How is monitor unit chamber reading related to dose delivered?
A
During TG 51. You’re doing a 100 cm, 10x10 cm, max depth, and saying 1 cGy/MU at that point. You change your MU chamber gain to relate current to MU
8
Q
What is the point of the second MU chamber for tracking MU delivered?
A
To terminate beam
Also, the second channel is allowed to over-run by 2%
9
Q
What is the purpose of the timer?
A
To terminate the beam after a certain time in case both chambers are failing
10
Q
The two dosimetry plates are sampling different parts of the beam so that a dose channel difference may be related to a change in __________ rather than to a change in _______ of the monitor ion chamber
A
The two dosimetry plates are sampling different parts of the beam so that a dose channel difference may be related to a change in __beam energy___ rather than to a change in __calibration__ of the monitor ion chamber
11
Q
True or False
Monitor chambers are sealed
A
True
12
Q
Described the general layout and orientation of the monitor chambers
A
- There are two
- Primary is MU1, secondary is MU2
- There are circular
- Each consists of two pairs of plates (4 plates total per MU chamber)
- MU1 is aligned to the radial axis of the beam, MU2 is aligned to the transverse axis
- Inner plates are completely within the field, meaning their summed output current is proportional to dose rate and the difference in their output currents tells you the beam angle symmetry errors. Interlock occurs if difference of more than 8V
- Outer plates are partially within the beam, on its edge. The difference in their output gives symmetry
13
Q
In the image, what is component #1?
Briefly: what does it do?
A
Pulsed Modulator
It’s essentially a big power supply tasked with supplying power to all components of the linac
14
Q
In the image, what is component #2?
Briefly: what does it do?
A
Electron Gun
Pulsed injection of electrons to the accelerator waveguide
15
Q
What component of the LINAC is responsible for generating microwave frequency. What frequency does it generate at? Why is that frequency chosen?
A
RF Driver
3000 MHz
It’s the resonant frequency of the main waveguide
16
Q
In the image, what is component #3?
Briefly: what does it do?
A
Waveguide transport systems
Metal tube (rectangular in cross section) used to transport the microwave power to the accelerator
17
Q
In the image, what is component #4?
Briefly: what does it do?
A
Circulator
Prevents any reflected microwaves in the system from reaching the klystron and RF driver which could potentially damage them. It sends any reflected power down another path, usually to a water load to dissispate heat, or back into the waveguide so it isn’t wasted.
18
Q
In the image, what is component #5?
Briefly: what does it do?
A
Klystron
Microwave amplifier
Receives a microwave power from the RF driver and steps it up to about 5-10 MW
19
Q
How is electron bunching, amplification and efficiency of a klystron improved?
A
By increasing the amount of cavities
20
Q
How is a klystron tuned? What are the effects of tuning on gain and bandwith?
A
If cavities are tuned to resonate at same frequency, this results in higher gain and narrow bandwidth
If cavities are tuned to resonate at slightly different frequencies, the cavity has reduced gain but a larger bandwidth
21
Q
In the image, what is component #6?
Briefly: what does it do?
A
Target
Conversion of electron energy into x-rays via bremsstrahlung production (roughly 20% efficiency in MV range)
22
Q
Why do varian linacs use a 270 degree bending magnet? (two main reasons)
A
- Redirect beam from parallel to patient, to perpendicular to patient (allowing for more compact design)
- Eliminate electron spatial energy spectrum (make beam achromatic)
23
Q
In the image, what is component #7?
Briefly: what does it do?
A
Primary Collimators
Round diverging collimators used to initially define the “useful” beam.
24
Q
In the image, what is component #8?
Briefly: what does it do?
A
Flattening Filter (for photons) or scattering foil (for electrons)
Flattening filter - provide a more or less flat beam at 10 cm depth in water over a wide range of field sizes
Scattering foil - Take a thin pencil beam and spread it out to a usuable size for a patient’s treatment while maintaining uniformity in the beam (may use multiple foils to improve homogeneity across larger field sizes)
25
What are scattering foils typically made of?
Either lead or copper
26
What are flattening filters typically made out of?
Varying materials such as lead, aluminum, steel and brass
27
True or false
Scattering foils and flattening filters are energy dependent?
True
So there's multiple on the carousel that get rotated out depending on the energy
28
In the image, what is component #9?
Briefly: what does it do?
Ion/MU Chambers
Used to monitor output, symmetry and (possible flatness)
29
In the image, what is component #10?
Briefly: what does it do?
Light source and mirror
Light source for field size and ODI
30
In the image, what is component #11?
Briefly: what does it do?
Secondary collimators (jaws)
Field shaping
31
In the image, what is component #12?
Briefly: what does it do?
Tertiary beam shaping (MLCs, blocks, electron cones, wedges, etc.)
As the name would suggest, field shaping
32
If the field light in a Linac was ever off, what do you adjust?
The mirror
33
True or false
Ion chamber and field light are on different axis
False
Field light and ion chambers are on same axis, so the mechanism retracts the ion chamber when the field light is required
34
What is a Cal Check Cycle and what does it check?
Everytime a beam is prepared, the BGM system checks the dosimetry circuit to ensure it's working properly
This includes: cable connectivity, dosimetry interlocks, field programmable gate arrays, BGM software, ion chamber power supply
35
What is a typical MLC maximum leaf speed? Where is it defined?
2.5 cm/s at isocenter
36
What is the maximum leaf extension from bank?
15 cm
37
What is the requirement for leaf position accuracy?
Within 1 mm
38
What is a typical MLC leaf transmission?
1-2%
39
What is used as the leaf position reference?
Optical beam (collimated infrared beam)
40
What aspect of the Linac nominally defines beam energy?
Acclerating voltage
41
Given a 10 MV beam, what is the maximum photon energy? What is the mean energy?
10 MeV
42
What is the parameter that defines photon beam quality?
PDD(10 cm)
43
What is the parameter that defines electron beam quality?
Distance of 50% (R50)
44
Which jaws are located above the other? X or Y?
Y jaws are located above the X jaws
45
How do Y jaws move? How do X jaws move?
Y jaws move on an arc trajectory
X jaws move on a linear trajectory but rotate slightly as they move to match beam divergence
46
Which jaws have a better penumbra? X or Y jaws?
X jaws because they are closer to the patient
47
What are the jaws in a Linac typically made out of?
Tungsten
48
How many MLCs are typically in a Varian Linac?
120 leaves total
40 inner, 20 outer per bank
Two banks
49
How many MLCs are typically in an Elekta Linac?
80 leaves
50
How many leaves are typically in a Siemens Linac?
82 or 160 leaves
51
Which component of the Linac head has the greatest influence in determining the shape of the isodose curves?
Flattening Filter
52
What is the thickness of inner leaves defined at isocenter? What about outer?
Inner leaves: 1 mm at isocenter
Outer leaves: 2.5 mm at isocenter
53
What strategies do Siemens, Varian and Elekta employ for MLC beam focusing?
Siemens: double focused MLC - Leaves rotate in and out on an arc to match beam divergence
Elekta and Varian - Rounded leaf ends match beam divergence to some extent across wide range of angles. Not as good matching as Siemens
54
Which MLCs yield the smallest penumbra? Which yield the largest?
Varian yields smallest penumbra. Elekta yields largest.
Varian MLCs are closest to patient of the three manufacturers. This overwhelmingly improves penumbra more than any other design choice
Elekta leaves are neither double focused, nor close to the patient, so their penumbra is in general the worst
Siemens is in the middle. Their design is the most geared towards beam divergence, but they're MLCs are higher up than Varian. They're roughly same distance to patient as Elekta's MLCs are
55
What are some advantages to having MLCs closer to target?
They're allowed to be thinner and shorter, and they don't have to move as fast (due to projections at isocenter)
Also the Linac head is allowed to be smaller, meaning less clearance issues for rotating Linac heads
56
What are the major disadvantages to having MLCs closer to target?
Projections at isocenter magnifies everything, including positioning errors and penumbra thickness
57
What is the MLC transmisison requirement if the MLCs are meant to be replacements for upper or lower jaws?
What about if they're just meant to be tertiary beam shapers?
Replacing upper or lower jaws: \< 0.5% transmission
Tertiary beam shaper: \< 5% transmission
58
What is the most commonly used method for energy selection used in Linacs? How does it work?
Two methods...
Method 1) You design the waveguide for a maximum allowed energy (Ex. 20 MeV), then for lower energies, you don't utilize part of the waveguide. The shorted area acts as a "drift tube" with reduced RF power, yielding minimal acceleration. This method is solely used to get to low energy (6 MeV)
But if you want subtle variations (6,9,10,12,16,20, etc.) you need to also utilize method 2 in addiiton to shorting the waveguide
Method 2) varying RF driver output to klystron, result in variation of final product of RF power leaving klystrong
59
Roughly what percentage of the useful beam do flattening filters absorb?
50-90%
60
Approximately how much higher must beam currents be for photon modes, versus electron mode?
200-1000x higher to compensate for lost efficiency with bremsstrahlung (20%) and lost fluence from flattening filter (50-90%)
61
Why are photon beams specified as MV but electron beams are specified as MeV?
Photon beams have a wide energy spectrum due to bremsstrahlung production, so MV is just used to resemble max energy of the beam
Electron beams are nearly monoenergetic (narrow 3% energy window), as such they're denoted by MeV
62
For what energies are magnetrons used for? What about Klystrons?
**Magnetrons** are typically used for **lower energies**
**Klystrons** are typically used for **higher energies**
63
What treatment machines typically utilize magnetrons?
Tomo
64
True or False
Magnetrons require a RF driver
False
Since magnetrons can be used to generate microwaves by themself, they do not require an RF driver
65
Do magnetrons result in longer or shorter waveguides?
Tricky question...
Their output power is limited, so if you try to use magnetrons for higher energies, you will need a longer waveguide
Typically though, magnetrons are only used for low energy units, so the waveguides on these units are typically shorter to begin with
66
Up until what energies are Magnetrons useful for?
18 MeV
67
Which manufacturer utilizes magnetrons?
Elekta
68
Explain how a magnetron works
A magnetron functions by boiling electrons fof a central pole which are attracted to alternating charges placed on the exterior walls.
A magnetic field is applied, forcing the electron to spiral outwards
As they pass each cavity openings, they set up a resonance phenomenon (similar to blowing across a glass bottle's topc) and generate microwaves at the resonant frequency
69
What type of waves do modern linacs utilize? Why?
Standing waves
Allows for shorter waveguides
70
Which wave designs result in better energy stability? Standing or traveling?
Standing
71
Which waveguides result in more efficient acceleration and less shunt impedance? How?
Standing
They have fewer cavities (which means fewer surfaces to dissipate power)
72
Which coils in the waveguie run the length of the waveguide and ensure that the electrons are focused in a pencil beam and don't repel each other off?
Solenoid (or focusing coil)
73
Which coils are used to inject the beam into the bending magnet at the proper location so they eventually strike the target or scattering foils in the proper location?
Beam steering coils
74
Which coils are adjusted when adjusting symmetry of a beam?
Beam steering coils
75
What are the first few cavities of a waveguide typically used for?
Bunching
76
What ar ethe latter cavities of a waveguide used for?
Accelerating
77
What is the spacing between cavities?
1/2 the wavelength
78
How does a standing wave allow for shorter waveguide design?
Magnetic waves set up very strong electroc fields in cavities due to a resonance phenomena
The oscillating electric fields are in phase with electrons as they travel down the guide
79
What is the approximate order of magnitude of acceleration? (energy/distance)
10-25 MeV/m
80
What are the two major subsystems of the modulator?
Thyratron and Pulse Forming Network (PFN)
81
What does a thyratron do?
It's essentially a big fancy switch
Linacs require too much power to run continuously, so capacitors are charged and periodically dumped (controlled by a thyratron) through the klystron to power the linac in a pulsed manner (also helps with cooling)
It's filled with a gas, typically hydrogen, that ionizes forming a conductive plasma when on
The main thyratron sits after the pulse forming network and controls the timing of pulses leaving the system
82
What do buncher cavities do, and how? And where are they usually found?
Usually found in klystron
Take a steady stream of electrons and bunch them
Electric field oscillates in the cavity, switching from positive to negative. Electrons that arrive early feel a retarding force and slow down. Electrons arriving in the middle feel no force. Electrons arriving late feel an accelerating force speeding them up.
83
What are purpose of catcher cavities in a klystron?
Bunched electrons from the buncher cavity travel down the drift space, where they cause the catcher cavity to ring at its resonant frequency
The oscillating electric fields from the ringing then induce an opposing electric field on the electron bunches which decelerate them, emitting microwave energy (conservation of energy)
84
What is component #1 in the klystron image?
Cathode
85
What is component #2 in the klystron image?
Microwave Input
86
What is component #3 in the klystron image?
Filament
87
What is component #4 in the klystron image?
Anode
88
What is component #5 in the klystron image?
Buncher Cavity
89
What is component #6 in the klystron image?
Catcher Cavity
90
What is component #7 in the klystron image?
Amplified Microwave Output
91
What is component #8 in the klystron image?
Electron beam
92
What is component #9 in the klystron image?
Collector
93
Why for TB1 are we not allowed to irradiate using a 40x40 field to treat?
This is sort of a trick question
You can use 40x40 to treat, just not for imaging.
94
Why for TB1 are we not allowe dto use a 40x40 field for imaging?
You can damage the EPID electronics (since the EPID only has an imageable dimension of 40x30)
95
When adjusting dose rate at the LINAC console, what is being adjusted, dose per pulse or timer interval between pulses?
Time interval between pulses
96
What is the most common process that a LINAC head knows the position of an MLC?
Through the use of linear encoding
97
Are the MLCs in the Truebeam single or double focused?
Single focused
98
How is Varian able to have the smallest penumbra out of the LINACs, even though Siemens has a double focused design?
Because Varian puts their MLCs closer to the patient
99
How many MLCs does our LINAC have?
80 small and 40 big
120 total, 60 pairs
100
What is typical...
Intraleaf transmission:
Interleaf transmission:
Leaf end leakage transmission:
Intraleaf transmission: 1%
Interleaf transmission: 1.5 - 2%
Leaf end leakage transmission: 15%
101
Which of the following have the best MLC light vs radiation field results?
Varian, Elekta, Siemens
Siemens
Double focused MLCs
102
How much does neutron production increase going from 10X to 15X?
What about 10X to 18X?
10X to 15X - neutron contamination increases by 10x
10X to 18X - neutron contamination increases by 20x
So in theory, from 15X to 18X - neutron contamination increases by 2x
103
What are MLC leaves typically made out of?
Tungsten
104
Which of the following has the smallest penumbra? Which as the worse?
Varian MLCs
Elekta MLCs
Siemens MLCs
Varian has the smallest penumbra
Elekta has the largest penumbra. Not only does it not double focus, it also has the MLC bank closest to the target out of all three options
105
Fill in the blanks in the diagram
106
Why is the purpose of SF6 gas in a linear accelerator?
To prevent arcing of electrons caused by the electric fields from the microwaves
107
Where is the RF driver located?
With the Klystron
If the LINAC has a magnetron, then there is no RF driver
108
What is the purpose of the RF driver?
To generate microwaves and input into the klystron
109
Generally how long is an accelerating waveguide?
1 meter
110
True or False
Modern LINACs do NOT use voltage differentials to accelerate electrons in the accelerating waveguide
True
They use oscillating electric fields instead. Voltage differentials are only good for lower energies typically. Not as efficient.
111
Why are the bending magnet and the electron gun in a vacuum?
The electron gun is in a vacuum to avoid electrons interacting with medium **and also burning out the filament** (imagine a light bulb)
The bending magnet is in a vacuum to avoid electrons interacting with the medium
112
What is the most important feature of a material that is to be used as a target?
**Ability to withstand heat**
That's why tungsten is used, it has the highest melting point out of all elements. If Z was the most important feature, you would use something with a higher Z than tungsten, but we don't.
113
Why is oil sometimes used in cooling instead of water?
(This isn't just the case with LINACs, it also applies to kV imaging sources as well)
Because oil has a higher boiling point and it's an electrical insulator, making it ideal to be used in high voltage equipment
114
What is the approximate bremmstrahlung production efficiency in MV beams?
20%
115
Where is SF6 gas located in the LINAC?
Transport waveguides
116
In terms of using "Golden Beam Data" or your own commissioning measured data, which does TG-106 recommend you use?
TG-106 recommends the use of user measured data
The golden beam data, although less prone to human error, is also not an exact match to your specific machine. Some installers will attempt to match the golden beam data as closely as possible, but it's not gonna be 100%
117
What is the "Golden Beam Data" useful for, if not to be used clinically?
It's a useful comparison for the acceptance physicist to make sure their measured data makes sense to what it should resemble
118
For commissioning, at an absolute minimum, what does TG-106 recommend measuring for photons? (6)
PDD values
In-plane and cross-plane profiles
Wedge factors
MLC data (leakage, penumbra, tounge and groove, DLG, etc)
Scatter factors
Tray factors
119
For commissioning, at an absolute minimum, what does TG-106 recommend measuring for electrons? (5)
PDD values
In-plane and cross-plane profiles
Cone-factors
Insert factors
Virtual source positions
120
What is the max depth of PDD measurements conducted for commissioning?
40 cm
So you need a 3D tank that's atleast 40+5 cm deep
121
How do you adjust flatness of a beam when engineer comes in?
You can't
122
Which manufacturer has the best clearance on their LINACs? Why?
Elekta
Two reasons,
1. MLC banks closer to target
2. Bending magnet is not 270 deg. It's the other achromatic
