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Residency > First Year Exam: Linear Accelerator > Flashcards

First Year Exam: Linear Accelerator Flashcards

1
Q

How long does a pulse typically last?

A

A few microseconds

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2
Q

What is the time interval between pulses typically?

A

A few milliseconds

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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

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4
Q

What is a Duty cycle?

A

Fraction of time that a beam is actually on

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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)

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6
Q

What do the monitor unit chambers measure? (3 things)

A

Dose rate
Integrated dose
Field symmetry

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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

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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%

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9
Q

What is the purpose of the timer?

A

To terminate the beam after a certain time in case both chambers are failing

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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

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11
Q

True or False

Monitor chambers are sealed

A

True

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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
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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

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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

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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

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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

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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.

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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

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19
Q

How is electron bunching, amplification and efficiency of a klystron improved?

A

By increasing the amount of cavities

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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

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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)

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22
Q

Why do varian linacs use a 270 degree bending magnet? (two main reasons)

A
  1. Redirect beam from parallel to patient, to perpendicular to patient (allowing for more compact design)
  2. Eliminate electron spatial energy spectrum (make beam achromatic)
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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.

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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)

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25
Q

What are scattering foils typically made of?

A

Either lead or copper

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26
Q

What are flattening filters typically made out of?

A

Varying materials such as lead, aluminum, steel and brass

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27
Q

True or false

Scattering foils and flattening filters are energy dependent?

A

True

So there’s multiple on the carousel that get rotated out depending on the energy

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28
Q

In the image, what is component #9?

Briefly: what does it do?

A

Ion/MU Chambers

Used to monitor output, symmetry and (possible flatness)

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29
Q

In the image, what is component #10?

Briefly: what does it do?

A

Light source and mirror

Light source for field size and ODI

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30
Q

In the image, what is component #11?

Briefly: what does it do?

A

Secondary collimators (jaws)

Field shaping

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31
Q

In the image, what is component #12?

Briefly: what does it do?

A

Tertiary beam shaping (MLCs, blocks, electron cones, wedges, etc.)

As the name would suggest, field shaping

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32
Q

If the field light in a Linac was ever off, what do you adjust?

A

The mirror

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33
Q

True or false

Ion chamber and field light are on different axis

A

False

Field light and ion chambers are on same axis, so the mechanism retracts the ion chamber when the field light is required

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34
Q

What is a Cal Check Cycle and what does it check?

A

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

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35
Q

What is a typical MLC maximum leaf speed? Where is it defined?

A

2.5 cm/s at isocenter

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36
Q

What is the maximum leaf extension from bank?

A

15 cm

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37
Q

What is the requirement for leaf position accuracy?

A

Within 1 mm

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38
Q

What is a typical MLC leaf transmission?

A

1-2%

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39
Q

What is used as the leaf position reference?

A

Optical beam (collimated infrared beam)

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40
Q

What aspect of the Linac nominally defines beam energy?

A

Acclerating voltage

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41
Q

Given a 10 MV beam, what is the maximum photon energy? What is the mean energy?

A

10 MeV

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42
Q

What is the parameter that defines photon beam quality?

A

PDD(10 cm)

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43
Q

What is the parameter that defines electron beam quality?

A

Distance of 50% (R50)

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44
Q

Which jaws are located above the other? X or Y?

A

Y jaws are located above the X jaws

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45
Q

How do Y jaws move? How do X jaws move?

A

Y jaws move on an arc trajectory

X jaws move on a linear trajectory but rotate slightly as they move to match beam divergence

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46
Q

Which jaws have a better penumbra? X or Y jaws?

A

X jaws because they are closer to the patient

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47
Q

What are the jaws in a Linac typically made out of?

A

Tungsten

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48
Q

How many MLCs are typically in a Varian Linac?

A

120 leaves total

40 inner, 20 outer per bank

Two banks

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49
Q

How many MLCs are typically in an Elekta Linac?

A

80 leaves

50
Q

How many leaves are typically in a Siemens Linac?

A

82 or 160 leaves

51
Q

Which component of the Linac head has the greatest influence in determining the shape of the isodose curves?

A

Flattening Filter

52
Q

What is the thickness of inner leaves defined at isocenter? What about outer?

A

Inner leaves: 1 mm at isocenter

Outer leaves: 2.5 mm at isocenter

53
Q

What strategies do Siemens, Varian and Elekta employ for MLC beam focusing?

A

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
Q

Which MLCs yield the smallest penumbra? Which yield the largest?

A

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
Q

What are some advantages to having MLCs closer to target?

A

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
Q

What are the major disadvantages to having MLCs closer to target?

A

Projections at isocenter magnifies everything, including positioning errors and penumbra thickness

57
Q

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?

A

Replacing upper or lower jaws: < 0.5% transmission

Tertiary beam shaper: < 5% transmission

58
Q

What is the most commonly used method for energy selection used in Linacs? How does it work?

A

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
Q

Roughly what percentage of the useful beam do flattening filters absorb?

A

50-90%

60
Q

Approximately how much higher must beam currents be for photon modes, versus electron mode?

A

200-1000x higher to compensate for lost efficiency with bremsstrahlung (20%) and lost fluence from flattening filter (50-90%)

61
Q

Why are photon beams specified as MV but electron beams are specified as MeV?

A

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
Q

For what energies are magnetrons used for? What about Klystrons?

A

Magnetrons are typically used for lower energies

Klystrons are typically used for higher energies

63
Q

What treatment machines typically utilize magnetrons?

A

Tomo

64
Q

True or False

Magnetrons require a RF driver

A

False

Since magnetrons can be used to generate microwaves by themself, they do not require an RF driver

65
Q

Do magnetrons result in longer or shorter waveguides?

A

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
Q

Up until what energies are Magnetrons useful for?

A

18 MeV

67
Q

Which manufacturer utilizes magnetrons?

A

Elekta

68
Q

Explain how a magnetron works

A

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
Q

What type of waves do modern linacs utilize? Why?

A

Standing waves

Allows for shorter waveguides

70
Q

Which wave designs result in better energy stability? Standing or traveling?

A

Standing

71
Q

Which waveguides result in more efficient acceleration and less shunt impedance? How?

A

Standing

They have fewer cavities (which means fewer surfaces to dissipate power)

72
Q

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?

A

Solenoid (or focusing coil)

73
Q

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?

A

Beam steering coils

74
Q

Which coils are adjusted when adjusting symmetry of a beam?

A

Beam steering coils

75
Q

What are the first few cavities of a waveguide typically used for?

A

Bunching

76
Q

What ar ethe latter cavities of a waveguide used for?

A

Accelerating

77
Q

What is the spacing between cavities?

A

1/2 the wavelength

78
Q

How does a standing wave allow for shorter waveguide design?

A

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
Q

What is the approximate order of magnitude of acceleration? (energy/distance)

A

10-25 MeV/m

80
Q

What are the two major subsystems of the modulator?

A

Thyratron and Pulse Forming Network (PFN)

81
Q

What does a thyratron do?

A

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
Q

What do buncher cavities do, and how? And where are they usually found?

A

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
Q

What are purpose of catcher cavities in a klystron?

A

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
Q

What is component #1 in the klystron image?

A

Cathode

85
Q

What is component #2 in the klystron image?

A

Microwave Input

86
Q

What is component #3 in the klystron image?

A

Filament

87
Q

What is component #4 in the klystron image?

A

Anode

88
Q

What is component #5 in the klystron image?

A

Buncher Cavity

89
Q

What is component #6 in the klystron image?

A

Catcher Cavity

90
Q

What is component #7 in the klystron image?

A

Amplified Microwave Output

91
Q

What is component #8 in the klystron image?

A

Electron beam

92
Q

What is component #9 in the klystron image?

A

Collector

93
Q

Why for TB1 are we not allowed to irradiate using a 40x40 field to treat?

A

This is sort of a trick question

You can use 40x40 to treat, just not for imaging.

94
Q

Why for TB1 are we not allowe dto use a 40x40 field for imaging?

A

You can damage the EPID electronics (since the EPID only has an imageable dimension of 40x30)

95
Q

When adjusting dose rate at the LINAC console, what is being adjusted, dose per pulse or timer interval between pulses?

A

Time interval between pulses

96
Q

What is the most common process that a LINAC head knows the position of an MLC?

A

Through the use of linear encoding

97
Q

Are the MLCs in the Truebeam single or double focused?

A

Single focused

98
Q

How is Varian able to have the smallest penumbra out of the LINACs, even though Siemens has a double focused design?

A

Because Varian puts their MLCs closer to the patient

99
Q

How many MLCs does our LINAC have?

A

80 small and 40 big

120 total, 60 pairs

100
Q

What is typical…

Intraleaf transmission:

Interleaf transmission:

Leaf end leakage transmission:

A

Intraleaf transmission: 1%

Interleaf transmission: 1.5 - 2%

Leaf end leakage transmission: 15%

101
Q

Which of the following have the best MLC light vs radiation field results?

Varian, Elekta, Siemens

A

Siemens

Double focused MLCs

102
Q

How much does neutron production increase going from 10X to 15X?

What about 10X to 18X?

A

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
Q

What are MLC leaves typically made out of?

A

Tungsten

104
Q

Which of the following has the smallest penumbra? Which as the worse?

Varian MLCs

Elekta MLCs

Siemens MLCs

A

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
Q

Fill in the blanks in the diagram

A
106
Q

Why is the purpose of SF6 gas in a linear accelerator?

A

To prevent arcing of electrons caused by the electric fields from the microwaves

107
Q

Where is the RF driver located?

A

With the Klystron

If the LINAC has a magnetron, then there is no RF driver

108
Q

What is the purpose of the RF driver?

A

To generate microwaves and input into the klystron

109
Q

Generally how long is an accelerating waveguide?

A

1 meter

110
Q

True or False

Modern LINACs do NOT use voltage differentials to accelerate electrons in the accelerating waveguide

A

True

They use oscillating electric fields instead. Voltage differentials are only good for lower energies typically. Not as efficient.

111
Q

Why are the bending magnet and the electron gun in a vacuum?

A

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
Q

What is the most important feature of a material that is to be used as a target?

A

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
Q

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)

A

Because oil has a higher boiling point and it’s an electrical insulator, making it ideal to be used in high voltage equipment

114
Q

What is the approximate bremmstrahlung production efficiency in MV beams?

A

20%

115
Q

Where is SF6 gas located in the LINAC?

A

Transport waveguides

116
Q

In terms of using “Golden Beam Data” or your own commissioning measured data, which does TG-106 recommend you use?

A

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
Q

What is the “Golden Beam Data” useful for, if not to be used clinically?

A

It’s a useful comparison for the acceptance physicist to make sure their measured data makes sense to what it should resemble

118
Q

For commissioning, at an absolute minimum, what does TG-106 recommend measuring for photons? (6)

A

PDD values

In-plane and cross-plane profiles

Wedge factors

MLC data (leakage, penumbra, tounge and groove, DLG, etc)

Scatter factors

Tray factors

119
Q

For commissioning, at an absolute minimum, what does TG-106 recommend measuring for electrons? (5)

A

PDD values

In-plane and cross-plane profiles

Cone-factors

Insert factors

Virtual source positions

120
Q

What is the max depth of PDD measurements conducted for commissioning?

A

40 cm

So you need a 3D tank that’s atleast 40+5 cm deep

121
Q

How do you adjust flatness of a beam when engineer comes in?

A

You can’t

122
Q

Which manufacturer has the best clearance on their LINACs? Why?

A

Elekta

Two reasons,

  1. MLC banks closer to target
  2. Bending magnet is not 270 deg. It’s the other achromatic

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