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Rt Physics > Wk02 Linac > Flashcards

Wk02 Linac Flashcards

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1
Q

What is the source axis distance (SAD) of a Co-60 linac?

A

80 or 100 cm

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2
Q

What are the problems of Co-60 linac machines?

A
  1. Short half life (approx 5 years) —> need to change source every 5 year
  2. Difficult of dose calculation and computerisation
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3
Q

What are the different external beam radiotherapy (EBRT) modes and their machine respectively?

A
  1. Contact x ray = <50kvp x ray unit
  2. Superficial x ray = 50-150kvp x ray unit
  3. Orthovoltage x ray = 150-400 kvp x ray unit
  4. Teletherapy = Co-60 unit, Cs-137 unit
  5. MV x rays = linac
  6. MV electron beam = linac
  7. Hadron therapy = cyclotron / synchrotron / synchrocyclotron
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4
Q

When high speed electrons hit the high Z-material target, most of the electron’s kinetic energy is transformed into _____?

A

Heat

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5
Q

X ray photons which formed by X ray tubes are divided into which two categories?

A
  1. Characteristic x rays
    - when incident electrons ejected a K-shell electron
  2. Bremsstrahlung photons
    - distant interaction with the nuclei —> low energy
    - close interaction with the nuclei —> moderate energy
    - direct impact with the nucleus —> max. Energy
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6
Q

The relative proportion of the number of characteristic photons to bremsstraahlung photons in an x ray beam spectrum varies with:

A
  1. Kinetic energy of the electron beam striking the x ray target
  2. Atomic number of the target
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7
Q

High energy x ray machines have __________ targets; low energy x ray machines have _________ type targets.

A

Transmission ; reflection

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8
Q

diagnostic range photons are produced at ____ degrees from the direction of electrons striking. While in megavoltage range, photons are produced at ____________ direction of electrons striking.

A

90; the same (180 degree)

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9
Q

Name 4 x ray beam qualities specifiers.

A
  1. Complete x ray spectrum
  2. Half-value layer (HVL)
  3. Tissue-phantom ratio (TPR)
  4. Percentage depth dose (PDD)
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10
Q

What is the term ‘beam quality’ is used to indicate.

A

The ability of a beam to penetrate a water phantom.

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11
Q

What is linear accelerator (linac)?

A

A device use high frequency electromagnetic waves to accelerate electrons to high energies through a linear tube.

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12
Q

What are the major components of a linac?

A
  1. Drive stand
    - Klystron / magnetron
    - waveguide
    - water cooling system
  2. Gantry
    - electron gun
    - accelerator structure
  3. Treatment head
    - bending magnet
    - flattening filter
    - scattering foil
  4. Imaging systems
    - MV/ KV imaging
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13
Q

What are the components of a treatment head under x ray therapy mode?

A
  • x ray target
  • primary collimates
  • flattening filter
  • ion chamber
  • secondary collimated
  • slots for wedges / blocks / compensators
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14
Q

What are the components of a treatment head under electron therapy mode?

A
  • primary collimating
  • scattering foil
  • ion chamber
  • secondary collimating
  • accessory mount
  • electron applicator
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15
Q

Modern linacs collimating are achieved with which 3 devices?

A
  1. Primary collimator
    - provides fixed maximum aperture for the x ray beam
  2. Secondary collimator (X and Y jaws)
    - tungsten jaws
    - 1cm x 1cm to 40cm x 40cm
  3. Multileaf collimator
    - each lead are driven by motor independently
    - used in IMRT and VMAT
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16
Q

What is the function of a flattening filter?

A

The profile of x rays generated at the target has a cone shape distribution, metal flattening filter turns the cone shape profile into a flat profile, to facilitate manual and forward computer planning.

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17
Q

What are the benefits of flattening filter free (FFF) x ray beam?

A
  1. Increase output efficiency
    - a large % of the x ray beam produced is absorbed by the flattening filter originally
  2. Increase output dose rate
    - increase from 400-600 to over 1200
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18
Q

What is the function of in chamber in linac?

A
  1. Determine treatment beam dose
  2. Redundant check of dose rate and integrated dose
  3. Allow feedback for flatness and symmetry
19
Q

Why the edges of multi-leaf collimator are rounded?

A

To minimise transmission penumbra

20
Q

What are the problems with wedged beam?

A
  1. Reduce dose rate
  2. Slow down treatment flow
  3. Limits the useable maximum field size
21
Q

What are the components that achieved electron beam collimating?

A
  1. Primary collimator
  2. Secondary collimator
  3. Electron applicator (Cone)
  4. Electron inserts
22
Q

What are the difference between electron and photon beam collimation?

A
  1. X ray target is removed
  2. The flattening filter is replaced by a scattering flattening filter foil to produce a broad electron beam
  3. the use of electron applicator
  4. Electron beam current is only 0.1% of that for x ray beam.
23
Q

What do the optical light field simulates?

A

The geometry of the radiation beam

24
Q

What are the functionalities of on-board imaging (OBI) system?

A
  1. Radiographic imaging
  2. Fluoroscopic imaging
  3. Cone beam CT (CBCT)
25
Q

What are the characteristics of linac photon beams

A
  • surface dose
  • buildup region
  • %PP
  • Dmax
  • dmax
26
Q

What contributes to the surface dose?

A

Surface dose is caused by secondary electrons produced by interaction with air and objects like flattening filter, mirror and collimator

27
Q

The __________ the energy beam, the __________ the buildup effect

A

Higher; larger

28
Q

What it the definition of percentage depth dose (PDD) curve?

A

It is a measure of the variation of absorbed dose at different depth (d) along the central axis of the beam in water

29
Q

Surface dose __________ with beam energy, ________ with field size

A

Decreases ; increases

30
Q

For nominal beam energy (4-10MV), what is the equation to calculate the depth of maximum dose (dmax)?

A

Energy / 4

31
Q

What is the manufacturer specification of photon beam energy?

A

The percentage depth dose (PDD) at 10cm depth in water with beam size 10cm x 10cm.

32
Q

What is tissue phantom ratio (TPR)20,10

A

The ratio of doses on the beam central axis at depths 20cm and 10cm in water obtained with a constant SAD = 100 and FS = 10 x 10 at the position of the detector

33
Q

What is the definition of field size

A

Width of the beam at 50% dose level

34
Q

What is the definition of beam penumbra

A

Distance between 20% and 90% dose level in the beam profile

35
Q

Penumbra width contributed and affected by________________.

A
  1. Physical field size of radiation source
    2.geometry of collimator
  2. Energy of radiation beam
  3. Medium of beam travelled
  4. Depth of measurement
  5. Scattered radiation
36
Q

What is beam flatness?

A

F = 100 x [(Dmax-Dmin) / (Dmax+Dmin)]
With Dmax and Dmin values taken within 80% of the beam width and profile measured at 10cm depth

37
Q

What is beam symmetry

A

S = 100 x [(left area - right area) / (left area + right area)]

38
Q

Output factor ____________ with ___________ field size

A

Increases ; increases

39
Q

Why electron beams have lower penetrating power than photon beams?

A

An electron will start interacting with the medium immediately, including orbital electrons and the nucleus. Due to strong Coulomb force interactions, electrons lose energy much faster than photons

40
Q

Electron beams have _____________ surface dose than that of photon beams

A

Higher

41
Q

The rate of electron energy loss per cm is _______________ and to the _________________.

A

Proportional to electron energy ; square of atomic number of medium (Z2)

42
Q

What is R50

A

Electron beam quality is specified by R50.
R50 is the depth of water at which the absorbed dose falls to 50% of the maximum dose

43
Q

What are the sources of x ray contamination of electron treatment?

A
  1. Scattering filter
  2. Ion chamber
  3. Jaws
  4. Metallic walls of treatment applicator
  5. Cone and inserts
  6. The patient

X ray contamination cause tails in PDD curves of electron beams

Rt Physics (13 decks)

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