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RadPhys > Raphex 14-17 > Flashcards

Raphex 14-17 Flashcards

1
Q

How does the surface dose on an e- beam vary with energy?

A

It increases w/ energy, unlike for photons

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2
Q

How does e- beam obliquity change side scatter?

A

Increases it

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3
Q

How does the surface dose on an e- beam vary with energy?

A

It increases w/ energy, unlike photons

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4
Q

How does the range and Bragg peak of charged particle, like protons and e-, vary with particle mass?

A
  • The heavier the particle, the less it scatters
  • Less range uncertainty
  • Sharper Bragg peak
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5
Q

How does obliquity affect e- dose and PDD?

A
  • It increases the surface dose
  • Leads to less sharp dose fall off
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6
Q

What happens to the 90% isodose line as you increase the energy of e-?

A

It constricts

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7
Q

How does the e- current required to produce photons 𝛥 w/ the photon beam energy?

A
  • Photon production efficiency increases linearly w/ beam energy
  • Higher current is required at lower than higher beam energies to produce the same dose rate
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8
Q

Which F isotope is used for PET scans? Which F isotope is stable?

A
  • F-18 is used for PET scans
  • F-19 is the stable isotope
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9
Q

Is 3H stable or unstable?

A
  • Unstable and radioactive
  • It’s used in research
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10
Q

Is 3H stable or unstable?

A
  • Unstable and radioactive
  • It’s used in biomedical research
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11
Q

How does the energy of positrons compare w/ PET spatial resolution?

A
  • Positrons travel some distance before annihilating. This travel distance adds uncertainty to the PET scan and contributes to poorer spatial resolution.
  • Using positrons with lower energy w/ decrease the travel distance and improve uncertainty and resolution.
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12
Q

Are annihilation photons emitted exactly anti-parallel in a PET scan?

A

No. This contributes to the poorer spatial resolution of PET scans.

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13
Q

What is the dynamic range of a CT scanner?

A

It is the ratio of the largest signal value to the smallest signal value

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14
Q

What’s the normal shape of the dose-response curve?

A

Sigmoidal!

Note the difference between the dose-response curve and the survival curve!

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15
Q

What is the mechanical iso-center?

A

Point of intersection of the couch, collimator, and gantry axes of rotation.

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16
Q

How do you identify the dosimetric isocenter of a linac?

A

Star shot

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17
Q

Can intraop radiation be delivered using a linac?

A

Yes!

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18
Q

As it related to its energy, which brachytherapy source would require the most anisotropy correction?

A

Source w/ the lowest energy, eg Pd-103, since it will be most affected by absorption by the metallic seed casing.

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19
Q

How is the backscatter factor related to dose and time of treatment?

A
  • BSF ∝ Dose
  • BSF ∝ 1/ tx_time
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20
Q

Which dosimeter has the least energy dependence in the kV range?

A

Radiochromic film

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21
Q

How is the total scatter factor measured?

A

It’s a combination of collimator and phantom scattered, and is measured in water.

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22
Q

Can EPIDs measure e- outputs?

A

No! They need photons, which are sequentially converted into e- and visible light, etv.

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23
Q

Can ion chambers be used in vivo?

A

No! They have a high bias voltage (300 V), which would endanger the patient.

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24
Q

Which dosimeter is unsuitable for SRS dosimetric measurements?

A

A farmer chamber, as its size is too large compared to SRS field sizes.

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25
Q

How often do radiation survey meters need to be calibrated at an accredited laboratory?

A

Annually

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26
Q

How often do radiation therapy dosimeters need to be calibrated at an accredited laboratory?

A

Bi-annually

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27
Q

How does the penumbra 𝛥 with ↑ beam energy?

A

↑ beam energy → ↑ penumbra 2/2 higher lateral range of secondary e-

28
Q

What’s the % attenuation for 6 MV and 15 MV photon beams?

A
  • 6 MV: 3%/cm
  • 15 MV: 2%/cm
28
Q

How does the degree of loss of charged particle eq at tissue boundaries related to field size?

A

↓ field size → ↑ loss of equilibrium

28
Q

Why do protons deposit almost all of their energy at the end of their range while e- do it in a more random manner?

A

Because proton are 2000 times heavier than e-, they travel in a nearly straight line.

29
Q

What does narrow beam geometry mean?

A

Means that the beam geometry is such that only the primary radiation can reach the detector.

30
Q

How does the penumbra of a Co-60 compare with a 6 MV Linac beam?

A

Co-60 will have a larger penumbra 2/2 small source size!

31
Q

What particles contribute to the skin dose of an MV photon beam?

A
  • e- contamination
  • incident beam
  • backscattered beam
32
Q

What happens to dose calculations in a patient with a metal prosthesis?

A
  • Metal prosthesis saturates CT value, resulting in an underestimation of the CT number
  • This underestimation leads to a falsely low attenuation value compared to the true value
33
Q

Can metal implants affect RF fiducial tracking?

A

Yes!

34
Q

Why do we use effective SSD for e- treatments?

A

To produce correlation with the inverse square law!

35
Q

Which DICOM format contains dose matrices?

A
  • DICOM-RT dose file
36
Q

What’s the occupancy factor of a room set to when calculating the maximum allowable dose rate?

A

1

37
Q

When designing shielding for a Linac room, should lead or steel be placed inside with concrete on the outside or the converse?

A
  • Lead or steel should be placed inside as they can produce photoneutrons
  • Concrete should be placed outside to block all neutrons
38
Q

What device is used for portal dosimetry for a Linac?

A

MV imager (NOT kV imager)

39
Q

What is a piezoelectric material and which imaging modality uses it?

A
  • Material that produces acoustic waver from radiofrequency
  • Used by ultrasounds
40
Q

What part of the dynamic jaw corresponds to the heel of a physical wedge?

A

The part that’s covered by the jaw for most of the treatment.

41
Q

How do you calculate ITV from CTV

A
  • ITV = CTV + Internal Margin
  • ITV; Internal Treatment Volume
  • IM; Internal Margin
42
Q

What is the most consistent and reproducible part of the breathing cycle?

A
  • End exhalation!
43
Q

Why is collimator rotation used during IMRT?

A

It decreases the dose banding effect from the tongue in groove design of the MLCs

44
Q

What’s the main advantage of using EPID vs. diode array for in vivo dosimetric measurements?

A

↑ resolution

45
Q

What’s the main advantage of VMAT vs. IMRT?

A

Shorter treatment time, less opportunity for intrafraction movement

46
Q

What’s the purpose of using a compensator for TBI treatments?

A
  • To compensate for differences in tissue
  • Improved dose homogeneity
47
Q

What’s the primary purpose of a beam spoiler, and how should it be positioned relative to the patient?

A
  • ↑ skin dose
  • Place as close to patient as possible
48
Q

What agreement between calculated and measured doses and dose uniformity is considered acceptable for TBI protocols?

A
  • Dose agreement: 5%
  • Dose uniformity: 10%
49
Q

What are fundamental particles?

A
  • Particles that are not made of smaller component particles
  • Include e-, and quarks (which make up protons, neutrons, etc)
50
Q

What does insufficient vacuum inside an XR tube cause?

A
  • Short-circuit, or tube arcing
  • air/impurities present in the tube cause current to flow between the cathode and usually the tube envelope
  • No useful particles are produced by the XR tube when this happens
51
Q

What’s the reasoning behind making the effective smaller than the actual focal spot for an XR tube?

A
  • ↑ actual focal spot: Heat distributed over a larger area → greater heat capacity
  • ↓ effective focal spot → improved image quality
52
Q

What’s the primary goal when using an FFF beam?

A

↑ dose rate and ↓ tx time

53
Q

How does the dmax of an FFF beam compare to the dmax of a filtered beam?

A
  • FFF beam dmax < Filtered beam dmax
    – FFF beams have lower average energy than filtered beams
54
Q

What XR energies are available for a tomotherapy unit?

A
  • 6 MV only
  • Used for tx & acquiring images
55
Q

What’s the SAD for a tomotherapy unit?

A

85 cm

56
Q

What kind of MLCs do tomotherapy units have?

A

Binary MLCs

57
Q

What’s the unique feature of tomotherapy XR shielding?

A

Tomotherapy units have internal shielding, which can attenuate the primary beam by 99.9%!

58
Q

In which kind of treatment is using a bolus over OSLDs inappropriate?

A

When doing TSET, since we want to measure skin dose itself!

59
Q

What’s the integral dose and how does it 𝛥 with photon energy, field size, incidence dose, & patient thickness?

A

Dose x Massirradiated

  • ↑ Photon energy → ↓ Integral dose
  • ↑ Field size, patient thickness, incident dose → ↑ Integral dose
60
Q

What’s the difference between a mask and a filter?

A
  • A mask blocks the beam, affecting intensity and field size but NOT energy.
  • A filter attenuates the beam, affecting its energy and intensity but NOT the field size.
61
Q

If a person has irregular breathing, how would it affect his 4D CT scan?

A

Some phases will image the same phases, leading to discontinuities b/w slices.

62
Q

How many gray levels in a x bit image?

A

2x

63
Q

What’s the formula for the dose rate?

A

It’s basically the denominator in the ΜU formula.

Dose Rate = O x PDD (or TMR) Scp x WF x TF

RadPhys (10 decks)

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