Characteristics of Clinical Beams: Photons Flashcards

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1
Q

What happens to the isodose plot in the patient as the energy of the beam increases?

A

Greater penetration

Less side scattered dose - loss of side lobes on the isodose plot.

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2
Q

How is a percentage depth dose (PDD) curve obtained from and isodose plot?

A

Linear profile of the dose down the central axis of the plot.

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3
Q

How does an increase in energy affect the PDD?

A
  • Greater depth of d(max)
  • Shallower drop off after d(max)
  • Shallower build up region
  • Lower entrance surface dose
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4
Q

What are the major characteristics of a clinical photon beam?

A
  • Build up region
  • Point of max dose
  • Normalisation point (5, 7 or10cm, depends on centre)
  • Dose reduction with depth
  • Penumbra
  • Beam profiles at depth
  • Dose from scatter (linac head and patient) and leakage
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5
Q

What causes the build-up effect in a clinical beam?

A
  • Photons interact at different depths in the tissue and generate secondary electrons.
  • At each interaction, the recoils electrons travel (mostly) forward and deposit dose
  • As more tracks overlap, the dose builds up until Charged Particle EquillibriumM (CPE) is reached.
  • A steady state would be reached if there was not photon attenuation (absorption or scatter)
  • Dose>0 at the surface due to back0-scattered electrons and electron contamination.
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6
Q

How dose increasing the beam size affect the PDD?

A

Increases the dose at depth due to:

  • More photons reaching the patient from the source
  • More scattered electrons to the measurement point from the irradiated volume.
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7
Q

How does increasing the SSD affect the PDD?

A

Increases dose at depth > d(max) (i.e. shallower drop-off)

Absolute dose would decrease with increase of SSD, but percentage dose relative to a fixed point increases

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8
Q

Define the Tissue-Phantom Ratio.

A

Ratio od the dose obtained at a fixed field size and SSD, but with different thicknesses of “tissue” between the surface and the detector.

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9
Q

How does the TPR vary with increasing field size?

A
  • Steeper build-up region
  • Lower depth of TPR(max)
  • Shallower fall-off after TPR(max)
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10
Q

Why is there a steeper fall-off from the TPR compared to a PDD?

A
  • TPR does not have an Inverse Square Law effect

- Scatter conditions of the TPR and PDD are different.

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11
Q

How is the beam size defined on a beam profile?

A

Defined at the Full-Width Half-Maximum

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12
Q

How are the penumbra defined on a beam profile?

A

Difference between the 80% and 20% dose points.

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13
Q

How does the beam profile change with increasing depth?

A
  • Beam and penumbra widen
  • Flattest beam at 10cm (with flattening filter)
    • Before 10cm beam is “horny”
    • After 10cm beam is peaked
    • Horniness and Peakedness increase with distance from 10cm depth.
  • Dose is reduced as with PDD.
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14
Q

How is the wedge angle defined?

A

Wedge angle is the angle between the isodose line and the normal to the central axis at 10cm deep.

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15
Q

What are the three causes of penumbrae in a clinical beam?

A
  • Geometric
  • Transmission trough collimators
  • Scatter in Patient.
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16
Q

How does a geometric penumbra arise?

A
  • Photon source is not a point source.

- Photons from opposite edges of source can pass through the collimators giving beams in overlapping locations

17
Q

What does the width of the geometric penumbra depend on?

A
  • Width of extended source

- Position of collimators

18
Q

What does the width of the transmission penumbra depend on?

A
  • Energy of the beam
19
Q

What happens if the jaw face is not parallel to the divergent beam?

A
  • At larger field sizes the path through the collimators is shorter
  • Penumbrae become larger.
20
Q

How do dosimetric penumbra change with increasing energy?

A
  • Lower energies have more lateral scatter.

- Dosimetric penumbrae reduce at higher energies as there is more forward scatter.

21
Q

What two factors affect the dose to patient when beam size is increased?

A
  • Incident beam (more photons incident to patient)

- Scatter within patient

22
Q

Whar percentage of the dose to the patient comes from head scatter?

A

3-4%

23
Q

Where does head scatter come from?

A

Mainly the flattening filter, but some contribution from collimators.

24
Q

How does the patient scatter factor (Sp) relate to the head scatter factor (Sc) and the field size factor (St)?

A

Sp = St/Sc

25
Q

What is a monitor unit?

A

Dose measured in the monitor chamber in the linac head that gives a fixed dose (1cGy) under the departmental reference conditions (fixed SSD, Field size and depth)