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Dose Tolerances > Tx planning - dose metrics/isodose distribution > Flashcards

Tx planning - dose metrics/isodose distribution Flashcards

1
Q

Materials wedges are made from.

A

Lead, copper, steel

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2
Q

Distance hard wedge should be from skin, why?

A

> 15cm; eā€™ contamination

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3
Q

New def of Wedge Angle:

A

angle defined by wedge at depth of 10cm

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4
Q

What effect does depth have on Wedge Angle, why?

A

WA decreases with depth; Scatter fills in (wedge scatter increases

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5
Q

How does a hard wedge effect PDD?

A

it creates Beam Hardening - PDD increases by 1-2%

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6
Q

Equation to solve for Wedge Angle:

A

WA = 90 - (Hinge Angle / 2)

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7
Q

Where do hotspots occur when using a wedge, what causes this?

A

Under the thin part of wedge (toe); Increase with FS and Angle, Differential Attenuation

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8
Q

How do you correct for an incorrect Wedge used?

A

look this up

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9
Q

What are common clinical uses for a wedge?

A
  1. Parotid Wedge Pair
  2. Breast
  3. 3-Field Rectum
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10
Q

If wedged properly, where does the hotspot occur for a 3-field rectum?

A

Posteriorly (pt is prone), due to the PA beam

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11
Q

What are the Air-Gap correction types?

A
  1. Effective Attenuation
  2. Effective SSD
  3. TAR Method
  4. Isodose Shift Method
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12
Q

Most accurate Air-Gap Correction, why?

A

TAR Method; uses ratio of TARs or TMRs as correction factor

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13
Q

What is the equation used for TAR-Method air-gap correction?

A

Db = Da x (TMRb / TMRa)

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14
Q

2 characteristics related to tissue inhomogeneity

A
  1. Change in absorption of primary & scatter

2. Change of secondary electron fluence

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15
Q

Where does event 1 for tissue inhomegeneity dominate?

A

Near the boundary

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16
Q

Where does event 2 for tissue inhomegeneity dominate?

A

Away from and In

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17
Q

What is TERMA

A

Total Energy Released in MAtter

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18
Q

What is the difference between TERMA and KERMA?

A

TERMA contains bremmstrahlung

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19
Q

Explain the 2 types of Convolution:

A
  1. Basic - kernal is spatially invariant (same tear-drop), not good with inhomogeneity, not good with divergence;
  2. Fancy (or Collapsed Cone Convolution)- kernal is spatially variant, better with divergence, better with inhomogeneity by scaling densities
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20
Q

Example of Superposition

A

Monte Carlo

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21
Q

Four things 3D Methods account for:

A
  1. Primary and Secondary Radiations
  2. 3D Proximity of Inhomogeneity
  3. 3D Contour Irregularities
  4. 3D Inhomogeneity shape
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22
Q

Correction Factors not accounting for tissue density change is measured as _____:

A

% per cm

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23
Q

What are the correction factors for lung in relation to beam energy?

A 
Orthovoltage: +8-10%
Co-60: +4-5%
4 MV: +3-4%
10 MV: +2-3%
18 MV: +1-2%
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24
Q

What are the correction factors for bone in relation to beam energy?

A 
Orthovoltage: - 10-15%
Co-60: - 5-7%
4 MV: - 3-4%
10 MV: - 2-3%
18 MV: - 1-2%
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25
How to solve for tissue correction factor?
Multiply the depth by the correction factor; ex. lung - 10 MV beam at 10cm = 10cm x 2%/cm = 20% or 20% over dose beyond the lung * *bone would result in an under dose
26
Explain how dose points near an Air Cavity is affected:
1. Upstream: Decreases - as dose approaches an air cavity it will be less due to lack of back scatter; 2. In Cavity: Increases - once in cavity dose will be greater due to fluence; 3. Distal Interface: Decreases - as dose exits the cavity, it will be less due to RE-buildup Region; 4. Beyond Cavity: Increases - dose will be greater due to fluence
27
Explain how dose points near a bone is affected:
1. Upstream: Increases - as dose approaches bone, it increases due to back scatter; 2. In Bone: Initially increases, then Decreases (**NOT Important) 3. Beyond Bone: Decreases - lower dose due to attenuation from bone
28
As photon energy _____, surface dose ______
increases; decreases
29
As e' energy ______, surface dose _____
increases; increases
30
How does bolus effect surface dose for photon and electron fields
Increases dose for both
31
Properties of bolus for Photon fields:
1. Increases surface dose 2. takes out irregular contours 3. bolus used for chest wall cases
32
Properties of bolus for e' fields:
1. Increases surface dose 2. takes out irregular contours 3. Maintains lateral scatter
33
Properties of Spoilers for photon fields:
1. Increases surface dose 2. as air gap increases, surface dose decreases 3. Accounts for transmission is large distance away
34
Common uses of beam spoiler for photon fields:
degraders - used for inflammatory breast and TBI
35
Properties of Spoilers for e' fields:
1. Increase surface dose 2. Increase penumbra 3. PDD shifts, dmax goes to surface 4. Changes energy spectrum
36
How is penumbra defined on a dose profile?
by lateral distance; either 80/20 or 90/10
37
For electrons, as gap increases, penumbra _____
increases
38
What creates Geometric Penumbra?
a finite source. ie. Co-60
39
Low energy e' (< 10 MeV) scatter more or less than high energy e' (> 10 MeV)
More
40
Factors affecting Penumbra:
1. as FS increases, Penumbra increases 2. as SCD increases, Penumbra decreases 3. as SSD increases, Penumbra increases 4. as depth increases, Penumbra increases
41
What energy ranges has the sharpest penumbra
4 MV to 6 MV
42
What causes penumbra outside the field
Compton Scattering and Pair Production
43
When should Arcs not be used?
1. Large Volume 2. Not a cylinder 3. Off center
44
What is Past Pointing, when is it used?
When the isocenter is placed beyond the center of the target when utilizing a partial arc; used to avoid hot spots close to the surface
45
When using arcs, what is the relationship between FS and Fall-off
As FS decrease, dose fall-off increases
46
Dose outside the field is caused by what?
leakage, scatter from tx head, Pt scatter
47
What is the predominant source of dose based on distance from field edge?
1. < 10cm - Pt Scatter and Collimator 2. 10 - 20cm - Pt Scatter 3. 20 - 30cm - Pt Scatter and Leakage 4. > 30cm - Leakage
48
What percentage of dose is given based on distance from field edge
1. 0cm - 50% (edge of field light = 50% dose) 2. 2cm - 5.0% 3. 10cm - 1.0% 4. 30cm - 0.2% 5. 100cm - 0.1%
49
By what factor can a wedge increase dose?
2 to 3%
50
As FS increases, dose _____
increases
51
e' density of prothesis
1. Stainless steel - 7 2. Co-Cr-Mo - 7 3. Titanium - 4
52
Should Physical or e' density be used when planning around prothesis?
electron
53
GTV
Gross Tumor Volume: Malignant and Macroscopic - Gross Disease
54
CTV
Clinical Target Volume: GTV + Microscopic
55
PTV (ICRU Report 50)
Planning Target Volume: CTV + Margin (pt motion, tumor motion, beam setup, location of OAR)
56
Treated Volume
Volume Enclosed by Rx Isodose Line
57
Irradiated Volume
Volume Receiving Significant Dose (>50%)
58
Mean Dose:
Average dose in Target/Tumor
59
ITV (ICRU Report 62)
CTV + IM (internal motion)
60
PTV (ICRU Report 62)
ITV + SM (Setup margin)
61
Define Conformity Index (CI) mathmatically
CI = Treated Volume / PTV Volume | **implies that tx'd vol totally encompasses the PTV

Dose Tolerances (23 decks)

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