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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; e’ 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
Q

How to solve for tissue correction factor?

A

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
Q

Explain how dose points near an Air Cavity is affected:

A
  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
Q

Explain how dose points near a bone is affected:

A
  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
Q

As photon energy _____, surface dose ______

A

increases; decreases

29
Q

As e’ energy ______, surface dose _____

A

increases; increases

30
Q

How does bolus effect surface dose for photon and electron fields

A

Increases dose for both

31
Q

Properties of bolus for Photon fields:

A
  1. Increases surface dose
  2. takes out irregular contours
  3. bolus used for chest wall cases
32
Q

Properties of bolus for e’ fields:

A
  1. Increases surface dose
  2. takes out irregular contours
  3. Maintains lateral scatter
33
Q

Properties of Spoilers for photon fields:

A
  1. Increases surface dose
  2. as air gap increases, surface dose decreases
  3. Accounts for transmission is large distance away
34
Q

Common uses of beam spoiler for photon fields:

A

degraders - used for inflammatory breast and TBI

35
Q

Properties of Spoilers for e’ fields:

A
  1. Increase surface dose
  2. Increase penumbra
  3. PDD shifts, dmax goes to surface
  4. Changes energy spectrum
36
Q

How is penumbra defined on a dose profile?

A

by lateral distance; either 80/20 or 90/10

37
Q

For electrons, as gap increases, penumbra _____

A

increases

38
Q

What creates Geometric Penumbra?

A

a finite source. ie. Co-60

39
Q

Low energy e’ (< 10 MeV) scatter more or less than high energy e’ (> 10 MeV)

A

More

40
Q

Factors affecting Penumbra:

A
  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
Q

What energy ranges has the sharpest penumbra

A

4 MV to 6 MV

42
Q

What causes penumbra outside the field

A

Compton Scattering and Pair Production

43
Q

When should Arcs not be used?

A
  1. Large Volume
  2. Not a cylinder
  3. Off center
44
Q

What is Past Pointing, when is it used?

A

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
Q

When using arcs, what is the relationship between FS and Fall-off

A

As FS decrease, dose fall-off increases

46
Q

Dose outside the field is caused by what?

A

leakage, scatter from tx head, Pt scatter

47
Q

What is the predominant source of dose based on distance from field edge?

A
  1. < 10cm - Pt Scatter and Collimator
  2. 10 - 20cm - Pt Scatter
  3. 20 - 30cm - Pt Scatter and Leakage
  4. > 30cm - Leakage
48
Q

What percentage of dose is given based on distance from field edge

A
  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
Q

By what factor can a wedge increase dose?

A

2 to 3%

50
Q

As FS increases, dose _____

A

increases

51
Q

e’ density of prothesis

A
  1. Stainless steel - 7
  2. Co-Cr-Mo - 7
  3. Titanium - 4
52
Q

Should Physical or e’ density be used when planning around prothesis?

A

electron

53
Q

GTV

A

Gross Tumor Volume: Malignant and Macroscopic - Gross Disease

54
Q

CTV

A

Clinical Target Volume: GTV + Microscopic

55
Q

PTV (ICRU Report 50)

A

Planning Target Volume: CTV + Margin (pt motion, tumor motion, beam setup, location of OAR)

56
Q

Treated Volume

A

Volume Enclosed by Rx Isodose Line

57
Q

Irradiated Volume

A

Volume Receiving Significant Dose (>50%)

58
Q

Mean Dose:

A

Average dose in Target/Tumor

59
Q

ITV (ICRU Report 62)

A

CTV + IM (internal motion)

60
Q

PTV (ICRU Report 62)

A

ITV + SM (Setup margin)

61
Q

Define Conformity Index (CI) mathmatically

A

CI = Treated Volume / PTV Volume

**implies that tx’d vol totally encompasses the PTV

Dose Tolerances (23 decks)

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