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Dose Tolerances > Tx planning - techniques > Flashcards

Tx planning - techniques Flashcards

1
Q

Factors that affect hot spots for parallel opposed beam

A
  1. as Pt thickness increases, hot spot increases
  2. as E decreases, hot spot increases
  3. as FS increases, hot spot decreases
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2
Q

What energies are optimal for 15cm separation

A

Co-60 and 4-6 MV

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3
Q

What energies are optimal for 20cm separation

A

10 MV or higher

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4
Q

Where are the hot spots of a POP beam found?

A

at the surface of each beam entry point

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5
Q

What is Integral Dose

A

Measure of total energy absorbed in the treated volume

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6
Q

Define Integral Dose mathematically

A

ID = mass(kg) x dose(rad)

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7
Q

Integral Dose relationship to energy for parallel opposed beams

A

As E increases, Integral dose decreases

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8
Q

List from tx techniques from highest to lowest in terms of how much integral dose is produced:

A

in general - VMAT > IMRT > 3D > Proton

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9
Q

Between a physical and dynamic wedge, which type of gives more dose, why?

A

Physical; more scatter due to being in path of beam longer

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10
Q

Advantages of IMRT: Forward Planning over Inverse Panning:

A
  1. simpler intensity patterns
  2. Easier to deliver
  3. Easier to verify
  4. Easier to calculate MU’s (able to use standard 3D TPS)
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11
Q

Advantages of IMRT: Inverse Planning over Forward Planning:

A
  1. Intensity Patterns (better distribution - not necessarily more uniform)
  2. Less dose to critical structures
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12
Q

For IMRT planning, what is the typical number of fields used?

A

5, 7, or 9

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13
Q

What energies should be used for IMRT, why?

A

6 MV or 10 MV - tighter penumbra, better coverage, no neutron shielding, no neutron pt dose

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14
Q

What energies should not be used for IMRT why?

A

energies over 10 MV - neutron production

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15
Q

IMRT causes more or less dose heterogeneity?

A

More

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16
Q

Advantages of Compensator based IMRT:

A
  1. better resolution than MLC
  2. Less MU required
  3. Less pt dose (outside field)
  4. Larger FS
  5. Tx planning may be easier
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17
Q

Disadvantages of Compensator based IMRT:

A
  1. Slower delivery (RTTs enter room for each field)
  2. Must be fabricated
  3. Accounting for transmission
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18
Q

What factors influence dose to contralateral breast?

A
  1. distance away from beam edge
  2. Use of HPP (increased dose if Cerro)
  3. Use of wedge (Medial get 3x more than lat)
  4. Alignment of Tangents
  5. Port film
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19
Q

What factors do not influence dose to contralateral breast?

A
  1. Gantry angle
  2. lung density
  3. breast volume
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20
Q

Ways to limit dose to contralateral breast

A
  1. Don’t use Cerro HBB,
  2. align tangents,
  3. decrease wedge angle (or only use lateral wedge;
  4. Use 2.5cm thick Pb shield
  5. Limit collimator for port films
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21
Q

Contralateral breast dose limit to women < 45 years old

A

200 cGy

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22
Q

When is TBI utilizied

A

as a radioimmuno-suppressant prior to bone marrow transplant

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23
Q

Common TBI Rx

A
  1. 5-8 fx
  2. 1000 - 1200cGy
  3. Rx to umbilicus
  4. Lung blocks used post 600-800cGy
  5. Dose Rate of 5 - 10 cGy/min
  6. SSD = 350cm and 450cm
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24
Q

Common TBI techniques:

A
  1. AP/PA
  2. Laterals
  3. Sliding tables
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25
Q

Physics goals of TBI:

A
  1. TPR: +-4%
  2. ISL: +-6%
  3. OAR: +-5%
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26
Q

For TBI inhomogeneity worsens with:

A
  1. Lateral technique
  2. Decreased E
  3. Increased Pt thickness
  4. Decreased distance (SSD)
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27
Q

For TBI, skin dose increases with:

A
  1. FS
  2. Distance (increased SSD)
  3. Spoilers (**want this for pediatric pts)
28
Q

When is IORT(e’) typically used?

A

originally for pancreas, now breasts

29
Q

Typical Rx for IORT(e’)

A
  1. Single fx
  2. 1000 - 2000 rad
  3. 6 MeV - 20 MeV
30
Q

For IORT(e’), use high or low Z material to collimate beam?

A

low, to not produce bremsstrahlung

31
Q

A decrease in width will improve/worsen flatness of beam for IORT(e’)

A

worsen

32
Q

When is Total Skin Irradiation used (TSCT)?

A

for Mycosis Fungoides, and t-cell lymphoma

33
Q

Common Rx for TSCT?

A
  1. 3-7 Mev
  2. SSD = 300 to 400cm
  3. 6-field tx
  4. dose prescribed to 1cm
  5. Do not exceed bone marrow tolerance
34
Q

For SRS, a large volume gets a smaller/larger dose?

A

smaller

35
Q

At what isodose line is Gamma Knife Rx’d?

A

50%…want a 200% hot spot

36
Q

At what isodose line is SRS on LINACs Rx’d

A

90%

37
Q

For SRS is penumbra better with Gamma Knife or LINAC

A

Gamma Knife

38
Q

For SRS, additional field/arcs will effect penumbra how?

A

worsens

39
Q

For SRS, what will an overlap of beams cause?

A

inhomogeneities (hot spots)

40
Q

How are dose profiles measured for SRS

A

Radiochromatic film

41
Q

How is output measured for SRS

A

small ion chamber or film

42
Q

For SRS, what is a shortcoming in regards to output measurements with chambers?

A

volume averaging

43
Q

What kind and how many sources are present on a gamma knife unit

A

201 Co-60 Sources - 6000Ci

44
Q

What is the size of the isocenter for a gamma knife unit

A

.3mm

45
Q

What is the gold standard of radiosurgery

A

Gamma Knife

46
Q

Gamma Knife is governed by what body

A

NRC

47
Q

Daily tests of Gamma Knife include:

A
  1. Interlocks
  2. Intercom
  3. Radiation Monitors - Geiger Muller
48
Q

Monthly Gamma Knife tests:

A
  1. Timer reproducibility
  2. Timer linearity
  3. Timer Accuracy
  4. Timer Error
  5. Mechanical Alignment
  6. Output (for 18mm collimator)
49
Q

Semi-Annual Gamma Knife tests:

A

Leak tests

50
Q

Annual Gamma Knife tests:

A
  1. Output for all Helmets
  2. Mechanical Iso - Rad iso
  3. Dose profiles
51
Q

Most accurate SRS delivery by LINAC is ____

A

cones, especially for lesions < 1mm

52
Q

Range of SRS cone diameters:

A

2.5mm to 50mm

53
Q

Typical number of arcs for LINAC SRS:

A

4-6

54
Q

Energies used for LINAC SRS:

A

< 10MV

55
Q

Typical Rx isodose line for CyberKnife:

A

70-80%

56
Q

SADs used for brain and body with CyberKnife:

A

80 SAD for brain; 90-100 SAD for body

57
Q

What is the name used to describe the fixed ranged of heavy charged particles?

A

Bragg Peak

58
Q

E range of Proton machines

A

30-250 MeV

59
Q

How is the Bragg Peak spread out (SOBP)

A

using multiple energies, or by a modulator

60
Q

Whats the purpose of an aperture in Proton therapy

A

to shape the beam

61
Q

Features of the Proton Head design:

A

Double Scattering Head: uniform scan or pencil beam scan

62
Q

How are protons accelerated

A

Cyclotrons or Acclerators

63
Q

What is the RBE of a proton, where is it largest?

A

1.1; can vary with E (depth) - larger at distal end of Bragg Peak

64
Q

Largest inhomogeneity corrections are for what kind of energy

A

Proton Energy

65
Q

How is Neutron Contamination associated with Proton Energy

A
  1. Increases with use of Modulator
  2. Increases with use of collimator
  3. Increases with higher E
  4. Increases if Mod/Coll is closer to pt
66
Q

Skin dose of Proton Beam is _____ than 15X or 18X

A

slightly higher

67
Q

For proton beam lateral dose penumbra is ______ than 15X or 18X

A

Larger

Dose Tolerances (23 decks)

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