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Rad Physics > Topic 9 - Advanced Treatment Planning > Flashcards

Topic 9 - Advanced Treatment Planning Flashcards

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1
Q

1 kB = ? bytes
1 MB =
1 GB =

A

2^10 or 1024 bytes
2^20
2^30

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2
Q

x-bit gray scale provides ___ unique shades

A

2^x

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3
Q

1 HU = ?% difference with respect to water

A

0.1%

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4
Q

HU of
Air
Water
Dense Bone

A

Air: -1000
Water: 0
Bone: 1300-1600

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5
Q

Higher T (field strength) MRIs do what to the acquisition time and signal noise ratio?

A

shorter acquisition

higher signal to noise ratio

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6
Q

Ultrasound equation

A

Vλ=cs where cs is the speed of sound in soft tissue 1540 ms-1
Typical frequency is 1-20 MHz; wavelength should be smaller than object in order to be reflected

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7
Q

18-FDG - half life

A

110 minutes; positron decay

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8
Q

Single kV portal film dose

A

.1-.5 mGy

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9
Q

CBCT dose

A

1-5 cGy

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10
Q

Block Edge =

A

PTV + Penumbra

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11
Q

irradiated volume

A

=Volume enclosed by the 50% isodose surface

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12
Q

Treated volume

A

=Volume enclosed by the 95% isodose surface (generally 2 cm margin between PTV and field edge

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13
Q

MUs for 3D vs IMRT

A

You need generally 2x the MUs for IMRT (and 10x for tomotherapy!)

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14
Q

MUs for VMAT vs IMRT

A

fewer MUs in VMAT

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15
Q

Low energy beam advantages

A

lower whole body dose, less buildup for superficial tumors, lower exit dose

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16
Q

in IMRT QA, with indivirual beam measurment what are the thresholds to pass?

A

if within 3% of dose or within 3 mm from point of agreement

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17
Q

disadvantages of IMRT

A 
2x treatment time 
greater dependence on setup uncertainty
more inhomogeneity
more whole body dose (2-3x)
greater room shielding requirements
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18
Q

positional accuracy of SRS

A

+/- 1 mm

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19
Q

Cyberknife technology (beam energy and creation, patient movement)

A

6 MV linac on robotic arm, frameless
uses X-band microwaves (10,000 MHz) compared to linacs (3000 MHz)
patient position monitored continuously with orthogonal X-ray beams and can track moving target volumes

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20
Q

Gamma-knife (Source, arrangement, pt movement, prescribtion)

A

200 Co-60 sources with a total activity of 60,000 Ci (220 Bq)
arranged in hemispheric chell of r= 40 cm
final collimation with bolted helmet
multiple isocenters
prescribe to the 50% dose line

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21
Q

Linac-bases SRS/SBRT (isocenters, MLCs, prescription)

A

usually a small number of isocenters (sometimes 1)
Mini or micro MLCs (3-4 mm leaf width)
prescribe to the 80-90% dose line

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22
Q

conformality index measures what? Acceptable range?

A

prescription isodose to target volume (PITV) is a measure of conformality
ranges 1-2 are acceptable 2-2.5 or 0.9-1 are minor deviations

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23
Q

How are beam profiles measured for srs/sbrt?

A

With film due to high spatial resolution

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24
Q

How is PDD, TMR, Sc,p measured for SRS/SBRT?

A

Small ion chambers <3mm

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25
Q

Winston Lutz Test

A

determines target accuracy
radiate small metallic ball and compare the difference between the center of the sphere shadow and the center of the field

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26
Q

proton SAD difference

A

Very high SADs (200 cm)

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27
Q

Proton dose depth vs photons

A

is increased compared to photons due to inverse square fall off of dose with distance

28
Q

Therapeutic energy rance of protons

A

30*250 MeV

29
Q

Protons are ____ susceptible to tissue heterogeneity than photons

A

More

30
Q

How often are the outputs for the SOBP and distal dose fall off measured for proton therapy?

A

Daily

31
Q

TSEB therapy beam energy

A

2-9 MeV

32
Q

Hyperthermia target temperature

A

41-45 degrees C over 30-60 minutes

33
Q

Thermal enhancement ratio

A

RT dose without heat/RT dose for equivalent effect with heat

34
Q

In IMRT optimization, “cost” quantifies ____.

A

The deviation from the desired dose distribution.

35
Q

Pencil beam convolution can be used to ___.

A

perform dose calculations for IMRT.

36
Q

beam spoiler in TBI is used to ___.

A

Increase surface dose in the skin’s blood vessels
This is also called a bolus! It is a 1-2 cm acrylic screen placed as close to the patient as possible.
Most protocols don’t require skin sparing

37
Q

The range of uncertainty in proton beams is approximately ___ the range of the beam.

A

2.5-4%

38
Q

Proton range straggling is caused by uncertainties in ___.

A

Energy Loss.
The sigmoidal shape at the distal end of the bragg peak is due to range straggling which is caused by small statistical fluctuation in the amount of E lost by individual protons.

39
Q

Range uncertainty in protons

A

Due to uncertainties in tissue composition and stopping power.

40
Q

Higher energy photon beams have _____ penumbra.

A

Increased

41
Q

prescribing to the 50-80% isodose lines in SRS is done in order to minimize ____.

A

Normal tissue exposure. You are prescribing into the penumbra. This introduces hotspots but will minimize normal tissue exposure.

42
Q

Benefits of MLC for SRS

A

no shape limitations for single isocenter

more flexibility in technique (static, dynamic conformal arc, IMRT, VMAT)

43
Q

Benefit of cones over MLC for SRS

A

Sharper penumbra as it is shaped closer to the patients head

44
Q

What is the frequency and tolerance to verify mechanical vs. imaging system iso on modern linacs?

A

daily; = 2.0 mm

45
Q

Can a linac accelerate protons?

A

Yes, but it takes greater lengths of the cavity.

46
Q

What is the function of a degrader in TSET?

A

To decrease the depth of e- beam penetration.

47
Q

What is a greater concern for PBS vs passive scatter?

A

Target motion

48
Q

Gating should be considered if target motion in any one direction is ____.

A

> 5mm

49
Q

____ cannot be ionized for use in particle therapy.

A

Neutrons (because they are neutral).

50
Q

QA tolerance of couch translation and rotation for SRS/SBRT vs non-IMRT?

A

1 mm; 0.5 degrees - SRS/SBRT

2 mm, 1 mm for non-IMRT

51
Q

QA frequency of door safety

A

daily

52
Q

QA frequency of audio/visual monitors

A

daily

53
Q

QA frequency of X-ray output consistancy of within 3%

A

daily

54
Q

QA frequency of distance indicator at iso within 2mm

A

daily

55
Q

QA frequency of “picket fence’ test for MLC positioning

A

Weekly

56
Q

What are medical events reportable to the NRC?

A 
Wrong body site
Wrong modality (15x vs 6x)
Any one fraction>50%, or total dose exceeding 20%
57
Q

What are good moderators of neutron energy?

A

Polyethylene, water, concrete – materials with a lot of hydrogen. They slow down neutrons because the protons in the material have a similar mass as the incident neutrons allowing efficient energy transfer.

58
Q

QA frequency of kV to MV isocenters?

A

daily

59
Q

kV imaging deposits most of the dose where?

A

at the surface

60
Q

What is the MIP in a 4D CT?

A

CT dataset where each individual pixel is assigned to its maximum value from all breathing phases

61
Q

Why is the collimator sometimes rotated for VMAT?

A

Dose banding from interleaf leakage can be reduced by rotating the gantry.

62
Q

Advantages of VMAT over IMRT

A

shorter treatment time (and thus decreased intrafractional movement)
(dose rate is the same or less than IMRT)

63
Q

physical compensators do what in TBI?

A

increase homogeneity. They attenuate the beam decreasing the dose rate to compensate for the varying separations of the body.

64
Q

TBI dose agreement and dose uniformity?

A

5% and 10%

65
Q

DIBH reduces the dose to the

A

heart and lungs

66
Q

The LET for protons is highest where?

A

At the distal end of the SOBP

67
Q

Passively scattered protons create out of field dose with _____.

A

Neutrons. Passively scattered proton beam interacts with the aperture and the nozzle creating neutrons and gammas.

Rad Physics (4 decks)

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