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Ford Flashcards

1
Q

mass rest of proton

A

938 MeV

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2
Q

frequency of gamma rays, xrays etc

A

gamma - 10^19 Hz
x-rays - 10^17 Hz
UV - 10^16 Hz
visible - 10^15 Hz
infrared - 10^ 14 Hz
thermal IR - 10^13 Hz
microwaves- 10^11 Hz
radio - 10^8 Hz

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3
Q

energy of wave

A

E = hv
lambda= h/p
E= pc

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4
Q

Plank constant

A

6.63 * 10^-34 m2kg/s

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5
Q

size of atom vs size of nucleus

A

atom is tens of nm
nucleus is 10^-15 m

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6
Q

Coulombs Law

A

F = kq1q2/r^2

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7
Q

ground state of hydrogen atom

A

13.6 eV

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8
Q

eV

A

1.6 *10^-19 J
charge of electron is 1.6 *10^-19 C

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9
Q

Coulomb constant

A

8.99*10^9 Nm2/C2

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10
Q

94.9 FM radio is what frequency?

A

94.9 MHz

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11
Q

mass of electron

A

9.11*10^-31 kg

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12
Q

magnetic force

A

F= qvB

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13
Q

centripetal force

A

F= mv^2/r

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14
Q

mass of proton

A

1.7*10^-27 kg

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15
Q

angular frequency

A

omega = v/r
f= omega/2pi

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16
Q

nuclei with Z> what are unstable

A

83

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17
Q

what elements are more neutron rich

A

higher Z

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18
Q

mass/energy of stable nucleus vs mass/energy of its parts

A

stable nucleus has lower mass/energy

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19
Q

where does beta minus decay occur

A

neutron rich nuclei
beta plus occurs for neutron poor nuclei

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20
Q

why is there a spectrum of energies for beta decay?

A

energy is shared with neutrino or anti-neutrino

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21
Q

half life of beta minus vs beta plus decay

A

beta-minus: long
beta-plus: short

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22
Q

how are neutron rich vs neutron poor isotopes made?

A

neutron rich: reactors (bombardment)
neutron poor: cyclotron

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23
Q

how to tell difference between beta plus and beta minus spectrum?

A

beta plus- no particles are created at very low energies and max energy is higher than that of beta-minus due to extra colomb force between nucleus and positron

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24
Q

mean life of radioactive source

A

1.44 * half life

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25
Q

activity of daughter is half life of parent is much longer than daugther

A

-activity of daughter about equal to activity of parent

Activity of daughter = parent activity (half life of parent)/(half life of parent - half life of daugther)

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26
Q

equation for mg Ra equivalent

A

(tau source/ tau Ra)* source activity

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27
Q

is beta in TG43 equation in rad or degrees?

A

radians

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28
Q

brachy inverse square law fall off- what source falls off fastest? Pt source, 3 mm, or 5 mm line

A

pt source falls off fastest, followed by 3 mm line and 5 mm line

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29
Q

range of electrons

A

half of its energy in MeV (cm)

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30
Q

binding energies of iodine:

K 33.168 keV
L1 5.188
L2 4.852
L3 4.557
M1 1.072

highest energy characteristic photon and auger electron for 30 kV incident photon

A

30 kV photon can ionize anything but k-shell
highest energy photon is L1-M1 = 4.12 keV

If this energy instead goes to auger, 4.12 - 1.072 = 3,04 keV is highest energy auger electron

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31
Q

offset between dose and kerma wrt energy

A

higher energy beams = higher energy electrons= longer range = bigger offset

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32
Q

why do higher energy beams have deeper dmax?

A

they produce higher energy electrons which travel further in the medium. It thus takes longer for equilibrium to be reached

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33
Q

how does linear attenuation coefficient depend on density?

A

increases linearly with density
-mass attenuation coefficient doesn’t depend on density

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34
Q

. How many HVLs of tungsten are required to attenuate a high-energy photon
beam down to 3% of its initial intensity?

A

(0.5)^n = 0.03

take ln of both sides and solve for n

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35
Q

is dmax deeper in lung than in water?

A

yes, because electron ranges are longer in lung

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36
Q

what does radiative stopping power depend on?

A

(Z/m)^2 and acceleration ^2 (ie Srad increases with energy)

this is in units of MeVcm^2/g
Z is of medium and m is of particle

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37
Q

what does collisional stopping power depend on?

A

decreases in higher Z material as there are less electrons to interact with
not a strong dependence on energy

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38
Q

what does stopping power of protons depend on

A

mostly collisional loss, radiative loss very small
-depends on Z of medium, z^2 of particle
-inversely proportional to velocity^2
-does not depend on particle mass

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39
Q

where does delta ray come from?

A

electron undergoes “hard” collision in which substantial amt of energy is transferred to electron
-delta ray can travel relatively long distance

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40
Q
  • For an ionization chamber in a water tank, which lling gas will provide the
    highest charge reading for a 6 MeV electron beam?
    a. Air
    b. Carbon dioxide
    c. Methane
    d. Approximately the same
A

The charge produced is from collisional losses. The collisional stopping powers for a 6 MeV electron in the above materials are, respectively, 1.870, 1.874,
2.474 MeV cm2/g, so methane should be the right answer. However, most of the
electron collision interactions are not in the gas itself but rather in the water
which then streams into the ion chamber. The readings, therefore, are approximately the same.

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41
Q

A 1 MeV electron beam interacts with a 1 × 1 × 1 cm block of material. Which type
of material will produce the highest exposure at 2 m from the block?
a. Muscle
b. Water
c. Aluminum
d. Tungsten

A

tungsten
The exposure at 2 m will be from bremsstrahlung photons. In higher Z materials the production of bremsstrahlung is much larger (much larger radiative
stopping power). If the block were very large, self-shielding may reduce the
ux of photons from a high-Z material.

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42
Q

what causes heating in the anode?

A

collisional losses of the electrons

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43
Q

key reason MV beams were developed

A

skin sparing

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44
Q

see chapter 8 diagrams in how linacs accelerate electrons

A

-oscillating voltage is applied to accelerating structures
-oscillation frequency is such that electron reaches the gap between cavities just as the voltage gradient is at its maximum
-frequencies are S or X band for linacs
-cavities are designed so that one wavelength of RF spans one cavity length- this means X band can be made more compact (however harder to make)

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45
Q

magnetron vs klystron

A

-magnetron is smaller, generates microwaves
-klystron is amplifier- amplifies W to MW- larger, bulky, and is stationary

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46
Q

how do magnetrons make microwaves?

A

-Cathode is heated- electrons boil off of it and accelerate towards anode
-magnetic field makes the electrons circle around the cathode instead of hit the anode directly
-electrons accelerating around the cathode produces microwaves which are picked up by the output antenna

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47
Q

why are linacs maintained at low vaccuum pressures/

A

to prevent arcing
-if there is no gas in system, there are no atoms to ionize and carry the current of a spark

-however, microwave generation system is exception as it has SF6 at high pressure

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48
Q

what is electron gun?

A

cathode through which electrons are emitted

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49
Q

what does modulator do?

A

introduces pulses into klystron/magnetron and also into gun
-electron system and RF system are switched on and off in synchrony with each other

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50
Q

what determines dose rate of linac?

A

-pulse repetition rate and current in peak of pulse

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51
Q

why is achromatic bending magnet used instead of 90 degree bending magnet (image pg 84)

A

-all electrons strike target at same position for achromatic
-otherwise, electrons strike target at different positions depending on their energy

achromatic = magnetic field increases going out radially from center. Therefore, as higher energy electrons travel through a larger orbit, they experience a larger magnetic force and are bent back to same location

-slalom magnet achieves same as achromatic but uses 3 magnet sectors- can make gantry head more compact

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52
Q

how is linac photon energy determined?

A

-not by flux of electrons but rather by energy of electron as it emerges from waveguide
-this is controlled by switching sections of the waveguide on or off

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53
Q

energy lost to heat in xray tube vs linac

A

The ratio of collisional stopping power in tungsten at 40 keV vs. 6 MeV is 3.17.
The implication of this is that relatively more energy is dissipated in collisional
losses in an X-ray tube vs. a linac. This energy goes into heating the anode.
More energy is converted to bremsstrahlung photons in a linac target (e.g. 6
MeV electrons) compared to an X-ray tube (e.g. 40 keV electron).

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54
Q

How does the copper housing of the linac target affect the spectrum of emerging
photons?
a. Hardens the beam
b. Softens the beam
c. Reduces overall output
d. Increases overall output

A

c. reduces overall output
The emerging photons are high energy and mostly undergo the Compton process in the copper surrounding the target. In this energy range, the Compton
cross-section is roughly independent of energy so the beam does not harden or
soften much, i.e. it does not preferentially lter out higher or lower energy photons. However, it does reduce the overall uence because it attenuates the beam.
Note that the very softest part of the spectrum (keV) energies will be ltered

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55
Q

What is the effect of not synchronizing the pulses of electrons from the gun with
the RF waves in the waveguide?
a. Lower energy electron emerging from the waveguide
b. Higher energy electron emerging from the waveguide
c. Lower average beam current
d. Higher average beam curren

A

a- lower energy electron emerges

If the electron bunches are not synchronized with the RF, they cross the gaps
when the potential between cavities is low. Therefore, they are not accelerated
efciently in the waveguide. The output decreases because high-energy electrons ultimately do not reach the the target

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56
Q

What would be the effect of increasing the overall magnetic eld in a 270°-bending magnet?

b. Target spot location moves away from the gun direction
c. Beam energy increases
d. Spot size decreases

A

A larger magnetic eld would bend the electrons more (i.e. into a tighter circle),
which theoretically should move the spot away from the gun.

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57
Q

equation for size of penumbra

A

W = source size (SSD-SCD)/scd
SCD is source to collimator distance

if penumbra is at some depth in the patient d, then SSD becomes SSD + d

-if SCD = SSD (ie collimator placed at skin) then penumbra at skin would be 0

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58
Q

tongue and groove effect

A

tongue and groove make dose fall-off less sharp than if these features were not present

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59
Q

why does tomo have increased output?

A

-no FFF
-85 cm SAD vs 100 cm

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60
Q

What are the key choices of linac operation that allow for the compact systems
such as TomoTherapy, CyberKnife, or Mobetron? Check all that apply.
a. High repetition rate
b. High frequency
c. Lower required MU
d. Low energy

A

high frequency and low energy

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61
Q

3 sources of penumbra

A

-source size
-shape of beam collimation device and transmission through edge of device
-scatter at edges of field- higher energy electrons can scatter further and thus penumbra are smeared out

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62
Q

size of horns for 10 MV vs 6 MV

A

horns are larger for 10 MV beam

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63
Q

depth of basal skin

A

0.07 mm

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64
Q

skin dose for FS 40X40 vs 10x10

A

2 x dose for 40x40

dose at skin is similar for different energies, slightly lower for high E
skin dose is increased if beam goes through couch or other device

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65
Q

why are there holes in thermoplastic mask?

A

decrease skin dose increase from plastic

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66
Q

maynord F factor

A

PDD2 = PDD1 * ((SSD2+dmax)/(SSD2+d))/((SSD1+dmax)/(SSD1+d))

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67
Q

equivalent FS for blocked field

A

FS unblocked * square root (1-f)
where f is fraction of field that is blocked

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68
Q

During an emergency treatment on the weekend, a PDD table is accidentally used
instead of a TMR table to calculate dose at a depth of 10 cm for an isocentric setup.
What is the approximate impact on the dose delivered?
a. Dose to isocenter 20% low
b. Dose to isocenter 10% low
c. Dose to isocenter 10% high
d. Dose to isocenter 20% high

A

PDD is smaller than TMR so the calculated MU will be larger than it would
have been if the TMR value were used. The delivered dose therefore will be
larger. The PDD differs by including an inverse square factor which for depth
of 10 is approximately 20%, i.e. (100/110)2

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69
Q

Sc for a blocked field

A

Sc will be for the “unblocked” field- i.e. the size of the collimator at iso

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70
Q

superposition convolution

A

convolve TERMA with kernal
-TERMA is total energy released in matter at each voxel. Considers attenuation, radiological pathlength
-kernel is calculated using MC and describes the contribution of terma in one voxel to neighbouring voxels
-kernels are also scaled based on density (ie in regions of lower density, kernel is larger because range of electrons is larger)

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71
Q

penumbra in inhomogeneities and for different energies

A

penumbra are wider in lung due to longer range of electrons and this effect is worse in high energy beam because electron range is longer

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72
Q

describe effect of bone on dose

A

-beam attenuation increases in bone due to higher density
-distal to bone there is increase in dose due to backscattered electrons
-within bone itself dose is not much different for 6 MV; at higher MV, pair production becomes significant and dose in bone increases

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73
Q

when are inhomogeneity effects worse?

A

small field
high energy

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74
Q

trick for remembering wedge direction

A

heels together, toes apart, like in tango

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75
Q

wedge angle

A

90- (hinge angle/2)

hinge angle is angle between 2 beams

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76
Q

For a treatment of breast cancer with two tangent eld (i.e. opposed oblique
beams), rank the following treatments in order of the smallest to largest hot spot
in the breast.
a. 6 MV, separation 15 cm
b. 6 MV, separation 25 cm
c. 15 MV, separation 15 cm
d. 15 MV, separation 25 cm

A

Order (smallest to largest hot spots): c, a, d, b. The hot spots in tissue are largest
for larger separation and lower energies. This is due to the PDD dropping off
more for larger separations and for lower energies.

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77
Q

where should ICRU reference dose be?

A

where dose can be accurately calculated
-not in build-up, steep gradient, or tissue interface

For IMRT, use dose-volume prescription and not dose to a pt since dose is not homogeneous

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78
Q

survival fraction for single fraction vs fractionated treatment

A

single fraction: SF = e^(-alpha D - beta D^2)

multiple fractions: SF = e^(-alpha D - beta* n*d^2)

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79
Q

advantages of compensators vs MLCs

A

-uses dose efficiently whereas with leaves, much of the beam is “wasted”

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80
Q

Rank the following treatment techniques in order from least to most MU required
to deliver the same dose to a target.
a. 3D-CRT
b. Step-and-shoot IMRT with MLC
c. Dynamic MLC IMRT
d. Compensator IMRT

A

Order is: a. 3D-CRT, d. Compensator IMRT, b. Step-and-shoot IMRT with
MLC, c. Dynamic MLC IMRT

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81
Q

compare 3DCRT DVH for OAR to VMAT DVH for OAR

A

The 3D-CRT plan delivers more doses to the OAR at the intermediate dose
levels. However, the situation is reversed at low-dose level, the VMAT plan
“spreading out the dose” more.

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82
Q

What advantage is provided by rotating the collimator away from zero degrees in
VMAT arcs?

A

With a rotation the leaves do not line up with each other. This prevents interleaf
leakages from overlapping exactly in the same place as the arc rotates around.

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83
Q

why is the elctron PDD not sharp, but instead spread out in depth?

A

higher energy electrons travel further and lower energy electrons travel less
-electrons scatter with air along the intervening path - some have lost energy

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84
Q

rules of thumb for electron PDD

A

R90 = E/3.3
R80 = E/3
R50 = E/2.33
Rp = E/2

higher energy beam has:
-deeper penetration
-higher surface dose
-more spread out fall-off
-broader region of dose max

surface dose is about 75+ E %

-Brems. contamination is 1-5 % of peak dose and is higher for high E electrons

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85
Q

what happens to electron PDD when FS < Rp?

A

-dmax closer to surace
-surface dose increases

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86
Q

electron beam penumbra

A

-penumbra for higher E is smaller than for lower E because higher E electrons are more forward directed

-as depth increases, penumbra become wide, especially for high energy beams. Higher isodose lines constrict, lower isodose lines bulge out

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87
Q

gap between applicator and patient for electron treatment

A

5 cm is typical
also leave 1 cm wider on each side than treatment area to account for penumbra

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88
Q

what effects happen when treating with electrons at extended SSDs?

A

-larger penumbra - more constriction and bulging
-output decreases- use virtual source for IS corrections
-light field no longer accurately tracks radiation field

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89
Q

what happens to electron beams with oblique incidence?

A

-dmax gets pulled closer to surface
-doses are higher on side where angle is sharper
ie dose lines pull closer to surface

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90
Q

what happens with inhomogeneities in electron beams?

A

-tissue prominence in beam center- dose under prominence is shifted up, regions on either side of prominence have more dose due to outscatter of electrons from prominence into these regions
–tissue defect- isodose lines under defect are shifted away from source. Inscatter of electrons from sides into region under the defect, creating hot spots on either side just inside edge of defect

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91
Q

Which energy and cutout combination might require a special measurement to
determine the PDD and MU to be used?
a. 6 MeV, 5 × 5 eld
b. 8 MeV, 10 × 10 eld
c. 12 MeV, 5 × 5 eld
d. 18 MeV, 10 × 10 eld

A

Lateral disequilibrium effects become important when the eld size becomes
smaller than the practical range of electrons (E/2). For a 12 MeV eld this is
approximately a 6 × 6 eld

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92
Q

When using an internal lead shield under the lip what would be the effect of
reversing the position of the low-Z absorber (i.e. absorber on the distal side of the
lead shield instead of the proximal side)?
a. Decreased dose to the gum mucosa distal to the shield
b. Increased dose to the gum mucosa distal to the shield
c. Decreased dose to the lip
d. Increased dose to the lip

A

The low-Z absorber is intended to prevent electrons from backscattering into
the tissue proximal to the shield. If the orientation is reversed the lip would
receive an extra dose. This dose increase could be as high as 70%.

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93
Q

where is brems. production highest in electron profile

A

along CAX b/c brems is more forward directed

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94
Q

volume of farmer vs scanning cs mini vs micro chamber

A

-0.6 cc
-0.125 cc
-0.06 cc
-0.006 cc

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95
Q

How much does the kQ factor in Problem 2 deviate if the incorrect PDD(10 cm) of
65% were used, i.e. the PDD for a 6 MV beam? Would the calibrated output be too
low or too high?
a. 2.2% low
b. 3.1% low
c. 2.1% high
d. 3.1% hig

A

From Fig 4 (or Table I) in TG-51 the factor at 65% is 0.993 for the chamber
model # NE2571. This value is higher than the one in Problem 2 by a factor
0.993/0.972 = 1.022.
Calibrated output will be 2.1% too low. Because of the high kQ the corrected
reading will be too high. In order to achieve 1 cGy/MU, the machine would
therefore be calibrated down to a lower reference dose

Kq decreases as % dd10x increases

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96
Q

How much will the dose calibration (cGy/MU) deviate if a 20 × 20 eld is accidentally used to measure output per TG-51 vs. a 10 × 10 eld for a 6 MV beam? (Refer
to the tables in Section 11.3.)
a. 0.96
b. 0.98
c. 1.025
d. 1.043

A

The ratio of doses for eld sizes 20 × 20 vs. 10 × 10 is TMR(20X20)/TMR(10X10) = 1.043.
Therefore, the measured dose will
be too large by a factor of 1.043. In order to achieve 1 cGy/MU, the machine
would therefore be calibrated down to a lower reference dose by a factor of
1/1.043 = 0.96.

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97
Q

pros and cons of diodes

A

pros: instant readout, sensitive, small, no bias voltage
‘cons: energy dependense, dose rate dependence, temperature dependence, radiation damage, directional dependence

98
Q

shielded diodes

A

for photon diodes
-provides shielding from low energy scattered photons, which diode overresponds to

99
Q

are diodes used as primary standard?

A

No
-neither are film or OSLDs
-only chambers are primary standard

100
Q

OSLD material

A

Al2O3:C
-wavelenght of photons released is 400 nm

-when radiation is absorbed, electrons are excited from valence to conduction band and become trapped. Upon exposre to light (or heat for TLD), the trapped electrons transition down and emit photons

-TLD material is LiF

101
Q

pros and cons of luminescent dosimeters

A

pros: small, don’t need wires, sensitivie, easy to use, minimal dependence on temperature, dose rate, or beam direction
cons: non-linear dose dependence (supralinear, must be careful while calibrating), fading, difficult to reuse, over-respond to low energy photons, readout is not instantaneous

102
Q

optical density of film

A

OD = log10 (Io/I)

103
Q

fog of film

A

OD with no exposure
-0.2

104
Q

pros and cons of radiographic film

A

pros: high resolution, low cost
cons: requires processor, scanner, has energy dependence

105
Q

pros and cons of radiochromic film

A

pros: high resolution, no processor, not energy dependent

cons: scanner required, complex calibration, expensive

106
Q

what happens if OSLD is placed outside of the field?

A

-scatter is softer fluence
-OSLD thus over-responds 10-25 %

107
Q

PDD measured with diode falls off faster or slower than that with chamber?

A

-slower because diode over-responds to low E photons

108
Q

dose reading of OSLD one minute after radiation?

A

40% high

109
Q

Z of OSLD- how does this affect reading?

A

effective Z of Al2O3:C OSLD is 10- higher than tissue
-OSLD can over-respond by factor of >3 for low kV energies

110
Q
  • Compare the dose response of an Si diode placed at the beam entrance on a
    patient vs. the beam exit.
A

The beam spectrum is much softer on the exit side of the patient due to scattered photons which are lower energy. The diode will therefore provide an
articially high reading on the exit side vs. the entrance side. This effect will
depend on the thickness of the patient.

111
Q

FMEA

A

failure mode and effect analysis
multiply severity, occurrence, and detectability

112
Q

patient-specific QA

A

dose verifications (IMRT QA)
in vivo QA
transmission devices: mounted on head of linac and beam is transmitted through them
calculation based- use log file fro patient and calculate fluence and compare to plan

113
Q

After an earthquake which linac QA test would be most likely to yield an error?
a. Output
b. Laser
c. Light eld-radiation eld coincidence
d. Graticule alignment

A

Earthquakes can cause a movement of lasers which are mounted on the wall.
All the other systems are physically integrated systems tied to the linac beam
itself and are less likely to be affected.

114
Q

. Which QA measure could potentially detect a change in dose to a spine tumor as
a result of ascites occurring after simulation?
a. Pre-treatment phantom-based IMRT QA
b. Pre-treatment secondary verication of MU calculations
c. In vivo diode measurements
d. In vivo EPID dosimetry measurements

A

This would require a measurement and that measurement would have to be
performed on the patient, not on a phantom. In vivo diode measurements are
typically performed on the entrance side of the patient, so anatomical changes
are typically not registered. EPID dosimetry in vivo (i.e. with the patient under
treatment) may identify the changes, since the transmitted dose changes.

115
Q

Rank the following IMRT QA tests by expected number of points passing (lowest
to highest).
a. Diode array, 2%/2mm criteria
b. Diode array, 3%/3mm criteria
c. Film, 2%/2mm criteria
d. Film, 3%/3mm criteria

A

This asks about the number of total points. Ranking is: a, b, c, d. Film has more
points than the diode array. 2%/2mm is a stricter criterion than 3%/3mm so the
pass rate will be lower.

116
Q

Which device used in QA will be most sensitive to deviations in the high-gradient
region of a head-and-neck plan?
a. OSLD
b. Film
c. Diode array
d. Ion chamber array

A

Film has the highest spatial resolution and so will be more sensitive in the
high-gradient situation where the change in dose per cm is large.

117
Q

In the IROC-H prostate phantom which detector is used to measure gamma values
TPS vs. measured?
a. OSLD
b. TLD
c. Film
d. Chamber

A

film

The gamma metric is a combination of dose difference at a point and distanceto-agreement (DTA). This requires some spatially resolved measurement, i.e.
more than a single point measurement from a TLD (or OSLD). There are no
chambers in the mail order phantom.

118
Q

Which IMRT QA method might detect a problem of an MLC leaf sagging under
gravity? (Check all that apply.)
a. Beam-by-beam QA on a at phantom, gantry 0
b. Beam-by-beam QA of uence with a lm on the gantry head
c. Composite QA with an ion chamber
d. Composite QA with lm

A

a and d
This problem requires the QA to be measured at the actual gantry angle of
delivery (not at gantry angle 0). A single ion chamber only provides a point
measurement which is not sensitive to this problem

119
Q

equation for magnification

A

SID/SOD

120
Q

equation for geometric penumbra width

A

source width * ((SID-SOD)/SOD)

121
Q

what happens as you move object away from detector?

A

-magnification increases but resolution decreases

122
Q

epid layers

A

-copper plate yields electron from photon due to compton
-electron interacts in scintillator- Gadolinium oxysulfide
-light are created
-photons are registered in detection layer - photodiode- amorphous silicon

-detector is read ouit row by row
-voltages on gate lines control what row is being read out

123
Q

equation for SNR

A

N/root(N) = root(N)

124
Q

DQE

A

number of optical photons produced for each xray photon that enters

125
Q

exposure to patient vs mA, kVp

A

mA kVp^2 at skin of patient
mA kVp^5 transmitted through patient

126
Q

MTF

A

responsiveness of detector as a function of frequency of features in image

127
Q

does scatter increase signal at detector?

A

yes, but does not provide any useful information

128
Q

dicom

A

digital imaging and communication in medicine
-communication

129
Q

pacs

A

picture archiving and communications system
-storage

130
Q

pitch

A

D/S
-D is distance table travels in one rotation
S is slice thickness
-pitch> 1 is undersampled
pitch < 1 is oversampled

131
Q

what does noise in CT depend on

A

square root of mA s
slice thickness

132
Q

what does resolution depend on in CT

A

focal spot size
detector resolution
pixel size

mA affects noise but NOT spatial resolution

133
Q

acquisition time for CBCT vs CT sim

A

1 min vs 1 s
-more motion with CBCT
-also more scatter with CBCT
-because of scatter, HU from CBCT not accurate (measured signal no longer depends on attenuation but also on scattered photons)

134
Q

CT ring artifact

A

faulty pixel in detector reconstructs as a ring

135
Q

CT cupping artifact

A

center of image appears darker than periphery
-larger scatter contribution to center
-beam hardening is more through center

136
Q

CT streaking artifact

A

due to beam hardening in projection angles that pass laterally through the high density objects - yields a hardened spectrum- detector registers this as fewer photons - gives dark streak

137
Q

how many bits per byte

A

8

138
Q

for 512 pixels, is pixel size the limiting factor for resolution in CT?

A

no

139
Q

How long does it take to acquire a 50 cm scan for the following CT settings: tube
rotation speed 0.5 sec, pitch 1.0, slice thickness 1.5 mm?

A

The distance the table travels per rotation is d = × = × = p S . . .1 0 1 5 1 5 mm/rotation,
where p is pitch and S is slice thickness. The table therefore travels at a speed
of 3.0 mm/s. For a scan length of 500 mm the required time is 167 sec

140
Q

Describe the artifacts in cone-beam CT if a full rotation is acquired but too few
projection angles are used.

A

This results in streaks due to missing information from various projection
angles.

141
Q

earth’s magnetic field

A

0.0001T

142
Q

Larmor frequency

A

gyromagnetic ratio * B/(2 pi)

1 H is 42.6 MHz/T

143
Q

equation for gyromagnetic ratio

A

gyro = magnetic dipole moment / (spin * hbar)

144
Q

CSF bright vs dark in MRI

A

dark on T1 weighted and bright on T2 weighted

145
Q

what does Gd do

A

shortens T1 relaxation rate
-gives brighter signal on T1 weighted images

146
Q

inversion recvoery

A

the spins are inverted 180 degrees and then allowed to decay. tarting the remaining sequence at a specific time nulls the signal from a particular tissue

147
Q

distortions in MRI

A

-changes in local magnetic field are equivalent to moving the pixel in space

148
Q

MRI magnetic susceptibility artifact

A

metal object distorts field (or really any hetereogneous object)

149
Q

MRI gradient distortion

A

if gradient is larger than expected, the image would be compressed in that direction

150
Q

MRI chemical shift artifact

A

-interface of 2 materials
-signal is “mis-mapped” because it has a different frequency- this is interpreted as being mapped to a different location

151
Q

What does 99mTc emit?

A

140 keV photons

152
Q

geometry of SPECT camera

A

-radiopharmaceutical emits photons
-photons interact with scintillator- convert gamma rays into optical photons
-photons are registered by PMT tubes

-collimator in front of scintillator for spatial localization

153
Q

18F decay

A

-decays to 18O and beta plus
-110 min half life

-18F-FDG

-15-20 mCi per patient

154
Q

summarize PET

A

-positron produced in decay wanders in tissue and annihitles with an electron, producing 2 0.511 MdV photons opposite each other
-these photons are registered as events coincident in time in the crystals of the PET detector ring and a line of response can be calculated

-as more decays occur, more lines of responses are acquired at different angles

155
Q

spatial resolution of PET -issue with momentum

A

-annihilation photons are not exactly colinear because some momentum imparted to nucleus must be balanced by momentum vector of photons in opposite direction
-LOR does not intersect exactly with position of annihilation event and thus affects spatial resolution with PET

156
Q

PET resolution

A

-4-5 mm FWHM
-non-colinearity of annihilation photons
-size of crystals in detector ring
-energy of positrons- they travel in tissue before annihilating and thus blur image
-algorithms and filters used in reconstruction

157
Q

standardized uptake value in PET

A

activity in image/(injected activity/body mass)

158
Q

PET attenuation correction

A

-some tissues (ex lung) attenuate less than others- would make it look like there is more activity
-use CT to do attenuation correction

159
Q

List three advantages and three disadvantages of a low-eld MRI (e.g. <0.5 T)

A

Advantages: Reduced artifacts (e.g. chemical shift or magnetic susceptibility),
lower cost, allows for open design (less patient claustrophobia), lower fringe
elds (i.e. low Gauss lines extending out from magnet), low SAR (specic
absorption rate, i.e. energy deposition in tissue). Also less effect on electron
paths when used during therapy, i.e. the “electron return effect.”
Disadvantages: Lower signal-to-noise (approximately proportional to B0), longer scan times, some pulse sequences not available, more distortions due to B0
inhomogeneities since permanent magnets are often used, less enhancement
with contrast agents like Gd.

160
Q

. What physical factor improves PET resolution in a small animal scanner for mice
and rats vs. a human scanner?
a. Range of the positron in tissue
b. Lower injected activity
c. Non-colinearity of annihilation photons
d. Time of ight

A

The PET detector ring in a small animal scanner is much smaller. Therefore,
the non-colinearity of photons has a much smaller effect than in a patient scanner because of less divergence in the paths of the two photons. The range of the
positron is about the same in both cases which limits resolution.

161
Q

how does energy of positron in PET affect resolution?

A

The resolution is affected by the range
of the positron. Resolution is worse with increasing range, i.e. increasing energy.

162
Q

What is the depth resolution of a 3.5 MHz ultrasound unit? Assume the speed
of sound in the tissue is 1540 m/s and that three cycles are used in each pulse.
Discuss how this changes for a frequency of 5 MHz and what the disadvantages
of that are

A

ultrasound can resolve features in the depth
direction that are one-half the distance of a spatial pulse length. Here the spatial
pulse length is three cycles, i.e. 3 lambda = 3v/f = 31540/(3.510^6)mm. Therefore, the smallest feature that can be resolved in the depth dimension is half of that or 0.66 mm.
Increasing the frequency to 5 MHz would improve the depth resolution further but there would be less penetration of the ultrasound wave in depth.

163
Q

Describe what SAR is in the context of MRI, what affects it, and the relevant regulatory limit

A

SAR is “specic absorption rate.” It is a measure of energy deposited in tissue
and has the units of W/kg. SAR is affected by many parameters including eld
strength (quadratic dependence), the gradient pulses used, the repetition rates,
the ip angles, and frequency. To maintain safety, the FDA limits SAR levels
to 4 W/kg for a 15-minute whole body scan (other similar values are in place for
specic sites). The IEC limits are similar though different “operational levels”
are dened and a 6-minute scan length is considered.

164
Q

Describe how the sensitivity of a PET scan depends on the injected activity. Is it
monotonically increasing/decreasing? Why?

A

Sensitivity increases at rst with injected activity. At very low activity, the
signal is photon limited, and increasing the number of photons improves the
signal-to-noise ratio. As the activity increases, however, the number of random counts registered in the detectors increases. These are event pairs that are
agged as simultaneous but which are actually not from the same annihilation event. Also the number of scatter events increases. Therefore, at very large
injected activity the sensitivity decreases again.

165
Q

rigid registration

A

assumes the anatomy of the patient is completely rigid and that a translation and rotation can be performed to make the patient align perfectly with the reference scan
-not always case due to changes in anatomy, neck flexing (for example)

166
Q

examples of systems with IGRT

A

-CBCT linac
-tomo
-halcyon
-cyberknife
-exactrac
-US guidance
-Calypso (transponders)
-surface imaging- AlignRT

167
Q

electron return effect

A

linac MRI
-electrons don’t travel in straight lines in magnetic field
-electrons can bend back toward source and create regions of high dose (happens in lung for example- get high dose at chest wall)

168
Q

types of IGRT

A

-online (image prior to treatment)
-offline (image at time of treatment but apply corrections at next treatment)
real time- image continuously through treatment

169
Q

what does catphan stand for

A

custom acceptance test phantom

170
Q

image quality phantom for kV

A

QCkV-1

171
Q

motion management examples

A

-use 4DCT to make ITV
-gating
-breath hold
-compression
-tracking

172
Q

. Which of the following are advantages of kV planar imaging over CBCT for use in
IGRT? (Select all that apply.)
a. Improved visualization of soft tissue
b. Lower dose
c. Potential for real-time tracking
d. Ability to support adaptive replanning

A

kV planar images are a single exposure and much lower dose (<1 mGy/image)
while CBCT delivers approximately 2 cGy. Multiple planar images acquired
over time (“uoro mode”) potentially allow for tracking of features that can be
visualized. This is used, for example, in CyberKnife treatments

173
Q

How far does the edge of a target region 10 cm from isocenter move for a rotation
of 3 degrees

A

10 mm * tan ((pi/180)*3) = 5.2 mm

174
Q

How does the CT number uniformity compare for a 20 cm diameter phantom vs.
a 40 cm diameter phantom in a QA test of CBCT QA?

A

. Less uniform for 40 cm
Less uniform due to an increased “cupping artifact” for larger phantom sizes.
Recall that this is due to scatter in the CBCT geometry.

175
Q

How does noise vary with mA in a QA test of CBCT?

A

noise scales like root(N) , where N is the number of
photons. Increasing the mA increases the number of photons.

relative noise decreases (rootN over N)

176
Q

List three advantages and three challenges of breath-hold treatment for radiation
therapy of left-sided lung cancer

A

Advantages: Reduces intra-fraction movement, the possibility to eliminate the
iGTV and therefore have a smaller treatment volume, inates lung (if breath
is held at end inspiration) which results in more normal lung sparing, moves
heart away from the treatment eld potentially.
Challenges: Achieving the same lung ination at each breath-hold may be a
challenge (residual air in the lung at tidal exhale can confound the perceived
ination volume); it may be challenging for some patients to hold their breath
especially those with compromised lung function; treatment times are longer
since the beam is off while you wait for the patient to breathe in and hold their
breath

177
Q

In 4DCT with spiral acquisition of a patient with a 6 sec breathing period how
much does the table move in one breathing cycle if the following parameters are
used: pitch p = 0.5, tube rotation speed Trot= 1.0 sec, slice thickness S = 3 mm? How
many independent slices can be reconstructed

A

The distance per breathing cycle is pitch* S * Tresp/Trot, where Tresp is the respiratory
period. The distance is then 9 mm. Note that in theory then you
could reconstruct three independent slices in this period (3 × 3 × 3 = 9 mm). To
get more independent slices you would need to either decrease the pitch (which
increases dose) or make the tube rotation speed smaller

178
Q

In 4DCT with spiral acquisition of a patient with a 6 sec breathing period how
much does the table move in one breathing cycle if the following parameters are
used: pitch p = 0.5, tube rotation speed Trot= 1.0 sec, slice thickness S = 3 mm? How
many independent slices can be reconstructed

A

The distance per breathing cycle is pitch* S * Tresp/Trot, where Tresp is the respiratory
period. The distance is then 9 mm. Note that in theory then you
could reconstruct three independent slices in this period (3 × 3 × 3 = 9 mm). To
get more independent slices you would need to either decrease the pitch (which
increases dose) or make the tube rotation speed smaller

-remember L is length of table travelled per cycle
-and breathing is 6 s- ie 6 cycles

179
Q

technical characteristics of SRS

A

-single fraction
-> 5 Gy
-target diameter > 3.5 cm
-brain
-accuracy < 1 mm
-no PTV margins, CTV in some cases

180
Q

3 collimation sizes available in gamma knife

A

-4,8,16 mm
-collimators are tungsten

181
Q

conformity index

A

volume of prescription isodose line/ volume of PTV

182
Q

R50%

A

volume of 50% isodose line/ volume of PTV

-smaller PTV= smaller R50%

183
Q

D2cm

A

max dose at 2 cm from PTV

-smaller PTV = smaller D2cm

184
Q

CI and V20Gy in lung SBRT

A

CI < 1.2
V20Gy < 10%

185
Q

recommendations for SBRT

A

-MLC width = 5 mm
-detectors for small fields
-E2E tests
-motion assessment for thoracic and abdominal sites
-dose calc grid < 2 mm
-use IGRT for alignment

186
Q

What is an approximate dose gradient in a linac-based SBRT plan at its steepest
point?

A

1 Gy/mm

187
Q

How does the value for D2cm (dose at 2 cm from the PTV) change if a lung SBRT
prescription is modied from 18 Gy × 3 to an isodose line at 75% of the maximum
dose vs. 10 Gy × 5 prescribed to an isodose line at 80% of the maximum dose?

A

d. Increased to 123% of original
If the plan is not changed aside from the prescription, the D2cm will track with
the maximum dose. In the original plan the maximum dose was 18x3/0.75 = 72

the rescaled plan the maximum dose is 10x5/0.8 =62.5
´
which is 87% of the original. The overall dose is decreased (54 Gy to 50 Gy). Also increasing the prescription isodose line will decrease the overall dose.

188
Q

According to the HyTEC Report and also QUANTRC and cooperative group protocols, what is the recommended method for dening the lung structure in SBRT
plans?

A

lung_r + lung_l - iGTV

189
Q

TBI dose

A

12-15 Gy in 6-10 fractions BID for myeloablative (ie kill stem cells in bone marrow)
-want uniform dose with +/- 10% variation

190
Q

typical features of TBI

A

-big SSD so patient fits in field
-spoiler- creates extra superficial dose in patient (for example to get bone marrow in ribs, which is at shallow depth)
-lung blocks
-compensator- corrects for fact that some parts of patient are thin (neck) and others thicker (abdomen)

191
Q

what factors control dose homogeneity for TBI? (POP beams)

A

-higher energy and bigger SSD = bigger PDD= more homogeneous
-smaller patient separation = more homogeneous

192
Q

different TBI setups

A

AP/PA with patient standing- patient can’t tolerate
AP/PA with patient lying down - patients move, moving lung blocks. Also, lung on downward side is compressed
lateral beams with patient sitting or lying down- cannot apply lung blocks and dose is less uniform in depth direction compared to AP/PA because of larger separation

193
Q

how to verify TBI dose

A

measure with TLDs, OSLD, MOSFET, diode
-make sure detector is calibrated to TBI geometry

194
Q

TSEI

A

total skin electron therapy
used to treat cutaneous T-cell lymphoma
-36 Gy in 36 fractions over 4 days per week
-usually 6 MeV is used so you don’t get too much Bremsstrahlung in patient

195
Q

how is TSEI delivered?

A

-2 beams aimed at angle at patient- reduces x-ray component which is highest along CAX of beam
-use extended distance so whole patient is in field
-at extended distance there is lots of scatter and attenuation in air, therefore, operate linac in high dose rate mode

-use multiple fields from different angles around the patient
-3 beams from 3 different angles on one day, followed by 3 beams from 3 different angles the following day

196
Q

how is uniformity achieved in TSEI?

A

-use a scatterer
-place near exit window of linac instead of near patient as in TBI
-placing scatterer near exit window of patient instead of near patient results in narrower angle for electrons that are scattered which results in a deeper depth-dose curve

197
Q

what about TSEI for soles of feet, scalp, chin?

A

-these regions do not see direct electron beam
-treat with electron boost field with conventional techniques
-skin folds can create regions of low dose

198
Q

. What is the impact of accidentally delivering a TBI treatment at 400 cm SSD if the
standard setup is meant to be at 450 cm SSD?

A

Overall the dose will be higher due to inverse square falloff. Because it is closer
the dose is less homogeneous (faster PDD falloff). The head and feet may be
cold because the eld size may be too small especially if the patient is tall

199
Q

how much heavier is proton vs electron

A

2000X

200
Q

where are protons typically used

A

-pediatric cancer
-CNS tumors
-eye cancer
-sarcomas
-unresectable H and N cancer
-liver cancer
-re-irridiation cases

201
Q

range of energies for therapy proton beams

A

90-230 MeV

202
Q

SOBP

A

-spread out bagg peak
-modulator (low Z) reduces energy of beam and therefore range
-range modulator wheel has different thicknesses of plastic, tuned to provide SOBP when wheel is spun
-beams with larger modulation have higher surface dose because each Bragg peak contributes surface dose

203
Q

compensator

A

in proton therapy, used to create 3D coformity to target- thickness is adjusted to modulate beam at different points
-can be very sensitive to patient movement

204
Q

what does inhomogeneity do to proton beam PDD?

A

-if it is bone, actally pulls the edge of the SOBP to a shallower depth
-if it is lung, pushes the edge of the SOBP to a deeper depth

205
Q

why low Z material for proton beam compensator?

A

reduces scattering of the beam

206
Q

components of proton beam delivery system

A

-first scatterer
-second scatterer
-range shifter
-range modulator
-patient aperture
-patient compensator

207
Q

penumbra of proton beam

A

-similar (not much better) to photon beam
-increases with depth

208
Q

pencil beam scanning for proton beams

A

-magnetic steering system scans beam back and forth over tumor
-one layer is scanned
-energy of beam is changed, which moves depth of bragg peak
-another layer is scanned
-and so on

209
Q

why can’t protons used linac?

A

2000X heavier than electrons
linac not strong enough to accelerate it

210
Q

cyclotron

A

-apply electric field to 2 half circles
-particle moves due to E field
-magnetic field is applied perpendicular to proton path, forcing proton to curve
-as proton travels, its energy increases due to E field
-pick off proton at specific radius to select energy

211
Q

synchotron

A

-particle beam travels in a vaccuum tube
-bending magnets bend beam into a circle
-each time beam goes around, it travels through an accelerating cavity, which applies E field to accelerate particle
-proton is extracted when it reaches desired energy

212
Q

differences between cyclotron and synchotron

A

-average beam current is higher in cyclotron
-cyclotron can yield continuous beam whereas synchotron yields beam in pulses
-in synchotron, can switch energy of beam quickly whereas cyclotron usually operate at fixed energy
-cyclotrons are more compact
-synchotron- particles can reach extremely high energies

213
Q

PTV margin sizes in proton treatment

A

-not uniform like photons
-direction of beam must be accounted for
-more margin needed at distal end (where bragg peak falls off suddenly) because uncertainties have bigger impact here due to bragg edge
-added margin to account for uncertainty from CT numbers, beam energy from accelerator

214
Q

major difference between photon and proton therapy

A

-effects of motion can have more impact for proton beams due to the bragg peak and its distal edge location…

215
Q

differences between heavy ion beams and proton beams

A

-range in tissue for heavy nucleon is less than that of proton, for the same energy
-LET for heavy nucleon is higher
-for heavy nucleon, dose is deposited beyond bragg peak because of nuclear fragments created through inelastic scattering of carbon ion
-penumbra of carbon ion beam is smaller than that of proton beam, due to less multiple coulomb scattering of heavier particle

216
Q

how does number of double strand breaks vary with LET?

A

-actually independent of LET
-reason why high LET ions do more damage is because the spacing of DSB is closer in high LET vs low LET, and high LET produces clustered lesions, which is more difficult to repair

217
Q

what accounts for increase in penumbra of proton beam with depth?

A

multiple Coulomb scattering

218
Q

What proton beam parameter affects the width, W, of the pristine Bragg peak

A

Spread in the energy of the protons. Some stop earlier and contribute more at
the proximal edge while some are at higher energy and stop deeper at the distal
edge. The range in energies can arise from the accelerator and beamline itself
or from range straggling in a range modulator

219
Q
  • In a cyclotron system how does the width, W, change when a smaller range beam
    is used?
A

A shorter range beam requires lower energy which is achieved by using a range
modulator (plastic slabs in the beam to modulate the energy down). Since there
is range straggling in this modulator, the width of the peak increases

220
Q

quimby system

A

uniform loading
not uniform dose

221
Q

machester system

A

uniform dose within 10%
uses peripheral loading

222
Q

where can quimby and manchester system be used?

A

-high energy sources only

223
Q

post-implant verification

A

verify location of brachy seeds

224
Q

LDR seed QA

A

10% of seeds are assayed and must be within 3 % of activity specified by manufacturer

225
Q

when is HDR for cervical cancer used?

A

-monotherapy for early cancers
-later stage cancers in combination with external beam radiotherapy

226
Q

manchester pts for cervix

A

-pt A is 2 cm from top of ovoid and 2 cm lateral from tandem
-pt B is 3 cm lateral from Pt A (pelvic sidewall)
-bladder pt is on posterior of foley balloon
-rectal pt is 0.5 cm post to vaginal wall

227
Q

common prescrption to pt A in brachy

A

-7 Gy X 4
-6 Gy X 5

228
Q

common dose limits for pt B, rectum, bladder, and mucosa for brachy

A

-pt B: 30-40% of Pt A
-rectum: < 4.1 Gy/fx
-bladder: < 4.6 Gy/fx
-mucosa: < 120 Gy

229
Q

examples of radionuclide therapy

A

-I131
-90Y- liver
-223Ra-chloride- castrate-resistant prostate

230
Q

implant size vs activity needed

A

For a smaller implant, dose falls off more rapidly with depth so more
activity is needed.

(i.e. IS is less significant for large cylinder vs small cylinder)

231
Q

How does the D90% for a prostate implant on the day of implant compare to the
D90% at 30 days post-implant?

A

lower
Due to swelling on the day of implant, seeds are farther apart than planned.
This results in an overall underdose at the periphery

232
Q

how does dose fall off for vaginal cylinder?

A

line source, so falls off as 1/r

233
Q

Co-60 decay

A

1%/month

234
Q

ortho rule

A

dose at 0 is 100
dose at 2 cm = 60+ E/10
dost at 5 cm is 30+ E/10
dose at 10 cm is 0 + E/10

235
Q

PHOTON pdd RULE

A

d5cm =80+E
d10=60+e
D20=35+e
D30=15+e

236
Q

electron shells

A

2,6,10 in s, p, and d
2n^2 in any shell

237
Q

h bar times c

A

197.3 MeV * fm

238
Q

mass of proton and mass of neutron

A

935 and 938 MeV

239
Q

alp-ha

A

e^2/ (4 pi epsilon knot hbar c)

240
Q

radius of electron

A

2.82 fm
(e^2/(4 pi epsilon knot me c^2)

241
Q

unit of barn

A

10^-28 m2

242
Q

electron shells

A

Shells do not have specific, fixed distances from the nucleus, but an electron in a higher-energy shell will spend more time farther from the nucleus than does an electron in a lower-energy shell.
Shells are further divided into subsets of electrons called subshells. The first shell has only one subshell, the second shell has two subshells, the third shell has three subshells, and so on. The subshells of each shell are labeled, in order, with the letters s, p, d, and f. Thus, the first shell has only a single s subshell (called 1s), the second shell has 2s and 2p subshells, the third shell has 3s, 3p, and 3d and so forth

s holds 2 electrons, p holds 6
total number of electrons is 2n^2,n is shell number

Valence electrons are the electrons in the highest occupied principal energy level of an atom

Ford (3 decks)

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