Ford Flashcards
mass rest of proton
938 MeV
frequency of gamma rays, xrays etc
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
energy of wave
E = hv
lambda= h/p
E= pc
Plank constant
6.63 * 10^-34 m2kg/s
size of atom vs size of nucleus
atom is tens of nm
nucleus is 10^-15 m
Coulombs Law
F = kq1q2/r^2
ground state of hydrogen atom
13.6 eV
eV
1.6 *10^-19 J
charge of electron is 1.6 *10^-19 C
Coulomb constant
8.99*10^9 Nm2/C2
94.9 FM radio is what frequency?
94.9 MHz
mass of electron
9.11*10^-31 kg
magnetic force
F= qvB
centripetal force
F= mv^2/r
mass of proton
1.7*10^-27 kg
angular frequency
omega = v/r
f= omega/2pi
nuclei with Z> what are unstable
83
what elements are more neutron rich
higher Z
mass/energy of stable nucleus vs mass/energy of its parts
stable nucleus has lower mass/energy
where does beta minus decay occur
neutron rich nuclei
beta plus occurs for neutron poor nuclei
why is there a spectrum of energies for beta decay?
energy is shared with neutrino or anti-neutrino
half life of beta minus vs beta plus decay
beta-minus: long
beta-plus: short
how are neutron rich vs neutron poor isotopes made?
neutron rich: reactors (bombardment)
neutron poor: cyclotron
how to tell difference between beta plus and beta minus spectrum?
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
mean life of radioactive source
1.44 * half life
activity of daughter is half life of parent is much longer than daugther
-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)
equation for mg Ra equivalent
(tau source/ tau Ra)* source activity
is beta in TG43 equation in rad or degrees?
radians
brachy inverse square law fall off- what source falls off fastest? Pt source, 3 mm, or 5 mm line
pt source falls off fastest, followed by 3 mm line and 5 mm line
range of electrons
half of its energy in MeV (cm)
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
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
offset between dose and kerma wrt energy
higher energy beams = higher energy electrons= longer range = bigger offset
why do higher energy beams have deeper dmax?
they produce higher energy electrons which travel further in the medium. It thus takes longer for equilibrium to be reached
how does linear attenuation coefficient depend on density?
increases linearly with density
-mass attenuation coefficient doesn’t depend on density
. How many HVLs of tungsten are required to attenuate a high-energy photon
beam down to 3% of its initial intensity?
(0.5)^n = 0.03
take ln of both sides and solve for n
is dmax deeper in lung than in water?
yes, because electron ranges are longer in lung
what does radiative stopping power depend on?
(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
what does collisional stopping power depend on?
decreases in higher Z material as there are less electrons to interact with
not a strong dependence on energy
what does stopping power of protons depend on
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
where does delta ray come from?
electron undergoes “hard” collision in which substantial amt of energy is transferred to electron
-delta ray can travel relatively long distance
- 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
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.
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
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.
what causes heating in the anode?
collisional losses of the electrons
key reason MV beams were developed
skin sparing
see chapter 8 diagrams in how linacs accelerate electrons
-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)
magnetron vs klystron
-magnetron is smaller, generates microwaves
-klystron is amplifier- amplifies W to MW- larger, bulky, and is stationary
how do magnetrons make microwaves?
-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
why are linacs maintained at low vaccuum pressures/
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
what is electron gun?
cathode through which electrons are emitted
what does modulator do?
introduces pulses into klystron/magnetron and also into gun
-electron system and RF system are switched on and off in synchrony with each other
what determines dose rate of linac?
-pulse repetition rate and current in peak of pulse
why is achromatic bending magnet used instead of 90 degree bending magnet (image pg 84)
-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
how is linac photon energy determined?
-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
energy lost to heat in xray tube vs linac
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).
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
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
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- 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
efciently in the waveguide. The output decreases because high-energy electrons ultimately do not reach the the target
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 larger magnetic eld would bend the electrons more (i.e. into a tighter circle),
which theoretically should move the spot away from the gun.
equation for size of penumbra
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
tongue and groove effect
tongue and groove make dose fall-off less sharp than if these features were not present
why does tomo have increased output?
-no FFF
-85 cm SAD vs 100 cm
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
high frequency and low energy
3 sources of penumbra
-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
size of horns for 10 MV vs 6 MV
horns are larger for 10 MV beam
depth of basal skin
0.07 mm
skin dose for FS 40X40 vs 10x10
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
why are there holes in thermoplastic mask?
decrease skin dose increase from plastic
maynord F factor
PDD2 = PDD1 * ((SSD2+dmax)/(SSD2+d))/((SSD1+dmax)/(SSD1+d))
equivalent FS for blocked field
FS unblocked * square root (1-f)
where f is fraction of field that is blocked
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
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
Sc for a blocked field
Sc will be for the “unblocked” field- i.e. the size of the collimator at iso
superposition convolution
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)
penumbra in inhomogeneities and for different energies
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
describe effect of bone on dose
-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
when are inhomogeneity effects worse?
small field
high energy
trick for remembering wedge direction
heels together, toes apart, like in tango
wedge angle
90- (hinge angle/2)
hinge angle is angle between 2 beams
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
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.
where should ICRU reference dose be?
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
survival fraction for single fraction vs fractionated treatment
single fraction: SF = e^(-alpha D - beta D^2)
multiple fractions: SF = e^(-alpha D - beta* n*d^2)
advantages of compensators vs MLCs
-uses dose efficiently whereas with leaves, much of the beam is “wasted”
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
Order is: a. 3D-CRT, d. Compensator IMRT, b. Step-and-shoot IMRT with
MLC, c. Dynamic MLC IMRT
compare 3DCRT DVH for OAR to VMAT DVH for OAR
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.
What advantage is provided by rotating the collimator away from zero degrees in
VMAT arcs?
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.
why is the elctron PDD not sharp, but instead spread out in depth?
higher energy electrons travel further and lower energy electrons travel less
-electrons scatter with air along the intervening path - some have lost energy
rules of thumb for electron PDD
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
what happens to electron PDD when FS < Rp?
-dmax closer to surace
-surface dose increases
electron beam penumbra
-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
gap between applicator and patient for electron treatment
5 cm is typical
also leave 1 cm wider on each side than treatment area to account for penumbra
what effects happen when treating with electrons at extended SSDs?
-larger penumbra - more constriction and bulging
-output decreases- use virtual source for IS corrections
-light field no longer accurately tracks radiation field
what happens to electron beams with oblique incidence?
-dmax gets pulled closer to surface
-doses are higher on side where angle is sharper
ie dose lines pull closer to surface
what happens with inhomogeneities in electron beams?
-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
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
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
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
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%.
where is brems. production highest in electron profile
along CAX b/c brems is more forward directed
volume of farmer vs scanning cs mini vs micro chamber
-0.6 cc
-0.125 cc
-0.06 cc
-0.006 cc
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
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
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
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.
pros and cons of diodes
pros: instant readout, sensitive, small, no bias voltage
‘cons: energy dependense, dose rate dependence, temperature dependence, radiation damage, directional dependence
shielded diodes
for photon diodes
-provides shielding from low energy scattered photons, which diode overresponds to
are diodes used as primary standard?
No
-neither are film or OSLDs
-only chambers are primary standard
OSLD material
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
pros and cons of luminescent dosimeters
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
optical density of film
OD = log10 (Io/I)
fog of film
OD with no exposure
-0.2
pros and cons of radiographic film
pros: high resolution, low cost
cons: requires processor, scanner, has energy dependence
pros and cons of radiochromic film
pros: high resolution, no processor, not energy dependent
cons: scanner required, complex calibration, expensive
what happens if OSLD is placed outside of the field?
-scatter is softer fluence
-OSLD thus over-responds 10-25 %
PDD measured with diode falls off faster or slower than that with chamber?
-slower because diode over-responds to low E photons
dose reading of OSLD one minute after radiation?
40% high
Z of OSLD- how does this affect reading?
effective Z of Al2O3:C OSLD is 10- higher than tissue
-OSLD can over-respond by factor of >3 for low kV energies
- Compare the dose response of an Si diode placed at the beam entrance on a
patient vs. the beam exit.
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
articially high reading on the exit side vs. the entrance side. This effect will
depend on the thickness of the patient.
FMEA
failure mode and effect analysis
multiply severity, occurrence, and detectability
patient-specific QA
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
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
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.
. 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 verication of MU calculations
c. In vivo diode measurements
d. In vivo EPID dosimetry measurements
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.
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
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.
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
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.
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
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.
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 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
equation for magnification
SID/SOD
equation for geometric penumbra width
source width * ((SID-SOD)/SOD)
what happens as you move object away from detector?
-magnification increases but resolution decreases
epid layers
-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
equation for SNR
N/root(N) = root(N)
DQE
number of optical photons produced for each xray photon that enters
exposure to patient vs mA, kVp
mA kVp^2 at skin of patient
mA kVp^5 transmitted through patient
MTF
responsiveness of detector as a function of frequency of features in image
does scatter increase signal at detector?
yes, but does not provide any useful information
dicom
digital imaging and communication in medicine
-communication
pacs
picture archiving and communications system
-storage
pitch
D/S
-D is distance table travels in one rotation
S is slice thickness
-pitch> 1 is undersampled
pitch < 1 is oversampled
what does noise in CT depend on
square root of mA s
slice thickness
what does resolution depend on in CT
focal spot size
detector resolution
pixel size
mA affects noise but NOT spatial resolution
acquisition time for CBCT vs CT sim
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)
CT ring artifact
faulty pixel in detector reconstructs as a ring
CT cupping artifact
center of image appears darker than periphery
-larger scatter contribution to center
-beam hardening is more through center
CT streaking artifact
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
how many bits per byte
8
for 512 pixels, is pixel size the limiting factor for resolution in CT?
no
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?
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
Describe the artifacts in cone-beam CT if a full rotation is acquired but too few
projection angles are used.
This results in streaks due to missing information from various projection
angles.
earth’s magnetic field
0.0001T
Larmor frequency
gyromagnetic ratio * B/(2 pi)
1 H is 42.6 MHz/T
equation for gyromagnetic ratio
gyro = magnetic dipole moment / (spin * hbar)
CSF bright vs dark in MRI
dark on T1 weighted and bright on T2 weighted
what does Gd do
shortens T1 relaxation rate
-gives brighter signal on T1 weighted images
inversion recvoery
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
distortions in MRI
-changes in local magnetic field are equivalent to moving the pixel in space
MRI magnetic susceptibility artifact
metal object distorts field (or really any hetereogneous object)
MRI gradient distortion
if gradient is larger than expected, the image would be compressed in that direction
MRI chemical shift artifact
-interface of 2 materials
-signal is “mis-mapped” because it has a different frequency- this is interpreted as being mapped to a different location
What does 99mTc emit?
140 keV photons
geometry of SPECT camera
-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
18F decay
-decays to 18O and beta plus
-110 min half life
-18F-FDG
-15-20 mCi per patient
summarize PET
-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
spatial resolution of PET -issue with momentum
-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
PET resolution
-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
standardized uptake value in PET
activity in image/(injected activity/body mass)
PET attenuation correction
-some tissues (ex lung) attenuate less than others- would make it look like there is more activity
-use CT to do attenuation correction
List three advantages and three disadvantages of a low-eld MRI (e.g. <0.5 T)
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 (specic
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.
. 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
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.
how does energy of positron in PET affect resolution?
The resolution is affected by the range
of the positron. Resolution is worse with increasing range, i.e. increasing energy.
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
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.
Describe what SAR is in the context of MRI, what affects it, and the relevant regulatory limit
SAR is “specic 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
specic sites). The IEC limits are similar though different “operational levels”
are dened and a 6-minute scan length is considered.
Describe how the sensitivity of a PET scan depends on the injected activity. Is it
monotonically increasing/decreasing? Why?
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.
rigid registration
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)
examples of systems with IGRT
-CBCT linac
-tomo
-halcyon
-cyberknife
-exactrac
-US guidance
-Calypso (transponders)
-surface imaging- AlignRT
electron return effect
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)
types of IGRT
-online (image prior to treatment)
-offline (image at time of treatment but apply corrections at next treatment)
real time- image continuously through treatment
what does catphan stand for
custom acceptance test phantom
image quality phantom for kV
QCkV-1
motion management examples
-use 4DCT to make ITV
-gating
-breath hold
-compression
-tracking
. 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
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
How far does the edge of a target region 10 cm from isocenter move for a rotation
of 3 degrees
10 mm * tan ((pi/180)*3) = 5.2 mm
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?
. 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.
How does noise vary with mA in a QA test of CBCT?
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)
List three advantages and three challenges of breath-hold treatment for radiation
therapy of left-sided lung cancer
Advantages: Reduces intra-fraction movement, the possibility to eliminate the
iGTV and therefore have a smaller treatment volume, inates 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 ination at each breath-hold may be a
challenge (residual air in the lung at tidal exhale can confound the perceived
ination 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
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
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
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
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
technical characteristics of SRS
-single fraction
-> 5 Gy
-target diameter > 3.5 cm
-brain
-accuracy < 1 mm
-no PTV margins, CTV in some cases
3 collimation sizes available in gamma knife
-4,8,16 mm
-collimators are tungsten
conformity index
volume of prescription isodose line/ volume of PTV
R50%
volume of 50% isodose line/ volume of PTV
-smaller PTV= smaller R50%
D2cm
max dose at 2 cm from PTV
-smaller PTV = smaller D2cm
CI and V20Gy in lung SBRT
CI < 1.2
V20Gy < 10%
recommendations for SBRT
-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
What is an approximate dose gradient in a linac-based SBRT plan at its steepest
point?
1 Gy/mm
How does the value for D2cm (dose at 2 cm from the PTV) change if a lung SBRT
prescription is modied 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?
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.
According to the HyTEC Report and also QUANTRC and cooperative group protocols, what is the recommended method for dening the lung structure in SBRT
plans?
lung_r + lung_l - iGTV
TBI dose
12-15 Gy in 6-10 fractions BID for myeloablative (ie kill stem cells in bone marrow)
-want uniform dose with +/- 10% variation
typical features of TBI
-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)
what factors control dose homogeneity for TBI? (POP beams)
-higher energy and bigger SSD = bigger PDD= more homogeneous
-smaller patient separation = more homogeneous
different TBI setups
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
how to verify TBI dose
measure with TLDs, OSLD, MOSFET, diode
-make sure detector is calibrated to TBI geometry
TSEI
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
how is TSEI delivered?
-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
how is uniformity achieved in TSEI?
-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
what about TSEI for soles of feet, scalp, chin?
-these regions do not see direct electron beam
-treat with electron boost field with conventional techniques
-skin folds can create regions of low dose
. 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?
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
how much heavier is proton vs electron
2000X
where are protons typically used
-pediatric cancer
-CNS tumors
-eye cancer
-sarcomas
-unresectable H and N cancer
-liver cancer
-re-irridiation cases
range of energies for therapy proton beams
90-230 MeV
SOBP
-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
compensator
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
what does inhomogeneity do to proton beam PDD?
-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
why low Z material for proton beam compensator?
reduces scattering of the beam
components of proton beam delivery system
-first scatterer
-second scatterer
-range shifter
-range modulator
-patient aperture
-patient compensator
penumbra of proton beam
-similar (not much better) to photon beam
-increases with depth
pencil beam scanning for proton beams
-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
why can’t protons used linac?
2000X heavier than electrons
linac not strong enough to accelerate it
cyclotron
-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
synchotron
-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
differences between cyclotron and synchotron
-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
PTV margin sizes in proton treatment
-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
major difference between photon and proton therapy
-effects of motion can have more impact for proton beams due to the bragg peak and its distal edge location…
differences between heavy ion beams and proton beams
-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
how does number of double strand breaks vary with LET?
-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
what accounts for increase in penumbra of proton beam with depth?
multiple Coulomb scattering
What proton beam parameter affects the width, W, of the pristine Bragg peak
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
- In a cyclotron system how does the width, W, change when a smaller range beam
is used?
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
quimby system
uniform loading
not uniform dose
machester system
uniform dose within 10%
uses peripheral loading
where can quimby and manchester system be used?
-high energy sources only
post-implant verification
verify location of brachy seeds
LDR seed QA
10% of seeds are assayed and must be within 3 % of activity specified by manufacturer
when is HDR for cervical cancer used?
-monotherapy for early cancers
-later stage cancers in combination with external beam radiotherapy
manchester pts for cervix
-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
common prescrption to pt A in brachy
-7 Gy X 4
-6 Gy X 5
common dose limits for pt B, rectum, bladder, and mucosa for brachy
-pt B: 30-40% of Pt A
-rectum: < 4.1 Gy/fx
-bladder: < 4.6 Gy/fx
-mucosa: < 120 Gy
examples of radionuclide therapy
-I131
-90Y- liver
-223Ra-chloride- castrate-resistant prostate
implant size vs activity needed
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)
How does the D90% for a prostate implant on the day of implant compare to the
D90% at 30 days post-implant?
lower
Due to swelling on the day of implant, seeds are farther apart than planned.
This results in an overall underdose at the periphery
how does dose fall off for vaginal cylinder?
line source, so falls off as 1/r
Co-60 decay
1%/month
ortho rule
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
PHOTON pdd RULE
d5cm =80+E
d10=60+e
D20=35+e
D30=15+e
electron shells
2,6,10 in s, p, and d
2n^2 in any shell
h bar times c
197.3 MeV * fm
mass of proton and mass of neutron
935 and 938 MeV
alp-ha
e^2/ (4 pi epsilon knot hbar c)
radius of electron
2.82 fm
(e^2/(4 pi epsilon knot me c^2)
unit of barn
10^-28 m2
electron shells
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
