therapy answers
T44.
C.
The spinal Y2 jaw abuts the cranial fields. The cranial fields would, therefore, require a
collimator angle of arctan(13 cm/100 cm)= 7.4 °.
T4S.
D.
Skin gap= 0.5 x 30 x (d/SAD) + 0.5 x 30 x (d/SAD)= 30 x 5 / 100 = 1.5 cm.
T46.
E.
The POD increases with increasing SSD, field size, and beam energy. The POD decreases with
depth.
T4 7.
E.
The increase in penumbra with energy is due to the increased range of the secondary electrons
created from the higher-energy photons. Penumbra actually decreases as the energy increases
until about 6 MV, and then it starts increasing again. The minimum penumbra occurs at about
4-6 MV.
T48.
B.
Beam symmetry r_efers to the beam profile being the same on each side of the beam central
axis. Flatness is measured by finding the maximum and minimum dose values on a beam
profile within the central 80% of the beam. Penumbra is a measure of the fall-off between the
80% to 20% dose shoulders of a beam profile.
T49.
C.
The size of the buildup region is related to the range of the secondary charged particles
released by the incident photons. As the photon energy increases, so does the secondary
charged particle range and, hence, the length of the buildup region. This region is
characterized by a lack of charged particle equilibrium.
TSO.
B.
The POD incorporates changes in the inverse square law, beam attenuation, and scattering so
as the SSD increases, the POD increases. However, the TMR is just a measure of beam
attenuation and scattering, so as the SSD increases, the TMR remains the same.
TS I.
C.
Small fields at the interface between two heterogeneities is the situation where model-based
algorithms, such as convolution/superposition, are likely to break down and Monte Carlo will
be beneficial.
TS2.
A.
In a three-field beam arrangement, the wedges would be placed on the parallel opposed fields,
and the thick ends of the wedges should be oriented toward the beam that is entering
perpendicular to the parallel opposed fields.
TSJ.
D.
When doing an SAD treatment, it is easier to calculate the MU with the TMR. The equivalent
square for a collimator setting of 13 x 18 cm2 is 15 cm2 , and Sc(15 x 15 cm2)= 1.015.
The field is blocked to a FS e of 10 x 10 cm2 , so the Sp( l 0 x 10 cm2)= 1.000 and the
q
TMR(d=8, 10 x 10 cm2 )= 0.842
MU = dose per field / (output for SAD setup x S c x SP x
TMR)= (300/2) / (1.0 x 1.015 x 1.0 x 0.842) = 176.
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T54.
E.
When doing an SSD treatment, it is easier to calculate the MU with the PDD. The equivalent
square for a collimator setting of 8 x 13 cm2 is 10 cm2 , and Sc(lO x 10 cm2) = 1.000. The field
is blocked to a FSeq of 8 x 8 cm2 , so the S/8 x 8 cm2 ) = 0.989 and the PDD(d=5, 8 x 8 cm2 )
= 86.2% MU= Dose per field / [output for SSD setup x Sc x SP x (PDD/100%)) = (400/1) I
(0.971 X 1.000 X 0.989 X 0.862) = 483.
TSS.
C.
The PDD is defined as (dose at depth / dose at dmaJ x 100%. In the previous question, the
dose at depth was prescribed as 400 cGy, and the PDD(d=5, 8 x 8 cm2 ) = 86.2%. Therefore,
the dose at dmax = 400 I 0.862 = 464 cGy.
T56.
B.
The side of the equivalent square is calculated as 4 x area/perimeter = [4 x (5 x 20)) /
[2 x (5 + 20)] = 8.0 cm. E quivalent squares are used to convert an irregularly shaped photon
field to a regularly shaped field that has approximately an equivalent scatter contribution to the
point of interest. This is done to simplify lookup tables for scatter factors, PDD, and TMRs.
TS 7.
B.
In order to deliver a flat field at a depth of IO cm, a beam profile at dmax will feature high dose
"horns" lateral of the central axis.
T58.
B.
The depth of the distal side of the 90% isodose line can be approximated as E/3.2 of the
electron energy. So, a 9 MeV beam will treat to approximately 2.8 cm. Use of a higher energy
will unnecessarily expose more tissue.
T59.
B.
The practical range of electrons in water (in cm) is approximately half of their energy (in
MeV). The thickness of lead necessary is approximately IO times less than water.
T60.
C.
There is little change in the output of an electron beam if the cutout size is larger than or equal
to the practical range. The practical range of a 9 MeV beam is about 4.5 cm (E/2). The output
will have significant changes only when the minimum cutout dimension is less than 4.5 cm.
T6 I.
A.
The dose fall-off gradient becomes less steep with increasing energy, as does the
bremsstrahlung production (radiative collisions) from the higher-energy electrons.
T62.
B.
A star shot is taken by exposing film from a number of different collimator, gantry, and couch
positions in order to determine the radiation isocenter. An end-to-end test measures the
uncertainty for a given treatment technique by performing the technique in a phantom from
simulation, through planning, and treatment delivery. A picket fence test is used for MLC QA.
CTDI is a measure of dose from a CT scan.
T63.
B.
AAPM Task Group 142 sets linac QA tolerances as a function of intended treatment technique
on each linac with the thought that there should be an increase in accuracy and precision when
you go from non-IMRT to IMRT to SRS/SBRT treatments.
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T73.
C.
As long as the physician is approved as an authorized user, specific for the Gamma Knife by
either the NRC or the appropriate state regulatory agency, any licensed physician may
prescribe radiation treatments. Neurosurgeons, for example, may be approved as an authorized
user provided they have the proper training. In practice, however, it is difficult for anyone
other than a board certified radiation oncologist to obtain designation as an authorized user.
T74.
E.
See NCRP Report 151 section 1.4.2. The design goal is 0.02 mSv week-1 [(0.02 mSv week-1)
x (50 week yr-1) = 1 mSv yr-1], the annual dose limit for a member of the public.
T75.
A.
By applying the fraction of time the beam is on and the use factor, one determines that
25 mR/hr x 0.5 x 5 min/hr / 60 min/hr= 1.0 mR in any 1 hour. Although the circumstance that
the location is an uncontrolled area is not paramount to the answer, note that the resultant
estimated dose to this location is less than 2 mR in any hr, a public dose limit.
T76.
C.
IMRT increases the number of MU and increases leakage, but does not increase the dose to
isocenter.
T77.
C.
FMEA stands for "failure modes and effects analysis" and its applications to radiotherapy are
described in Huq et al. "The report of Task Group 100 of the AAPM: Application of risk
analysis methods to radiation therapy quality management," Med Phys 23:4209-62, 2016.
T78.
B.
RO-ILS stands for "radiation oncology incident learning system." For more details see Evans
and Ford, "Radiation Oncology Incident Leaming System: A Call to Participation." JJROBP
90:249-50, 2014.
T79.
A.
As described in Huq et al. "The report of Task Group 100 of the AAPM: Application of risk
analysis methods to radiation therapy quality management," Med Phys 23:4209-62, 2016,
RPN = 0 x S x D where 0, S, and D are the likelihoods of occurrence, clinical harm, and
detection, respectively.
TSO.
D.
Compared to performing a deformable image registration between image sets of the same
modality, inter-modality deformable image registration, such as CT to MR, is less reliable.
T81.
C.
4D CT allows determination of the tumor motion. Tumors near the diaphragm (lung, liver,
etc.) are most affected by respiratory motion.
T82.
A.
The storage requirements, processing time, and patient dose are actually increased in order to
maintain the same level of noise.
T83.
B.
A larger focal spot facilitates better heat dissipation, which is required when high mAs
(milliamp seconds) exposures are required. Otherwise the tube would overheat. In all other
respects, however, a large focal spot image is inferior in image quality to a small focal spot.
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T84.
B.
T l relaxation is related to relaxation in the z-direction, and T2 relaxation is related to
relaxation in the x-y direction.
TBS.
C.
X-rays in the kV range have interactions with tissue that are predominantly photoelectric
effect, which is proportional to z3.
T86.
E.
The wavelength 0-) is 'A,= elf, where c is the velocity of the wave and/ is its frequency. The
average velocity of ultrasound in soft tissue is 1540 mis. Therefore, 'A, = c If= 1540 (m/s) I
5 x 10 6 Hz = 0.00031 m = 0.31 mm.
T87.
A.
The CT and PET portions of the diagnostic PET/CT share DICOM coordinates. In other
words, the images are acquired and automatically aligned based on the couch positions, under
the assumption that no patient motion occurred between scans. To correctly fuse the simu­
lation CT and the diagnostic PET/CT, the CT from the diagnostic PET/CT scan is registered to
the simulation CT. That registration matrix is then applied to the PET from the diagnostic
PET/CT so that the PET information can be shown overlaid upon the simulation CT.
T88.
B.
The width of the CT window is a measure of the range of CT number values displayed. When
the window is narrow, as in picture a, the visual transition from dark to light structures in the
image will happen quickly. Narrow windows are used when trying to differentiate between
tissues with similar attenuation properties, whereas wide windows are used in areas where
tissues with very different attenuation properties are adjacent to one another.
T89.
C.
The SNR increases as the number of photons absorbed by the detector increases. Increasing
the mAs and the slice thickness will both increase the number of photons incident upon a
particular detector. However, increasing patient thickness will result in greater attenuation of
the x-ray beam, resulting in fewer photons reaching the detector.
T90.
A.
A V20 of 25% means 25% of the volume receives at least 20 Gy, or 75% of the volume
receives at most 20 Gy.
T9 I •
B.
Since brachytherapy sources are implanted into the patient, the patient motion errors are
smallest. The proton range depends critically on radiological path length, and changes in
patient position could have a large effect on the types of tissues traversed by the beam.
Although photons are less sensitive to radiological path length than protons, IMRT x-rays
result in larger errors than 3D conformal x-rays because of the concave dose distributions
created by IMRT.
T92.
E.
The misalignment between the radiation and imaging isocenters is a systematic error that
cannot be corrected with image guidance.
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therapy answers
T93.
A.
Each image-guided treatment system performing patient positioning and/or repositioning
based on in-room imaging system, either 2D or 3D, relies upon vendor software that compares
and registers on-board images and reference images. Quality assurance of this process could
be easily done by a phantom study with known shifts, and this is recommended for each
system used clinically. The accuracy of this process should be tested on the daily basis,
especially for SRS/S BRT.
T94.
A.
Flattening filter free beams are capable of higher dose rates, which counteract the low duty
cycle.
T95.
E.
Images should be acquired during the respiratory gate in order to validate the correlation
between the external gating signal and the internal anatomy. The setup alignment should be
based on fiducial markers, high-contrast objects that move with the tumor. Orthogonal
kilovoltage images cannot reliably resolve soft tissue.
T96.
B.
kV fluoroscopy can be used on a linac gantry-mounted kV system in order to verify that an
internal fiducial or landmark, such as a stent, is in the correct place when the external marker
used for respiratory gating would trigger the MV beam on. Since fluoroscopy uses the kV
beam longer than a single radiograph, the dose is greater. In a linac-mounted kV imaging
system, the kV beam is orthogonal to the treatment field, so it cannot image the treatment
aperture, and the MV beam would be used for that.
T97.
A.
Linac gantry-mounted kV imaging systems are capable of radiography, fluoroscopy, and cone­
beam computed tomography (CBCT). An image is generated by the flat panel area detectors
mounted opposite the x-ray tube and can be used for target localization before (radiography
and CBCT) and during (fluoroscopy) treatment. However, fluoroscopy-based tracking systems
are not commonly used due to the potential for excessive radiation exposure.
T98.
D.
Since an IMRT fluence pattern is designed to be heterogeneous, the uniformity of the beam
incident upon the MLC is unimportant.
T99.
D.
During the optimization, various fluence patterns are checked to calculate the objective
function and minimize it. The beam energy and VMAT arc length are usually manually
determined prior to an optimization. The PTV is the target defined by the physician, which is
an input to the optimization.
TI 00.
A.
For the same patient geometry, more monitor units usually implies smaller average field size
during treatment delivery and therefore more modulation.
TIO I.
D.
Such a treatment would require at least 10 steps with field widths for each step of 10, 9, 8, ....,
2, and 1 cm, respectively. Even with 10 steps, the dose intensity variation would not be smooth
across the field, but rather would have discrete steps of 3% every cm. Thus, more than 10 steps
would result in a smoother dose gradient.
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TI 02.
D.
Increasing the number of fractions and the number of treatment fields or arcs will decrease the
likelihood of synchronization between the MLC motion and the target motion. Even if it is
synchronized for a particular fraction or beam, the effect would get washed out over the course
of many fractions or beams.
TI 03.
B.
The MU for a VMAT plan is generally lower than that for a static IMRT plan for the same
patient target and anatomy because VMAT treatments usually have larger beam apertures and,
thus, higher MU efficiency.
TI 04.
A.
Contribution of each objective to the total objective function is proportional to its priority.
TI 05.
A.
By allowing the target heterogeneity to increase, you can prescribe to an isodose line closer to
the penumbra of each beam, minimizing the amount of normal tissue exposed in the beam's­
eye view projection of each treatment field.
TI 06.
E.
Assuming the treatment plan is properly calculated, the range of a proton beam can be
precisely controlled, thus minimizing the heart dose for proton therapy.
TI 07.
A.
The spoiler needs to be placed close to the patient in order to increase the surface dose. If it is
placed too far away, an insufficient number of electrons will reach the patient. Lungs are at the
greatest risk for complication, and they are usually blocked. Patient separations are generally
smaller in the anterio-posterior direction, so the dose homogeneity will be better with that
beam arrangement. Although the light and radiation fields may be congruent at 100 cm SAD,
this is not the case for extended distances.
TI 08.
E.
Treatment time is quite long, which can be a disadvantage since this population of patients
is often physically weak. However, some TBI treatment protocols actually recommend a
lower dose rate. Extended SSDs are used in order to cover the patient in a single field. A
consequence of extended SSDs is increased dose uniformity due to the inverse square law. The
dose rate decreases with an increase in SSD.
TI 09.
A.
Currently, the administered activity is 1.49 µCi/kg.
TI I 0.
C.
Y-90 undergoes beta- decay with a decay energy of 2.28 MeV. 0.01% of the decays produce
1.7 MeV photons.
TI I I.
A.
Radium-223 goes through a decay chain yielding 4 alphas, 3 betas, and numerous gammas.
About 95% of the dose to the target cells is from alpha particles.
TI I 2.
E.
Therapeutic electron beams are typically of energies between 4 and 20 MeV, which translates
to treatment depths between 1.3 cm and 7 cm. Therefore, they are not typically used for lesions
that could potentially be deeper than this. However, total skin irradiation is a good site for an
electron beam because it treats the entire skin while keeping the whole body dose low.
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