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preface
The RAPHEX 2009 Exam Answers book provides a short explanation of why each answer is correct, along
with worked calculations where appropriate. An in-depth review of the exam with the physics instructor is
encouraged.
In cases where more than one answer might be considered correct, the most appropriate answer is used.
Although one exam cannot cover every topic in the syllabus, a review ofRAPHEX exams/answers from three
consecutive years should cover most topics.
We hope that residents will find these exams useful in reviewitig their radiological physics course.
RAPHEX 2009 Committee
Copyright © 2009 by RAMPS, Inc., the New York chapter of the AAPM. All rights reserved. No part
of this book may be used or reproduced in any manner whatsoever without written permission from the
publisher or the copyright holder.
Published in cooperation with RAMPS by:
Medical Physics Publishing
4513 Vernon Boulevard
Madison, WI 53705-4964
1-800-442-5778
E-mail: mpp@medica1physics.org
Web: www.medicalphysics.org
therapy answers
Tl.
E
Tl.
B
An element is defined by the number of protons in its nucleus. Protons and electrons must
be equal in number in a neutral atom, so it is the number of neutrons that differs between
isotopes of the same element.
Tl.
A
In the symbol, the subscript is the atomic num6er, Z, which is the number of electrons.
The number of protons must be equal to Z to balance positive and negative charges. The
superscript is the mass number, A, which is the total number of protons and neutrons, N,
in the nucleus. Thus N =A - Z = 14 - 6 = 8.
T4.
D
Iodine-125 decays by electron capture to an excited state of 125Te. From this it decays
to the ground state by emission of 35.5 keV gammas. Internal conservation gives rise to
characteristic x-rays that have energies in the range of 27 to 35 keV
TS.
D
After n half-lives, the activity of a source is reduced to (l /2t times its initial activity.
T6.
A
The treatment time increases by the inverse of the source decay. Decay time = 14 days.
Half-life= 74 days. Time= 282 s x exp(0.693 x 14/74) = 322 s.
T7.
D
Bremsstrahlung x-rays of any energy up to that ofthe projectile electron can be created.
For characteristic x-ray emission, the electron must have an energy greater than or equal to
the binding energy of an orbital electron. An electron from an outer shell then fills the
vacancy, and a characteristic x-ray of energy equal to the difference in electron binding
energies is emitted.
T8.
B
T9.
B
Electrons travel from the cathode to the anode.
TIO.
A
Increasing filtration in a beam decreases the total number of photons, and hence
decreases the dose rate. However, filtration decreases more of the low-energy photons,
thus increasing the HVL.
Til.
D
Co-60 does emit betas, but these are absorbed in the source housing.
Raphex 2009
therapy answers
Til.
B
The beam current is higher in the photon mode. The bending magnet is fixed in position
and is required to point the electrons toward the isocenter. The thicker the scattering foil,
the greater the number of interactions and the higher the percentage of bremsstrahlung.
Cone design and collimator settings are both important for electron beam flatness.
Til.
C
When the linac is calibrated, 1 MU is adjusted to be equivalent to a specific absorbed dose
at a reference point, e.g., 1 cGy at the isocenter, at depth <Imax for a 10 x 10 em field. The
monitor chamber monitors the beam during treatment, by collecting charge as the beam
passes through the chamber. Thus, variations in absolute dose rate do not affect the dose
delivered. Symmetrical segments of the chamber are also used to monitor beam symmetry
during treatment.
Tl4.
A
The light field matches the point in the penumbra of the photon beam that is 50% of the
dose on the beam axis.
TIS.
A
Cerrobend blocks generally have slightly greater primary transmission than MLCs. The
greatest disadvantage of MLCs is the stepped edge, which can create a broader penumbra
and which may not conform as well to the required block shape.
Tl6.
C
Leaf calibration accuracy is critical for the accuracy of IMRT delivery.
Tl7.
E
Imagers are generally smaller than film, requiring multiple overlapping images for large fields.
TIS.
D
Although this feature can be used for diagnostic scans, all therapy simulation scans are
taken in a plane perpendicular to the couch.
Tl9.
C
Even at low kV, coherent scatter contributes only a small part of the total scatter. The
characteristic x-rays created by photoelectric interactions within the patient are of very low
energy (because of the low Z of tissue) and have an extremely small range. Compton electrons and photoelectrons also have a short range and are unlikely to leave the patient's body.
TlO.
C
Although pair production becomes possible at 1.02 MeV, Compton scatter is the most
probable interaction in soft tissue between 25 ke V and 25 MeV
Tll.
B
Although there are numerically more Compton than photoelectric interactions at typical
CT energy (120 kV), the probability of a Compton interaction per gram of tissue is
independent of Z (with the exception of hydrogen), whereas the probability of a
photoelectric interaction is proportional to Z 3, causing a large differential attenuation
for higher Z materials such as bone and contrast media.
2
Raphex 2009
therapy answers
Tll.
B
By similar triangle geometry, 40 em projects to 180 em at (180/40) x 100 = 450 em. The
dose rate at 450 em (by the inverse square law) is (100/450f x 600 = 29.6 cGy/min.
Tll.
C
HVL x !l = 0.693.
T24.
B
CT number= 1000 X [(J.lmaterial - !lwater) I !lwater1 where !l is the linear attenuation
coefficient.
TlS.
B
Electrons lose most of their energy in soft tissue by ionization and excitation of the tissue
atoms. Collisions with atomic nuclei resulting in radiative losses are also possible, but less
likely in low-Z media.
T26.
D
Charged particles are directly ionizing, while neutrons are indirectly ionizing: their energy
is given to charged particles, such as protons, which then directly ionize the atoms along
their paths.
T27.
A
Neutrons have no charge, and are indirectly ionizing. They transfer energy to protons,
which have a large mass, and are densely ionizing, especially at the ends of their tracks
(the "Bragg Peak").
T28.
B
The average energy is about 2 MV
T29.
B
Exposure, the ability of photons to ionize a mass of air, is only defined for photons below
3 MeV, and not for particulate radiation. The absorbed dose resulting from an exposure of
1 R depends on the photon energy, and the effective Z of the absorbing material, which is
higher for bone than for muscle. The roentgen, R, is a unit of exposure; the SI unit is C/kg.
T30.
C
1.0 Gy = 1.0 Sv I Quality Factor.
Tll.
B
Kerma (Kinetic Energy Released per unit Mass) is a maximum at the surface and decreases
with depth, since the photon fluence decreases. The kerma curve is slightly lower than the
depth dose curve beyond dmax·
T32.
E
The f-factor, also known as the "roentgen-to-rad" conversion factor is 0.876 in air, under
conditions of electronic equilibrium. For other media it varies with photon energy and
effective Z. For water at 1 MeV, f= 0.971.
Raphex 2009
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therapy answers
TJJ.
D
All diagnostic and therapeutic photon and electron beams have QF = 1.0. For neutrons
it is higher, but the value depends on the neutron energy. High-energy charged particles
(protons, alphas) also have QF > 1.0, since it is related to LET and RBE.
T34.
C
Kenna is an acronym for Kinetic Energy Released per unit MAss. It is defined as the sum
of the initial kinetic energies of all the charged particles liberated by indirectly ionizing
radiation in an element of material with unit mass.
TJS.
E
For cylindrical chambers in photon beams, the effective point of measurement is
0.6 rcav upstream from the chamber center.
T36.
C
Unlike the previous TG-21 protocol, TG-51 protocol mandates an annual calibration in
water only. Monthly QA can be performed in another phantom such as Solid Water®, but
the appropriate factors must be determined by intercomparison at the time of the calibration in water.
T37.
D
It is good practice to calibrate the output monthly and to adjust the calibration of the monitor
chamber if it has drifted by more than 2%. (Large changes, however, should be investigated
to determine the cause.) Daily output checks should also be done by a fast, simple method. If
they are outside a given range, an investigation is required to solve the problem.
T38.
A
Currently, chambers and electrometers must be calibrated every 2 years at an ADCL.
T39.
A
T40.
C
Thermoluminescent dosimeters require a reader in which the TLD is heated and the light
emitted is collected, so there is a delay between measurement and results. However their
small size and tissue equivalence makes them useful as in vivo dosimeters.
T41.
E
It requires a higher dose than radiographic film. At present, the disadvantages of
radiochromic film are that it is more expensive, it only comes in limited sizes, and
it has a slightly noisier image than radiographic film.
T42.
D
The PDD increases with increasing SSD, according to Mayneord's f-factor, because the
inverse square factor becomes relatively less important at greater SSD. The depth dose
curve increases between the surface and <Iroax, then decreases approximately exponentially.
PDD increases with increasing field size because the proportion of dose due to scatter
increases with depth and does so more quickly as field size increases.
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Raphex 2009
therapy answers
T43.
A
As SSD increases, PDD increases, because of the difference in the inverse square
component of the PDD. The correction factor is
[(SSD + d)/(SSD + <Imax)f X [(SSDext + <1max)/(SSDext + dW.
In this case: (106/101.6)2 x (136.6/141) 2 = 1.022.
T44.
D
The equivalent square of a rectangular field is that square field which has the same scatter
contribution on the axis, and hence the same POD and TMR. One square centimeter near
the axis contributes more scatter than the same area at the corner of the field. Hence, the
equivalent square of a rectangle has a smaller area than the rectangle. An approximate rule
for the side of the equivalent square is 4 x NV where A = area, P = perimeter.
T45.
D
TMR is independent of SAD. It is the ratio of dose rate at depth (d) to dose rate at
depth <Imax, both measured at the same distance.
T46.
D
The fraction of dose due to scatter increases with both depth and field size.
T47.
B
The hot spot will now be (107/95) = 112.6% of the prescribed dose of 4500 cGy.
T48.
D
The equivalent square of 6 x 28, using 4 NP, is 9.9; use 10.
MU = [dose/fx]/[(cGy/MU at SSD) x (PDD/100)]
= 180 I (1.0 x 0.871) = 207 MU.
T49.
C
Dose at dlldose at d2 = PDD1/PDD2. Dose at <Imax = 180 x (100/87.1) = 207 cGy.
TSO.
A
Equivalent square of 18 x 22 = 20.
MU = [dose/fx]/[(cGy/MU at SAD) x TMR x TF]
= 125/(1.089 X 0.869 X 0.96) = 138 MU.
TSI.
C
The higher the photon energy, the lower the total dose at dmax· However, AAPM's Report
No. 85, which discusses inhomogeneity corrections, recommends treating lung lesions with
an energy of 12 MeV or less, because of the possibility of lack of buildup on the surface of
a lesion adjacent to lung with higher-energy photons.
T52.
A
The flattening filter "overfiattens" at shallow depths, and ''underfiattens" at greater depths.
The dose at <Imax is therefore greater toward the edges of the beam, and this effect increases
with field size.
Raphex 2009
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therapy answers
TSJ.
B
A midline block makes a much greater change in the equivalent square than does a corner
block of the same area. For midline blocks, in fact, the 4AIP rule does not accurately predict
the true equivalent square. A Clarkson type SAR calculation is the best way to calculate
equivalent squares for such fields. Note that under a cord block, the dose is higher than the
block transmission alone, due to scatter from the surrounding field.
T54.
C
The 90% isodose is shallower than dmax, so bolus less than dmax thickness can be used.
Also, the dose in the build-up region is slightly enhanced by the exit dose when treating
with parallel-opposed fields, pushing the 90% isodose closer to the skin.
TSS.
C
The PDD is greater for 18 MV than for 6 MV at 5.0 em depth. Therefore the MU required
for 300 cGy would be lower, and the dose delivered using the same MU will be higher. The
exit dose will be higher, due to the greater depth dose, and the dose uniformity across the
vertebral body will be greater.
T56.
D
Wedge angle= 90°- (Hinge angle/2) = [90°- 30°] = 60°.
This holds if the surface is perpendicular to the beam axis; in practice, the wedge angle
may also have to compensate for an air gap.
T57.
E
With a dynamic wedge the wedge factor (measured on the beam axis) is a function of both
the starting and ending moving jaw position, i.e., both field width and offset.
T58.
D
10 em of lung is approximately equivalent to 3 em tissue, and 10 - 3 = 7 em of
"missing" tissue. Thus the dose beyond the lung is increased by approximately
7 x 3.5 = 24% (ignoring scatter effects).
Transmission corrections are greater at lower energy, where the attenuation per centimeter
of tissue is greater.
T59.
C
Historical data reported in the literature without inhomogeneity corrections actually used a
variety of beam energies and path lengths through inhomogeneities. The doses reported
therefore included these uncertainties. Although different planning systems use different
algorithms, the use of corrections is expected to reduce the uncertainty in dose reporting.
T60.
A
The increased transmission through lung at the chest wall side of the field generally
requires a smaller wedge for a homogeneous distribution. The effective patient thickness
will be less, resulting in a lower maximum tissue dose.
T61.
C
Using similar triangle geometry:
gap= (1/2)(colll + coll2) x (match depth/SAD)= 0.5 x 53 x 6/100 = 1.6 em.
6
Raphex 2009
therapy answers
T62.
D
The angle required to rotate the brain fields so that their inferior edge matches the
divergence of the upper border of the spine field is: tan- 1 [(36/2)/( 100)] = 10 degrees.
T63.
B
2 Gy out of 40 Gy is 5%. This occurs at about 2 em from the field edge.
T64.
C
The dose to the fetus depends on its distance from the field edge, but from 10 to 20 em the
dose at 10 em depth is between about 2% and 0.6% of the dose on the beam axis. (Ref:
AAPM Report No. 50, "Fetal Dose from Radiotherapy with Photon Beams," AAPM
Radiation Therapy Committee Task Group 36, Reprinted from Med Phys 22(1):63-82,
1995.) The dose is made up of patient scatter, head leakage, and radiation scattered from
the collimators and blocking tray.
T65.
A
Unlike photons, the surface dose increases with energy for electrons.
T66.
B
x:ray contamination is mostly due to interactions with high-Z components in the head of
the accelerator, such as the scattering foil, collimators, etc.
T67.
D
This is important when (a) ensuring adequate coverage of the tumor volume with
the 90% isodose curve at depth, (b) avoiding adjacent structures with the penumbra,
and (c) abutting electron and x-ray fields.
T68.
A
The range of an electron beam is about half its energy (MeV), in centimeters. The 90%
depth dose occurs at about 1/4 to 1/3 of the energy, in em, depending on the linac design.
T69.
D
Superficial x-rays have 100% of the dose on the skin; photon beams have much lower
skin doses, which depend mainly on energy and field size. Scattering foils introduce more
low-energy electrons into the beam, and hence increase surface dose.
T70.
C
For lead shielding, the rule of thumb is (MeV/2) mm. (For cerrobend, 20% is added.) The
transmitted dose will not be zero, because of bremsstrahlung generated by the electrons in
the shield.
T71.
C
Backscatter from the front surface should be considered. This varies with beam energy and
depth in the beam, and should be absorbed by an appropriate amount of low-Z material on
the surface of the lead.
T72.
C
The tolerance is 2%.
Raphex 2009
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therapy answers
T73.
D
There is generally no way to prevent catastrophic failure. Daily QA aims to verify the
accuracy of items used for patient alignment, such as lasers and 001. Output can drift, and
should be verified daily. Commercial devices such as diode or chamber arrays are designed
to measure several parameters in one exposure (e.g., output, depth dose, symmetry, and
flatness constancy). The door interlock and the operation of audiovisual devices are safetyrelated checks.
T74.
A
A check sum is the binary sum of all bytes in a computer file. These are checked
periodically to ensure that data and program files on disk have not been corrupted.
T75.
D
Although dose should be minimized, it will not affect the accuracy of the dose calculation
or beam placement, as A-C will.
T76.
C
Each slice contains approximately 512 x 512 pixels, and each pixel is 1 to 2 bytes.
Therefore the total is 100 x 512 x 512 x 2 bytes = 52.4 MB.
T77.
D
T78.
B
See N CRP report 94.
T79.
A
The average annual dose to members of the public is about 1.6 mSv, excluding the
dose from radon. Medical x-rays, cosmic, internal, and terrestrial radiation each contribute
0.3 to 0.4 mSv, and nuclear medicine contributes about 0.14 mSv.
T80.
D
(See NCRP report 116.) The maximum permissible whole-body dose for a radiation worker
is 5 rem or 50 mSv per year. Most radiation workers receive only a fraction of this, due to
the principle of ALARA (as low as reasonably achievable) used when designing shielding.
T81.
D
The maximum possible installed activity (typically I 0 Ci for an 192Ir source) is used for
shielding calculations.
T82.
A
Occupancy factor is always 1.0 in a controlled area. B-E are factors that, like occupancy
factor, affect the calculation of shielding thickness.
T83.
B
Lead is inefficient as a neutron moderator, but it is placed in the door downstream from the
borated polyethylene to attenuate the capture gammas. The neutron dose is about 10 times
higher for photons than for electrons. The threshold for neutron production is 8 MeV, but
supplemental shielding is not required below 10 MeV.
8
Raphex 2009
therapy answers .
T84.
D
The main advantages of EPIDs are that the images are in digital format and thus instantly
viewable, can be digitally enhanced, are easily archived to a PACS, require no film processing, and do not require the radiation therapist to enter the treatment room.
T85.
D
Although Compton interactions predominate in number at typical diagnostic CT
energies, the probability of photoelectric interactions is proportional to Z3, creating a
large differential between high-Z dental work anc,! low-Z soft tissue. While this differential
is advantageous in plane images, it can cause problematic artifacts in a CT image.
T86.
C
Increasing mAs will make the film darker, but have no effect on contrast. Contrast is
improved by reducing scatter. Reducing the collimator setting to the minimum necessary
field of view usually has the greatest effect. If different grids are available, the one with the
greatest grid ratio will "clean up" the most scatter. If these two techniques still do not
work, for patients with very large lateral separations one solution is to take orthogonal R
and L anterior obliques, which makes use of the somewhat reduced patient thickness (if
this is the case), and avoids the bones.
T87.
C
The more slices, the larger the file size, which may slow down computation. Contouring
may take longer (however, contouring on fewer slices and using interpolation could result
in contouring taking the same time). Slice thickness greater than 3 mm will result in poor
quality DRRs, and uncertainty in the position of volume edges and of small structures.
T88.
B
PET (positron emission tomography) relies on coincidence counting of the annihilation
photons that are emitted at 180 degrees from the point where a positron and electron
annihilate. The positron emitter is labeled to a substance that is taken up by, or metabolized
by, the tissue to be imaged.
T89.
C
Unlike CT and MRI, which have pixel sizes less than I mm, PET scan resolution is limited
to a few millimeters, mainly because of the finite range of the positrons, and also because
the 511 ke V annihilation gammas are not exactly anti parallel.
T90.
A
MR images can have greater distortion than CT images because of magnetic field
inhomogeneities.
T91.
C
The high visibility of the prostate on ultrasound images created with an intrarectal
transducer make this type of imaging highly useful for prostate brachytherapy.
T92.
D
Raphe_x 2009
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therapy answers
T93.
D
Beams that appear to be optimal sometimes cannot be physically set up on the linac. For
example, some posterior oblique fields with a couch rotation could result in a collision
between the head and the couch. Plans with this type of field must be checked on the linac
before the plan is finalized.
T94.
A
When the gross tumor volume is expanded to create the clinical target volume, it will
typically be limited by physical boundaries such as bone, which will limit the spread of
disease. However, the expansion of the CTV to create the PTV is to allow for patient
movement and setup error, so this margin is not limited.
T95.
E
The edge of a photon field is the 50% decrement line (the line joining points that are
50% of the dose on the axis). Therefore the field must be wider than the volume by twice
the distance from the 95% to the 50% decrement lines. For a 6 MV beam this is typically
7 mm on each side, but may be as low as 5 mm for smaller depths. For higher-energy
beams, the penumbra is often wider, so that a slightly larger margin is required; this margin
may also increase with depth.
T96.
E
In general, a higher beam energy will deliver a more homogeneous dose distribution,
with a lower maximum dose. However, if the PTV is close to the skin or to overlying lung,
a lack of buildup may underdose part of the PTV. Also, a 15 MV photon beam delivers a
small neutron leakage dose to the patient outside the beam, while a 6 MV photon beam
does not.
T97.
B
Most binning software uses 10% incremental phases for the 4D-CT reconstruction. The 0%
is end expiration, and the 50% is end inspiration.
T98.
D
By the inverse square law, (100.5/lOO.Of = 1.01 or (100.0/100.5) 2 = 0.99.
T99.
C
TIOO. D
Orthogonal fluoro systems take only 2-D images, and therefore are no better at imaging
soft tissue than conventional radiographs. Cone beam CT (CBCT) requires approximately
3 to 5 cGy per image. Orthogonal fluoro requires only a few mR per image, but multiple
images are taken during a typical treatment. However, the dose from CBCT is usually
greater than for fluoro.
-
TIOI. C
Ultrasound, cone beam CT, or orthogonal images are used to align the patient's anatomy
before treatment. Fiducials in the prostate are helpful for localization as the prostate cannot
be seen on a radiograph, and alignment on bony landmarks does not correct for differences
in prostate position due to bladder/rectal filling. However, if the prostate moves, e.g., due to
the passage of gas in the rectum, this will only be seen if the patient is continuously monitored during treatment.
10
Raphex 2009
therapy answers
TIOl. B
Spiral CT slice thickness is rarely less than 1.0 or 1.5 mm, but CBCT can have submillimeter
slice thickness. CBCT has a larger beam area than spiral CT (fan beam), and therefore more
scatter to degrade image quality.
TI03. C
TI04. B
Although rectal (and bladder) filling may alter the position of the prostate relative to
skin marks or bony landmarks, ultrasound localization customizes the daily setup to the
actual prostate position on that day. A full bladder is, however, necessary to transmit
the ultrasound.
TIOS. D
TI06. D
T I 07. D
This is not necessarily true-it depends on the volume and proximity of OARs. For
example, prostate plans can have very homogeneous doses in the PTV.
T I 08. A
In some IMRT planning systems, an ideal solution is calculated based on the dose
constraints, then the closest "deliverable" plan is calculated based on the design of the
MLC. The deliverable plan may be "worse" than the ideal (meaning that the dose
constraints are not met as closely). However, the plan tested on the diode array is the
deliverable plan, so this test is not testing the quality of the IMRT plan, only the ability
of the linac and MLC to reproduce the plan clinically.
T I 09. D
An ion chamber can only measure one point at a time and to measure the entire fluence
pattern would be too tedious.
T II 0. B
For AP/PA fields, lung transmission blocks are required to prevent overdosing the lungs.
For lateral opposed fields, the head and lower extremities require compensators to improve
dose homogeneity.
T I I I. C
Accuracy should be better than conventional RT, due to the small size of the volume
treated. Head frames improve registration between imaging scans, treatment planning,
and treatment delivery.
Raphex 2009
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therapy answers
Till. A
Gamma Knife radiosurgery is often prescribed to the 50% isodose, which means that some
point in the volume receives 200% of the prescribed dose. Linac-based SRS is generally
prescribed to the 70%-90% isodose, giving less variation across the treatment volume.
Till. D
Using multiple fields tends to increase skin dose and decrease depth dose. In general, the
more oblique fields used, the higher the x-ray dose to the patient's whole body but the
greater the dose uniformity, except to thin body parts such as fingers and toes.
Tll4. E
TIIS. C
Activity at daylO (0.5) =Current activity (A0 ) x exp(-0.693xl0/60).
Tll6. E
Tll7. E
Unlike curies or becquerels, the air-kerma strength (AKS) gives an indication of the dose
rate at a given distance from the source. Convert dose rate in air to cGy/min at 1 m:
Dose/min = 40,000 x 1/60 x 11(1 0 4) = 0.067 cGy.
Tll8. A
The shorter half-life of 103Pd (17 days) requires a higher initial activity. The dose is
delivered in a shorter time; hence the total dose is reduced accordingly.
Tll9. A
The exposure rate is: Activity x Exposure rate constant x 1/d 2 .
= 110 X 4.7 X 1/900 = 0.574 Rfh = 574 mR/h.
TllO. C
The MPD is 5000 mrern/y or 100 mrern/wk. For protection purposes this is taken to be
equal to 100 rnR/wk. Thus the physician should not spend more than 0.5 h at this location
(assuming there are no other sources of exposure). A lead shield at the bedside would
generally be used to reduce the dose rate for ALARA compliance.
Till. A
A greater activity of 1251 is required because it has a lower exposure rate constant than 192Ir.
Till. C
Till. B
ll
Since 1251 and 103Pd emit lower energy than 192Ir, the dose per seed is more localized.
Thus, peripheral loading of low-energy seeds delivers less dose to the center of the
prostate, including the urethra. Peripheral loading slightly increases the rectal dose but
can significantly reduce the urethral dose.
Raphex l 009
therapy answers
Tl24. C
Tl25. E
The largest diameter ovoids will decrease the mucosal surface dose, but increase the depth
dose below the surface, due to the inverse square law.
Tl26. D
Reducing the activity in the region closest to the rectum also reduces dose to the cervix
and does not improve the differential between the rectum and point A. Treating with many
HDR fractions would tend to decrease normal-tissue complications but increases cost and
complexity of treatment, tending to reduce the advantage of HDR. However, because of the
shorter treatment time, it is possible to displace the rectum away from the sources with
packing or a rectal retractor, thus reducing the relative dose.
Tl27. E
Paterson-Parker tables cannot be used for seeds of equal activity. They were designed for
Ra needles, with lower activity at the center of the plane to give a more uniform dose at the
treating distance. Commercial computer treatment planning systems can calculate the 3-D
dose distribution for any seed arrangement.
Tl28. E
Tl29. D
The Transport Index (TI), which is the maximum dose rate at 1 m, must be stated on the
label, along with the radionuclide and its activity; the surface dose rate is not required on
the label.
TllO. D
Till. D
The minimum balloon-skin distance is determined initially, when planning the treatment.
If it is too small (e.g., <7 mrn), the physician may decide not to treat. Balloon dimensions
can be verified to make sure that the balloon is not leaking. This can be done once per day
for BID treatments.
Till. C
T 133. B
18F is a positron emitter used in PET. 137 Cs is a reactor fuel by-product, created by fission.
192Jr is created by bombarding 191 Ir with neutrons in a reactor. 4°K is a naturally occurring
isotope of potassium, present in the human body.
T ll4. B
Raphex 2009
Answer A might give a reasonable value, but iodine can be cleared by other routes. Also,
the question asked for a calculation ofTb prior to administration.
ll
therapy answers
TIJS. A
Historically, the Nuclear Regulatory Commission (NRC) and Agreement States required
patients receiving radionuclide therapy to remain hospitalized until the retained activity in
the patient was less than 30 mCi or the dose rate at 1 m from the patient was less than
0.05 mSvlh (5 mremlh). However, in 1997 the NRC amended its regulations to allow for
the release of patients if the expected total effective dose to individuals exposed to the
patient is not likely to exceed 5 mSv.
Tl36. B
Microwave radiation is used to increase the temperature of the tumor and its immediate
vicinity, shortly after administering radiation therapy or chemotherapy. Special applicators
produce a microwave field for heating deep-seated tissues, but do not directly transfer their
own heat to superficial tissues.
Tl37. A
There can be significant neutron contamination in a passively scattered proton beam,
produced by scatter in collimators and energy degraders. This is reduced in a spot-scanned
proton beam. The lateral penumbra of a proton beam is superior to that of a photon beam
at shallow depths, but comparable to, or even wider than photons at very large depths.
Tl38. A
This is the only advantage for prostate treatment with protons.
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Raphex 2009