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preface

The RAPHEX 2011 Therapy 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 of RAPHEX exams/answers from three consecutive years should cover most topics.

We hope that residents will find these exams useful in reviewing their radiological physics course.

RAPHEX 2011 Committee

Copyright 2011 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@medicalphysics.org Web: www.medicalphysics.org

Tl. C

Tl. c

Tl. B

T4. A

TS. c

T6. c

T7. E

T8. c

T9. B

TIO. A

Til. A

Raphex 2011

therapy answers

The end result, in beta-minus decay, is a neutron converted to a proton and an electron. Thus, Z increases by 1, but the mass number, 60 in this case, remains the same. 60Ni is stable after the emission of 1.33 and 1.19 MeV gammas.

The total activity implanted must be the same.~The activity per seed will decay as: A= Ao x 0.5

therapy answers

Til. A

Til. D

Tl4. D

TIS. C

Tl6. D

Tl7. A

TIS. B

Tl9. C

TlO. C

Tll. B

Tll. C

Tll. A

T24. E

2

It would be appropriate to contour, and reassign the density of, any structures that do not represent the actual density of the patient at the time of treatment.

Photoelectric interactions increase with the 3rd power of the atomic number. Bone, with a significant amount of calcium, has a much higher photoelectric cross section than soft tissue.

The probability of the photoelectric effect per unit mass is proportional to Z 3

There is no energy loss in coherent scatter.

The Compton photon can be scattered at any angle, but the Compton electron is emitted at an angle limited to 0-90 with respect to the direction of the incident photon.

The kinetic energy of a photoelectron is equal to the photon energy-the binding energy of the electron.

The threshold is 1.02 MeV, equal to the sum of energies of annihilation photons, each of 0.511 MeV. The incident photon energy is transferred to the rest mass energies of the electron and positron plus their kinetic energies.

The probability is proportional to E-3 except when photon energy is near the electron shell binding energy.

Cerrobend blocks are both better focused and closer to the patient than MLC leaves, and therefore reduce penumbra.

Raphex 2011

TlS. D

T26. B

T27. A

T28. B

T29. A

TJO. B

TJI. C

Tll. B

TJJ. B

T34. D

Raphex 2011

therapy answers

The attenuation coefficient in lead, for photon energies from 100 ke V to 20 MeV, initially decreases with increased energy due to Compton interaction, then increases as pair production becomes more prevalent at higher energies. As the attenuation coefficient decreases, HVL increases.

For a 16 MV photon beam, 1 em depth is in the build-up region. Kerma (kinetic energy released to charged particles per unit mass) is greater than the absorbed dose in the build-up region.

HVL = 0.693/Jl.

Neutrons 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" for protons).

Coherent scattering does not change the beam energy. The pair production threshold is above the energy of a typical diagnostic x-ray unit. The Compton effect may slightly increase the effective beam energy but not nearly to the extent of the photoelectric effect, which has Z3/E3 dependence.

The uncertainty is given by the square root of the number of photons detected. There are 10,000 counts detected. The square root of 10,000 is 100, so the percent uncertainty is (100/10,000) X 100% = 1%.

If the voltage is too low, less charge is collected because ion recombination increases, and the reading will be low.

The AAPM recommends the use of a parallel-plate chamber for calibration of a 6 MeV electron beam. The parallel-plate chamber perturbs the electron beam less than a cylindrical chamber. Diodes, although small, are not used for direct calibration of any beam.

3

therapy answers

TJS. A

T36. E

T37. D

T38. C

T39. D

T40. D

T41. E

T42. C

T43. D

4

The charge collected is proportional to the mass of air in the chamber. As temperature increases, or pressure decreases, the air expands and the mass of air in the chamber decreases. Chambers are calibrated at standard temperature and pressure (22 C and 760 mm Hg), and the chamber reading must be corrected to the value that would have been obtained at 22 C, 760 mm.

A and C are commonly used for the calibration geometry, but there is no rule that states that a particular geometry must be used to define 1 MU. It is vital, however, that all data tables, treatment planning systems, and MU checking programs use data consistent with each other, and with the department's definition of 1 MU.

This can be verified by calculating a simple plan, delivering it to a phantom, and comparing the measured dose in the phantom with that prescribed in the plan and that obtained with hand calculations.

Parallel-plate chambers can also be used.

The HVL (expressed as mm of Al or Cu) defines the penetrability of a low-energy x-ray beam. Different combinations of kVp and filtration can produce beams with the same HVL, and hence the same depth dose characteristics. The SSD also affects the PDD and is important for superficial x-ray units that typically treat at short SSD.

Although Monte Carlo is the most accurate, it also takes the most computer calculation time and is therefore not yet universally used for treatment planning.

DVH plots show all statistical parameters about dose to a structure but provide no information of spatial position of dose values.

For points that lie beyond the inhomogeneity, the predominant effect is the attenuation of the primary beam. The changes in the secondary electron fluence affect the tissues within the inhomogeneity and at the boundaries.

D95 is typically used to evaluate target coverage, not hot spots which are often evaluated using the 0 05 0 95 should therefore be high for targets and low for normal tissues. D05 should ideally be no more than 110% of the D95 , which means that there is good dose homogeneity within the PTV. All doses in OARs should be as low as possible.

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T44. B

T45. D

T46. B

T47. B

T48. B

T49. C

TSO. B

TSI. B

TS2. D

TSJ. B

Raphex 2011

therapy answers

CSeq: 16x14 = 14.93, Sc = 1.015, FSeq 10x10, SP = 1.0 TMR (d = 8, 10.0) = 0.842 MU = 150/(1.0 X 1.015 X 1.0 X 0.842) = 176.

CSeq: 15x10 = 12, Sc = 1.006, SP = 1.007, PDD(5,12) = 0.873 Output= 0.971 cGy/MU MU = 300/(0.971 X 1.006 X 1.007 X 0.873) = ~49.

Typically, breasts with a separation over 25 em require higher energy photons to keep the highest dose below 110%. However, this should be balanced against lack of dose in the build-up region. The effective depth through lung at the chest wall will determine the maximum dose.

At any interface between high- and low-density tissues, there are dose build-down and build-up effects, which may result in lower dose to the surface of a lung tumor. This effect is worse for higher energy beams and AAPM recommends using

therapy answers

T54. D

TSS. B

T56. D

T57. D

T58. B

T59. C

T60. C

T61. D

T62. E

T63. B

T64. C

T65. A

6

By similar triangle geometry: (size on wall/size at isocenter) = (source to wall dist./source to isocenter dist.)

= 500 cm/1 00 em. Thus, size on wall = 40x5 em = 200 em. (Note: The distance to the wall must be measured from the source, not the isocenter.)

The equivalent square is equal to 4 x area/perimeter = 2 x AxB/(A+B).

The surface dose will be slightly less at extended SSD.

All isodose values and MUs will be increased by 5%.

Attenuation is about 3.5% per em for a 20x20 em 611V photon beam at d = 12 em. An increase of 4 em total, or 2 em per beam, will reduce the dose at the isocenter by about 7%.

The attenuation per centimeter at 6 MV is about 3.5% (B, D, and E). A is about 4%: 2% for the change in TMR and 2% for the output. C involves an inverse square correction of approximately (110/100? = 1.21.

The opposite is true. In a 3-field plan, the thick ends of the wedges point towards the 3rd field.

Because the wedging is achieved by closing a jaw, there is no limit to the collimator setting in the non-wedge direction. This is an advantage over physical wedges, for which size and weight can limit this dimension.

Divergence = tan-1(9/100) = 5 for each field. To eliminate divergence, the RPO gantry angle= 60 + 180- (2 x divergence)= 230.

2 Gy out of 40 Gy is 5%. This occurs at about 2 em from the field edge.

Target and flattening filter are used in X-ray mode.

A pencil electron beam, after passing through scattering foils, is spread into a broad beam that appears to diverge from a point that is closer to the patient than the photon beam source.

Raphex 2011

T66. c

T67. c

\ T68. c

T69. D

T70. B

T71. D

T72. D

T73. A

T74. c

T75. B

T76. D

T77. C

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therapy answers

A typical 6 MeV beam delivers over 90% in the build-up region at d = 1 em, 100% at d = 1.4 em, and 90% at d = 2 em. At d = 3 em, the depth dose is about 10%.

Theoretically, the range (6 em) would be the correct answer, but in practice its more like 5x5 before the 90% starts to decrease in depth.

The 90% depth dose occurs at about E(Me V)/3 em, except for very small fields.

Electron interactions with high-Z materials in the head of the linac generate bremsstrahlung x-rays.

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.

This is an advantage of superficial x-rays over electrons.

Ref: Klein et al. "Task Group 142 report: Quality assurance of medical accelerators." Med Phys 36(9):4197-4212, 2009.

However, the same report allows daily output measurements to be 3%.

Ref: Klein et al. "Task Group 142 report: Quality assurance of medical accelerators." Med Phys 36(9):4197-4212, 2009.

Collision interlocks and positioning/repositioning are also listed as daily tests.

The amount of radiation that delivers 1 Gy to water will only deliver 0.99 Gy to muscle. Therefore the beam-on-time needs to be increased by 1%.

The information in A, B, and Care sent with DICOM. DICOM-RT also sends the data in D.

Radiation workers can get a maximum of 50 mSv per year (or about 1 mSv per week), while the dose to the general public is restricted to 1 mSv per year (or 0.02 mSv/week). Radiation shielding is usually designed using the ALARA (As Low As Reasonably Achievable) principle, which suggests further reducing the dose to radiation workers by a factor of 10, making it 0.1 mSv/wk.

7

therapy answers

T78. B

T79. D

T80. B

T81. D

T82. D

T83. B

T84. A

T85. A

T86. C

T87. A

T88. D

T89. C

8

Attenuation of x-rays in the MV range is almost independent of atomic number and is determined mostly by equivalent path length (i.e., gm/cm2).

192Ir has an average energy of approximately 340 keV, which is too high to be effectively shielded by lead aprons, so they are not required for the patient. An advantage of remote afterloaders is that staff are not exposed.

In current NRC regulations the definition of a medical event includes thresholds for both an absolute dose error and a percentage difference from the prescribed dose. Total dose errors of 0.5 Sv to an organ, tissue, or skin and 20% errors in total delivered dose (or 50% in a single fraction) are reportable. (Errors of 10% are recordable.) See 10 CFR 35.3045: Report and notification of a medical event.

In borated polyethylene the polyethylene moderates the neutrons to thermal energies, and they are then captured by the boron, providing very effective neutron shielding. The outside of the door must also contain lead or steel to attenuate the gammas produced by this neutron capture event in boron.

The image resolution will degrade because of the blurring caused by lung motion.

Although treatment time may be increased, gating allows the beam to be turned on only during a specific fraction of the breathing cycle, when tumor motion is limited. Without gating, the lTV must include the full range of motion of the tumor.

X -rays produce electrons in the metal plate, and the electrons produce light in the phosphor screen.

The scatter increases the signal, noise (square root of signal), and thus SNR (square root of signal). The scatter degrades CNR because it increases the noise.

Raphex 2011

T90. A (

T91. C

T92. B

T93. B

T94. B

T95. B

T96. B

T97. C

T98. C

Raphex 2011

therapy answers

Generally, CBCT is acquired in one single full or partial rotation of the kV tube. Because it is a cone-beam acquisition, resolution in the cephalocaudad direction is superior or similar to a regular multislice scanner, as is dose. On a linear accelerator, the length of the scan volume is limited by the size of the kV detector and the single rotation acquisition. Generally, CBCT over lengths greater than approximately 15 em is not currently possible.

.

Small metallic fiducials are clearly visible with both 2D and 3D imaging techniques. 2D imaging techniques are not generally used to determine rotational errors. Only CBCT can be used to visualize soft-tissue anatomy.

The reconstruction CBCT volume increases with the imager size. The half-fan CBCT scan essentially increases imager size in the right-left direction and therefore increases the reconstruction volume in the axial plane.

Only B can provide 3D information in real time. A single kV or MV source can only provide real-time 2D information, while tomotherapy or CT units cannot provide real-time information.

CyberK.nife uses real-time orthogonal x-ray imaging.

CT-on-Rails provides the best image quality but is arguably the most inconvenient solution. kV image quality is always better than MV, and fan beam CT is always better than MV because of decreased scatter dose.

The PTV is larger than the CTV, which is larger than the GTV. CTV includes microscopic spread of tumor beyond GTV, and PTV must include setup uncertainties. PTV s need not be connected. Margins may differ in different directions depending on patient and tumor geometry and anatomy.

For certain treatment sites MR provides superior soft-tissue delineation as compared to CT, and modem treatment couch top additions allow the patient to be set up accurately in the treatment position. MRI images, however, still may have geometric distortions due to magnetic field perturbations, and also do not provide information on electron density or Hounsfield numbers needed to make inhomogeneity corrections in treatment planning.

9

therapy answers

T99. D

TIOO. D

TIOI. D

TIOl. C

TIOJ. B

TI04. B

TIOS. C

TI06. D

TI07. B

TI08. A

TI09. D

TIIO. E

10

The location of the RF beacons should be detected even if they move with breathing. In fact, that is one of the advantages of this type of tracking: the patient can be monitored during treatment, and the beam can be turned off if the beacons move outside a predetermined range.

Each MV portal image will require 2 to 3 MU, for a total of 160 to 240 MU over the course of treatment. Assuming 65% depth dose at 10 to 15 em, this will result in approximately130 cGy total dose at the isocenter.

DRRs typically have poorer resolution in all directions than do conventional simulation films, especially in the cranialcaudal direction because of CT slice thickness.

MV beams have greater penetrating power and thus deliver a higher midplane dose.

Within limited dose ranges, monitor unit settings for IMRT treatment can scale proportionally with dose, just like MU for non-IMRT treatments.

For many treatment sites, single arc IMAT usually requires fewer MU than IMRT, but the quality of the plans is highly variable.

Beam weighting is determined by the TPS based primarily on the treatment planning objectives specified.

It is imperative that the dosimeter used for these measurements be smaller than the radiation field. Otherwise the dosimeter will under-respond and patients will be overdosed. A 0.6 cc Farmer chamber has a length of at least 1 em and is too large.

This definition of CI only takes the respective volumes into account, not the amount of overlap between them.

~

Raphex 2011

Till. D

Till. A

TIIJ. D

Tll4. B

TIIS. A

Tll6. D

Tll7. E

Tll8. C

Raphex 2011

therapy answers

The projection of the largest jaw size must fall completely inside the circle, or radiation will leak through the aluminum plate and irradiate the patient. Further, since the aluminum plate blocks the light field but not the radiation field, it is possible to not notice this error.

2A2 = 102

The 10 em thick cone will attenuate a 6 MV beam to 1%. The proximity of the cone to the isocenter, 30 em, further reduces geometric penumbra.

About 80% of 6 MV radiation will be transmitted through 2 em of aluminum. The Winston-Lutz test, although intended for determining target accuracy, will show areas of leakage if the jaws are set too large, provided large enough film is used. Even with the jaws set so that there is no leakage beyond the cone, output (dose/MU) will be affected if the jaws are not set to the correct size. Once set and verified, jaw motion should be disabled to prevent the therapist from inadvertently changing jaw size while using the pendant to position the patient and gantry.

The conformality index in radiosurgery is defined as the tissue volume receiving the given dose over the target volume encompassed by the same dose. The amount of target volume receiving at least 14 Gy is 0.95 x 23.5 = 22.3 cc and hence the volume of brain tissue receiving at least 14 Gy is 1.3 x 22.3 = 29.0 cc.

The dose to be delivered is the same, but the time to deliver the dose decreases because of the higher activity.

Multi-lumen balloons allow for differential loading and therefore more control over the dose distribution, including less dose to the skin if needed.

Because the wrong magnification is used, the balloon radius will be assumed to be too small and lower dwell time will be prescribed, resulting in an underdose equal to (1.29/1.4)2

II

therapy answers

Tll9. A

TllO. C

Till. A

Till. D

Till. D

Tll4. A

TllS. A

Tll6. A

Tll7. E

Tll8. C

Tll9. E

ll

Titanium applicators are CT/MRI compatible and are available from several brachytherapy equipment providers and, as such, allow both modalities of scans. Inhomogeneity corrections are not usually done for T &0 planning.

The 1311 gamma rays have a range of energies. The most prevalent (82%) is 364 keY.

1251 decays via electron capture, and then emits a spectrum of characteristic x-rays in the range 27 to 35 ke V.

Total dose= Initial dose rate x 1.44 x Half-life. (1.44 x Half-life is called the Mean-life.) Total dose= 0.1 Gy/h x 1.44 x 60 d x 24 h/d = 207 Gy.

A greater activity of 1251 is required because it has a lower "U" (i.e., air kerma rate) than 192Ir. In pre TG-43 terminology, 1251 has a lower exposure rate constant than 192Ir.

10 min= 600 s. The treatment time will double when the activity has dropped to one-half, i.e., after one half-life, or 74 days.

On purely biological grounds, a higher dose rate for fewer fractions would be expected to cause greater normal tissue complications.

Increasing the number of fractions would reduce the complication rate but would tend to defeat the advantage of using HDR. In GYN, the rectal dose can be reduced to an acceptable level by improved geometry.

Due to the inverse square law, larger diameter ovoids decrease the mucosal surface dose but increase the depth dose. Since ovoids (except mini ovoids) have the same internal shielding, doses to bladder and rectum are not affected.

Ingested radioiodine rapidly crosses the stomach wall, where it enters the blood stream. Because the half-life of 1311 in the blood is several hours, the whole marrow space is continuously irradiated. Dose-limiting toxicity for whole marrow begins at doses in excess of 2 Gy. Some patients can experience salivary gland complications, but these are usually reversible and not life threatening.

Raphex lOll

TIJO. B

TIJI. C

Tl32. C

TIJJ. A

Tl34. C

TIJS. B

therapy answers

The dose in an unrestricted area must not exceed 0.02 mSv in any one hour according to Subpart 380.5: Radiation Dose Limits for Individual Members of the Public.1 To reduce 0.64 mSv/h to 0.02 mSv/h would require 5 half-value layers. 5 x 0.027 em= 0.135 em.

The average radiation dose from 37 mBq (1 0 mCi) administration of FDG in organs ranges from approximately 1-4 cGy, the highest dose being received by the bladder wall, a consequence of the fast excretion of FDG fron: the body.

1 Department of Environmental Conservation, Regulations, Chapter IV-Quality Services, Subpart 380.5.

Raphex 2011 13