Miscellaneous Physics

Front 1

128. As opposed to forward based 3D treatment planning, which of the following is typically needed for an inverse treatment plan?
(A) OAR dose limits
(B) Delineation of a ring structure
(C) Delineation of target and OARs
(D) Selection of incident beam angle

Back 1

Key: B
Rationale: Inverse plans rely heavily on the user defined cost functions and dose objectives; as such they tend to ignore regions that are not specifically defined in the objectives. As a result, high-dose regions are typically found immediately outside of the defined target. One technique implemented to avoid this issue is to create a ring structure around the target and define the dose constraints for this structure in the objectives. The remaining options must be defined for both forward and inverse based plans.
References: Van Dyk, J. The Modern Technology of Radiation Oncology, Volume 2, Medical Physics Publishing, Copyright 2005.

Front 2

60. In total photon body irradiation (TBI), the purpose of the beam spoiler is to:
(A) increase the dose at depth.
(B) decrease the dose at depth.
(C) increase the dose at the surface.
(D) decrease the dose at the surface.

Back 2

Key: C
Rationale: A common dosimetric goal in total body irradiation is to achieve dose uniformity within 10%. Dose in the buildup region of a photon beam is typically much less than 10% of the maximum dose, requiring steps to increase the skin dose. A 1-2 cm thick acrylic beam spoiler, placed close to the patient surface, is generally sufficient to increase the skin dose to at least 90% of the prescription dose.
References: AAPM Task Group 29 report.

Front 3

6. Management of respiratory motion should be considered if intrafraction motion is of a magnitude greater than:
(A) 2 mm.
(B) 5 mm.
(C) 8 mm.
(D) 10 mm.

Back 3

Key: B
Rationale: Per AAPM Task Group 76, errors caused by motions less than 5 mm are considered small relative to other errors in radiotherapy.
References: AAPM Task Group 76 report.

Front 4

9. For a certain x-ray technique, the exposure is measured to be 10 R. Given that the f factor is 0.876 rad/R, what is the dose in air?
(A) 0.01141 Gy
(B) 0.0876 Gy
(C) 8.76 Gy
(D) 11.41 Gy

Back 4

Key: B
Rationale: Using the f factor, the dose is (10 R* 0.876 rad/R) = 8.76 rad = 0.0876 Gy.
References: Khan FM, The physics of Radiotherapy, 3rd Edition, Chapter 8, pp. 109.

Front 5

61. Absolute output calibrations for a medical linear accelerator are BEST performed using:
(A) an ion chamber.
(B) a diode detector.
(C) a piece of radiographic film.
(D) a Geiger counter.

Back 5

Key: A
Rationale: AAPM Task Group 51 requires absolute dose calibration to be performed using a small ion chamber, approximating a Bragg-Gray cavity. Diode detectors and radiographic film are useful for relative measurements, but have dependencies on energy, exposure, and other factors making dose calibration less accurate. A Geiger counter is an ion chamber tuned to maximum sensitivity to allow the detection of very small signals, and would therefore be saturated and overwhelmed by the dose rate from a therapeutic beam.
References: AAPM Task Group 51 report.

Front 6

221. What standard protocol is used to facilitate the transfer of medical image data between different manufacturers?
(A) JPEG
(B) RTOG
(C) DICOM
(D) ALARA

Back 6

Key: C
Rationale: Developed by the ACR and National Electrical Manufacturers Association (NEMA), the DICOM (Digital Imaging and Communications in Medicine) standard was created to aid in the creation and transmission of medical imaging data.
References: Http://medical.nema.org.

Front 7

20. The relative exposure rate of an x-ray generator is proportional to:
(A) kVp.
(B) (kVp)2.
(C) (mAs)2.
(D) (mAs)3.

Back 7

Key: B
Rationale: The exposure rate is proportional to the mAs and the square of kVp.
References: Khan FM, The physics of Radiotherapy, 3rd Edition, Chapter 3, Fig 3.10.

Front 8

85. What accessory is used to lower the beam energy in total skin electron therapy?
(A) Bolus
(B) Wedge
(C) Lucite blocks
(D) Custom cerrobend cutout

Back 8

Key: C
Rationale: Lucite blocks are used to decrease beam energy and improve beam flatness.
References: Khan, The Physics of Radiation Therapy, Ch. 14 Electron beam therapy. 1994.

Front 9

92. In which interaction does the number of neutrons increase?
(A) Alpha decay
(B) Compton effect
(C) Electron capture
(D) Photoelectric effect

Back 9

Key: C
Rationale: Photoelectric effect and Compton scatter are atomic interactions that do not alter the nucleus. Alpha decay is a nuclear interaction where a helium nucleus is ejected from the nucleus. In electron capture, an orbital electron is captured by the nucleus and it combines with proton to from a neutron.
References: Attix, Introduction to Radiological Physics and Radiation Dosimetry, Ch. 5 Absorbed dose in radioactive material. Khan. The Physics of Radiation Therapy, Ch. 2 Nuclear Transformations, 1994.

Front 10

87. Gantries for proton therapy accelerators are much larger than for electron linacs because:
(A) protons have a smaller charge then electrons.
(B) protons have a larger magnetic moment than electrons.
(C) protons are much more massive than electrons.
(D) protons interact more readily with residual gas in the beam line than electrons.

Back 10

Key: C
Rationale: The magnitude of the deflecting force exerted on a charged particle traveling perpendicular to a magnetic field is qvB, where q is the charge, v is the speed and B is the strength of the magnetic field. These quantities are all roughly the same for protons and electrons in treatment machine gantries (except that the charge is the opposite sign). The mass of a proton however is 2000 times larger than an electron and therefore it follows a curved trajectory that is much larger. Options B and D are irrelevant.
References: P.N. McDermott and C.G. Orton, “The Physics and Technology of Radiation Therapy,” Medical Physics Publishing, Madison, WI, Copyright 2010.

Front 11

106. For the treatment of prostate carcinoma with low-dose rate brachytherapy, the American Brachytherapy Society recommends slightly different prescription doses depending on whether I-125 sources or Pd-103 sources are used. The reason for a slightly higher recommended prescription dose for I-125 sources is the fact that I-125 has a:
(A) longer half-life.
(B) smaller seed size.
(C) lower anisotropy constant.
(D) larger average photon energy.

Back 11

Key: A
Rationale: A longer half-life Dose is delivered over a longer period of time for a source with a longer half-life, necessitating a higher total dose.
References: Perez and Brady’s Principles and Practice of Radiation Oncology, 5th Edition, Lippincott Williams & Wilkins, Copyright 2008.

Front 12

151. What is the equivalent square for a 10 cm x 4 cm field?
(A) 2.8 cm
(B) 4 cm
(C) 5.7 cm
(D) 8.3 cm

Back 12

Key: C
Rationale: According to Day's rule, the area to perimeter ratio (A/P) of the square and rectangle are equal to each other. For a square of side a, (A/P) = a/4. For a rectangle with sides b and c, (A/P) = bc/(2(b+c)). Thus, a = 2bc/(b+c) = (80/14)=5.7 cm.
References: Khan FM, The physics of Radiotherapy, 3rd Edition, Chapter 9, pp 164-165.

Front 13

152. Which of the following is an advantage of using thermoluminescent dosimeters (TLDs) for in-vivo measurements?
(A) Their response is independent of energy.
(B) Their small size makes them convenient for use.
(C) They create a permanent record of the dose delivered.
(D) One dosimeter provides a 2D representation of the dose distribution.

Back 13

Key: B
Rationale: The small size of TLDs makes then very convenient to position accurately and they do not alter the dose distribution in any significant manner. They are energy dependent, with the response being much larger in the kV range than in the MV range. Once read, they lose most of their information and therefore do not create a permanent record of dose. Finally, one TLD also give a single reading, not a 2D distribution like film would.
References: Khan FM, The physics of Radiotherapy, 3rd Edition, Chapter 8, pp 144-148.

Front 14

202. What is the approximate effective dose to a patient receiving a 4D CT of the chest on a multi-slice CT scanner?
(A) 0.02 mSv
(B) 2 mSv
(C) 20 mSv
(D) 200 mSv

Back 14

Key: D
Rationale: The effective dose to a patient receiving a 4D CT of the chest on a multi-slice CT is approximately 200 mSv. During a 4D CT acquisition, a patient is scanned multiple times at each couch position; as such their exposure to radiation can be up to an order of magnitude higher than a standard multi-slice protocol (~ 20 mSv). The effective dose to patient receiving a standard chest x-ray is approximately 0.02 mSv.
References: J. Valentin, Annals of the ICRP 30 (4), 19-24 (2000). T. Li et al., Med. Phys. 32 (12), 3650-60 (2005).

Front 15

253. The largest source of scatter in a photon beam from a conventional linear accelerator treatment head is the:
(A) target.
(B) collimators.
(C) flattening filter.
(D) monitor chamber.

Back 15

Key: C
Rationale: The flatting filter (FF) is the largest source of scatter. The primary photon beam passes directly through the high density material of the flattening filter creating unwanted photon scatter. Understanding the effects of FF is important as new FF-free machines are becoming an option for IMRT.
References: Liu HH, Mackie TR and McCullough EC “A dual source photon beam model used in convolution/superposition dose calculations for clinical megavoltage x-ray beams. “ Med Phys. 24; 1960 (1997).

Front 16

178. What is the predominant mode of energy transfer for fast neutrons in tissue?
(A) Nuclear reactions
(B) Compton scatter with electrons
(C) Recoil collisions with protons
(D) Bremsstrahlung production

Back 16

Key: C
Rationale: Fast neutrons interact with tissue primarily by setting protons in motion. Nuclear reactions are of lesser importance especially for fast neutrons. Bremsstrahlung production only occurs for electrons.
References: P.N. McDermott and C.G. Orton, “The Physics and Technology of Radiation Therapy,” Medical Physics Publishing, Madison, WI, Copyright 2010.

Front 17

142. In a photon-producing linear accelerator, replacing a higher-Z target with a lower-Z target results in which of the following?
(A) Increased photon production and higher average beam energy
(B) Increased photon production and lower average beam energy
(C) Reduced photon production and higher average beam energy
(D) Reduced photon production and lower average beam energy

Back 17

Key: D
Rationale: X-rays are produced through a process called bremsstrahlung, which is more efficient for higher-Z materials. High-Z materials are also efficient at filtering out low-energy photons from the beam, leading to higher beam average beam energies. Therefore, replacing a high-Z target (typically tungsten, Z=74) with a lower-Z target (e.g. copper, Z=29) will result in less-efficient photon production and lower average beam energy.
References: F. Khan, “The Physics of Radiation Therapy,” Fourth Edition, Lippincott Williams & Williams, Baltimore, MD, Copyright 2010.

Front 18

98. Which of the following regulatory bodies oversees the medical use of byproduct materials (e.g., brachytherapy sources) in an agreement state?
(A) American Brachytherapy Society (ABS)
(B) Nuclear Regulatory Commission (NRC)
(C) U.S. Food and Drug Administration (FDA)
(D) Individual state agencies (e.g. department of human health)

Back 18

Key: D
Rationale: According to the NRC, an agreement state is a state that has signed an agreement with the NRC authorizing the state to regulate certain uses of radioactive materials within the state. In an agreement state, a specific state agency such as the department of human health or the radiation regulatory agency is responsible for regulating and overseeing the medical use of byproduct materials. In a non-agreement state, this responsibility is under the purview of the NRC.
References: http://www.nrc.gov/reading-rm/basic-ref/glossary/agreement-state.html.

Front 19

56. Compared to a 3D conformal treatment, a conventional linac-based IMRT treatment:
(A) is forward planned.
(B) uses 1 segment per beam.
(C) uses less total monitor units.
(D) can conform better to the target volume.

Back 19

Key: D
Rationale: An IMRT treatment is inverse planned and uses several segments per beam to create intensity modulation that can conform better to our target volume. Due to the large number of segments used, IMRT treatments typically uses more monitor units.
References: Khan FM, The physics of Radiotherapy, 3rd Edition, Chapter 20.

Front 20

109. A therapeutic neutron beam deposits doses most efficiently in which of the following materials?
(A) Fat
(B) Bone
(C) Lung
(D) Muscle

Back 20

Key: A
Rationale: Neutron beams transfer energy most efficiently in collisions with particles of similar mass (protons), and will therefore deposit dose most efficiently in hydrogen-rich materials. Fat is among the most hydrogen-rich tissues. Neutron doses in fat can be 20% higher than in muscle. Bone is among the least hydrogen-rich tissues in the body, so neutron beams are often described as “bone-sparing”.
References: F. Khan, “The Physics of Radiation Therapy,” Fourth Edition, Lippincott Williams & Williams, Baltimore, MD, Copyright 2010.

Front 21

111. Images produced with megavoltage (MV) x-rays show lower contrast between bone and soft tissue than kilovoltage (kV) x-ray images because:
(A) MV x-rays are more penetrating.
(B) the physical penumbra is smaller for MV x-rays.
(C) MV photons scatter through smaller angles.
(D) Compton scattering dominates at MV energies.

Back 21

Key: D
Rationale: One of the primary reasons for poor contrast in MV x-ray images is due to the fact that Compton scattering is the dominant interaction in tissue at these energies. Compton scattering is almost independent of atomic number for all elements except hydrogen. It is true that MV x-rays are more penetrating but it is differential penetration that is crucial for contrast. The penumbra is generally larger for MV x-rays. MV x-rays do tend to scatter through smaller angles than kV x-rays but they cannot be eliminated.
References: P.N. McDermott and C.G. Orton, “The Physics and Technology of Radiation Therapy,” Medical Physics Publishing, Madison, WI, Copyright 2010.

Front 22

64. When developing a plan to treat a free breathing lung lesion based on CT imaging, which of the following ICRU volumes is defined solely by oncological considerations?
(A) CTV
(B) ITV
(C) PRV
(D) PTV

Back 22

Key: A
Rationale: Per ICRU Report # 50 and 62, the CTV consists of the gross macroscopic disease and tumor infiltration. In contrast to the remaining volumes listed, the delineation of the CTV is solely dependent on oncological consideration. When defining the ITV, PTV and PRV, an adequate safety margin for organ motion and irradiation technique must also be considered.
References: ICRU Report #50 and 62.

Front 23

77. If a low-energy electron beam is matched to a photon beam at a patient’s surface, which of the following dosimetric phenomenon is observed between the two fields?
(A) Cold spot at depth in electron beam
(B) Cold spot at the surface of the electron beam
(C) Hot spot at depth in the photon beam
(D) Hot spot at the surface at field junction

Back 23

Key: C
Rationale: For field junction on a flat surface, the hot spot in always in photon beam side due to the out-scatter of the electrons. These areas overlap with the matched photon beam at depth, resulting in regions between the matched fields that have excessively high doses (hot spots). Avoiding hot spots at depth would require separating beams at the surface (i.e. using non-matching beams), leading to a cold spot at the surface.
References: F. Khan, “The Physics of Radiation Therapy,” Fourth Edition, Lippincott Williams & Williams, Baltimore, MD, Copyright 2010.

Front 24

78. The contamination at the end of the electron range in electron beam radiotherapy is due to what interaction?
(A) Alpha decay
(B) Beta decay
(C) Pair production
(D) Bremmstrahlung production

Back 24

Key: D
Rationale: Photons produced by bremsstrahlung interactions either in the treatment head or in the patient create an unwanted photon dose at the end of the electron range. Only these photons have enough energy to contribute to dose at depths beyond the electron range. Alpha and beta decay are modes radioactive decay. Pair production is a high energy photon interaction.
References: Khan, The Physics of Radiation Therapy, Ch. 14 Electron beam therapy.1994.

Front 25

75. What technology is used to create an intensity modulated radiotherapy (IMRT) beam in helical tomotherapy?
(A) Compensators
(B) Spot scanning
(C) Assymetric jaws
(D) Binary multileaf collimators

Back 25

Key: D
Rationale: Binary multileaf collimators are used in helical tomotherapy to modulate the intensity as the beam and the patient passes thru the plane of the beam. Compensators are used for static IMRT. Spot scanning is a technique used in proton therapy to deliver the dose to different region in the target. While tomotherapy utilizes asymmetric jaws, they are not responsible for the IMRT beam.
References: Mackie et al., Tomotherapy: a new concept for the delivery of dynamic conformal radiotherapy. Med Phys 20 (6) 1993.

Front 26

259. What is the main physical mechanism by which photons are produced in a Gammaknife?
(A) The photons are produced by the annihiliation of a positron and an electron within the circular collimators.
(B) The photons are produced during the de-excitation of a Ni-60 nucleus that is the result of radioactive beta decay of a Co-60 nucleus.
(C) The photons are produced by bremsstrahlungs generation in the circular collimators by a beam of high-energetic electrons accelerated in a waveguide.
(D) The photons are produced during the de-excitation of a Co-60 atom that has been stripped of one of its inner-shell electrons by an internal photoeffect.

Back 26

Key: B
Rationale: The Gammaknife contains Co-60 radioactive sources, therefore A and C are wrong. D is wrong because there is no internal photoeffect process.

Front 27

90. The fraction of the original nuclide remaining after 5 half-lives is:
(A) 1/5.
(B) 1/10.
(C) 1/24.
(D) 1/32.

Back 27

Key: D
Rationale: After 5 half-lives, the remaining amount of a radioactive nuclide is N*(1/2)^5 = N/32.
References: Khan FM, The physics of Radiotherapy, 3rd Edition, Chapter 2.

Front 28

242. Which of the following statements is correct concerning proton versus photon beam therapy?
(A) When defining the PTV, an additional margin in depth is required for proton versus photon beams.
(B) Skin sparing effects are more pronounced with proton versus high energy photon beam therapy.
(C) Tissue heterogeneities have a less significant impact on proton versus photon dose distributions.
(D) Tissue dose beyond the distal edge of the target is equivalent for a 200 MeV proton and a 20 MV photon beam.

Back 28

Key: A
Rationale: In contrast to photon radiotherapy, during proton beam therapy a margin in depth must be used to allow for uncertainties in the geometric relationship of the distal edge of the target relative to the 90% isodose line.
References: ICRU Report # 78.

Front 29

264. What is the recommended tolerance for daily output constancy of a linear accelerator beam?
(A) 1%
(B) 3%
(C) 5%
(D) 10%

Back 29

Key: B
Rationale: AAPM Task Group 142 recommends 3% tolerance for x-ray and electron beam constancy.
References: AAPM Task Group 142(2009) (an update to Task Group 40 in 1994).

Front 30

244. Which statement BEST characterizes absorbed dose and exposure?
(A) Absorbed dose and exposure are different measures of ionization in matter.
(B) The definition of both exposure and dose require charged particle equilibrium.
(C) Ionization chambers that are used to measure absorbed dose can also be used to measure exposure.
(D) Exposure is only defined in air for photon beams whereas dose is defined in all media and for all types of ionizing radiation (photons, electrons, protons, etc.).

Back 30

Key: D
Rationale: B is not true, as absorbed dose is not only a measure of ionization in matter, but generally of imparted energy that includes energy transfers that are below the ionization threshold. A is only true for photon radiation and if you have both corresponding calibration factors. C is wrong, because dose does not require charged particle equilibrium.

Front 31

32. At a depth of 5 cm in tissue, which of the following beams has the narrowest lateral penumbra?
(A) Co-60
(B) 6X photon
(C) 20 MeV electron
(D) 200 MeV proton

Back 31

Key: D
Rationale: Proton beams have a very narrow lateral penumbra, but its sharpness decreases with increasing beam energy, hence, depth of penetration. The width of the penumbra (80-20 percent isodose levels) is narrower for proton than for photon beams for penetrations up to approximately 17 cm. The next largest penumbra at a depth of 5 cm is the 20 MeV electron beam, followed by a Co-60 beam.
References: ICRU Report # 78.

Front 32

33. What parameter is currently used to quantify brachytherapy source strength?
(A) Air kerma
(B) Absorbed dose
(C) Specific activity
(D) Dose rate constant

Back 32

Key: A
Rationale: Brachytherapy source strengths are defined in terms of air kerma strength (cGy cm^2/ hr) according to AAPM Task Group 43. The dose rate constant defines the dose per unit air karma strength at a distance of 1 cm along the transverse axis in water. Absorbed dose rate is no longer used. Specific activity is the activity per unit mass and is not used to describe encapsulated brachytherapy sources.
References: AAPM Task Group 43 Report.

Front 33

2. How will removing the flattening filter from a photon beam alter its x-ray beam characteristics?
(A) It will not affect the beam characteristics.
(B) The peak energy of the beam will be higher.
(C) The average energy of the beam will be higher.
(D) The average energy of the beam will be lower.

Back 33

Key: D
Rationale: The flattening filter (FF) filters out a lot of the low energy photons. Therefore, removal of the FF results in a lower average energy (softer) beam because of the increased low energy component. The FF does not affect the peak energy.
References: Almberg SS, Frengen J, Lindmo, T Monte Carlo study of in-field and out of field dose distributions from a linear accelerator operating with and without a flattening filter. Med Phys 39 (8), 2012

Front 34

341. Which of the following IMRT features is theoretically associated with an increased risk for secondary malignancies?
A. It increases the whole-body dose.
B. It does not provide effective sparing of critical organs.
C. It is performed with smaller PTV margins.
D. It uses fewer monitor units than conventional radiation therapy.

Back 34

Correct answer is A. RATIONALE: Because of the increased treatment times and significant leakage through the MLC leaves, the dose to normal tissue is significantly increased with IMRT compared to non-IMRT therapy. As a result, this has raised concerns about the potential for increased incidence of secondary malignancies.