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.

Front 35

6. What fraction of the maximumenergy of the incident electron beam BEST approximates the energy of thebremsstrahlung beam?

(A) 1/4

(B) 1/3

(C) 1/2

(D) 2/3

Back 35

Key: B Rationale:The “1/3approximation” is a useful estimate of the average energy of a bremsstrahlungbeam. For example, a 6 MV linac beam has average photon energy of approximately2 MeV.

Front 36

9. Based on ICRU 83, the nearmaximum dose, D2%, is BEST defined as the:

(A) mean dose to 2% of thevolume of a structure.

(B) highest dose to 2% of thevolume of a structure.

(C) volume of a structurereceiving 2% of the mean dose.

(D) volume of a structurereceiving the highest 2% of the prescription dose.

Back 36

Key: B Rationale:Because of advancedcomputer-based treatment planning and 3D conformal delivery techniques, ICRUhas recommended using Level 2 reporting which is primarily based on volumemetrics instead of point dose metrics. The D2% is best defined as the highestdose to 2% of a given structure.

Front 37

15. What is the purpose ofrounded leaf ends on MLCs?

(A) Reduce radiation leakage (B) Reduce internal scatter

(C) Provide accurate MLCpositions, independent of leaf position

(D) Provide radiation field edgepenumbras that are similar, independent of leaf position

Back 37

Key: D Rationale:For single-focusedMLCs, radiation passes through different thicknesses of material depending onthe MLC position. This results in different radiation penumbra depending on position.In order to mitigate this effect, these MLCs are designed with rounded leafedges.

Front 38

19. According to AAPM TG-142,which statement BEST describes the tolerance values for periodic machine QAtests?

(A) They are specific to PTVmargins used.

(B) They are independent oftreatment techniques.

(C) They are specific to daily,weekly, and bi-weekly imaging.

(D) They are specific tonon-IMRT, IMRT, and SRS/SBRT treatment techniques.

Back 38

Key: D Rationale:An importantdistinction of the current QA guidance (TG-142) is the use of tolerance valuesspecific to treatment technique.

Front 39

24. In agreement states, whichtwo organizations are charged with enforcing radiation safety regulations inthe radiation oncology workplace?

(A) NRC and FDA

(B) EPA and OSHA

(C) ICRP and NCRP

(D) BEIR and UNSCEAR

Back 39

Key: A Rationale:The NuclearRegulatory Commission (NRC) has as part of its responsibilities the regulationof radioactive materials used medically, including diagnostic radionuclides andbrachytherapy sources. The Food and Drug Administration is charged with theregulation of “medical devices”, including those that produce radiation, suchas linear accelerators (and associated treatment planning software).

Front 40

37. In radiation shieldingcalculations, the use factor (U) is defined as the fraction of time that:

(A) the beam is turned on in aday.

(B) the isocenter is notblocked by a primary jaw or MLC.

(C) a radiation worker may bepresent when the beam is on.

(D) the radiation is directedat a particular shielding structure.

Back 40

Key: D Rationale:The Use factor (U)is the fraction of time during which the radiation under consideration isdirected towards a particular structure.

Front 41

39. What is the advantage ofusing a parallel-plate ionization chamber over a cylindrical ionization chamberfor radiation dose measurement?

(A) Reduced equipment cost

(B) Reduced beam directiondependence

(C) Reduced sensitivity toscatter radiation

(D) Reduced cavityperturbations due to smaller electrode spacing

Back 41

Key: D Rationale:Because of the smallelectrode spacing (typically ~2mm), a parallel-plate chamber minimizes cavityperturbations in the radiation field and serves as a good dosimeter forelectron beams and surface dose measurements.

Front 42

41. What commonly usedcomponent of the photon beam delivery system is NOT essential for linac basedIMRT delivery?

(A) Target

(B) Primary jaw

(C) Flattening filter

(D) Monitor chamber

Back 42

Key: C Rationale:An IMRT planmodulates the beam intensity to create a non-uniform beam. Therefore aflattening filter, whose purpose is to create uniformly flat beam intensities,is not needed. Flattening filters are still used in some IMRT plans, but theyare not required. Although the MLC or a compensator modulates the beam, theprimary jaws are still needed to block radiation outside the largest extent ofthe field. Monitor chambers are used in the treatment head to measure theoutput of the machine and are used for all external beam RT modalities.Scattering foils are used for electrons.

Front 43

52. A 10 MV photon beam traverses12 cm of lung tissue. The approximate correction factor for the increasedabsorbed dose due to this inhomogeneity is:

(A) 12%.

(B) 18%.

(C) 30%.

(D) 48%.

Back 43

Key: C Rationale:The correctionfactor for 10 MV radiation traversing lung tissue is approximately 2.5% per cm

Front 44

72. An electron passes througha potential difference of 120 V, causing the electron to acquire a kineticenergy equal to:

(A) 120 J.

(B) 120 eV.

(C) 120 MV.

(D) 120 kVp.

Back 44

Key: B Rationale:This questionrequires knowledge of energy units commonly used in radiation therapy.

Front 45

77. If a tertiary MLC is addedto a linear accelerator to enable the IMRT delivery, the improvement in thephysical penumbra will likely be due to a decrease in which component of thepenumbra?

(A) Scatter

(B) Leakage

(C) Geometric

(D) Transmission

Back 45

Key: C Rationale:The physicalpenumbra is the composite of the scatter, geometric and transmission penumbras.By adding a tertiary MLC, the source to diaphragm distance (SDD) increases andthereby reduces the geometric penumbra in the patient.

Front 46

78. Compared to CT-based IGRT,what is an advantage of integrated MRI-external beam radiotherapy?

(A) Increased RBE due to MRfield

(B) Decreased treatmentdelivery time

(C) Image guidance withimproved spatial resolution

(D) Image guidance withimproved soft tissue visualization

Back 46

Key: D Rationale:MRI guided RTprovides better soft tissue visualization. There is no increased RBE from MR.The treatment delivery is not effected by the presence of the MRI. Spatial resolutionof MRI is not an advantage compared to CT based IGRT.

Front 47

86. In a perfectly calibratedCT scanner, water is assigned a value of how many Hounsfield units? (A) -1,000 (B) 0 (C) 100 (D) 1,000

Back 47

Key: B Rationale:CT numbers are allrelative to the value of water that is defined as 0.

Front 48

96. If the temperature is 2%higher and the pressure is 1% lower than standard conditions, the reading of anopen-air ion chamber will be:

(A) 3% lower.

(B) 1% lower.

(C) 1% higher.

(D) 3% higher.

Back 48

Key: A Rationale: Radiationmeasurements in open-air ion chambers must always be corrected for changes intemperature and pressure. The signal from an ion chamber is proportional to thenumber of air molecules in the chamber available to be ionized. Increasing thetemperature and decreasing the air pressure both serve to reduce the number ofair molecules in the chamber, reducing the reading from the ion chamberproportionally

Front 49

106. Neglecting heterogeneitycorrections in the dose calculation for a tangent breast treatment results in:

(A) overestimation of skindose.

(B) underestimation of skindose.

(C) overestimation of dose inthe breast near the chest wall.

(D) underestimation of dose inthe breast near the chest wall.

Back 49

Key: C Rationale:Without accountingfor the low density of the lung tissue, the dose to the chest wall will be overcalculated. The presence of the air in the lung results in less in-scatter doseto the chest wall area. The skin area is not significantly affected by theheterogeneity since it is farther away

Front 50

117. Underwhich irradiation conditions will protons have a high LET?

(A) Allirradiation conditions

(B)Irradiation in the initial half of the SOBP

(C) If theproton energy drops below 10 MeV

(D) If thecells being irradiated are defective in HR

Back 50

Key: CRationale:Proton beams at clinical energies are low LET, averaging 2-3 keV/μm in thecenter of the spread out Bragg peak (SOBP). However, proton energy decreasestoward the end of the range of the SOBP, and very low energy stopping protonsdo have a high LET. DNA repair defects may alter the relative biologicaleffectiveness (RBE) of protons but not the LET

Front 51

118. Annihilationcoincidence detection is employed in PET scans in order to:

(A) provide SUV data.

(B) reject scatteredphotons.

(C) improve detectionefficiency.

(D) shorten the time of thescan

Back 51

Key: B Rationale: Scattered photons woulddetract from the contrast in the PET image and make quantitative assessmentdifficult. Annihilation coincidence insures that only unscattered annihilationphotons are counted. Photons not detected within the coincidence time windoware not counted.

Front 52

130. For a TBItreatment, a 75 cm x 50 cm field is required. The maximum field size setting onthe accelerator is 40 cm x 40 cm. What is the minimum SSD needed?

(A) 155 cm

(B) 164 cm

(C) 188 cm

(D) 210 cm

Back 52

Key: C Rationale: The field sizeincreases linearly with SSD. The 40x40 cm field size is defined at 100 cm SSD,so to treat this patient, the SSD must increase to (75 cm / 40 cm) * 100 cm =187.5 cm.

Front 53

133. 125I and 131I are examples of:

(A) isobars.

(B) isomers.

(C) isotones.

(D) isotopes.

Back 53

Key: D Rationale: Isotopes are atoms thathave the same number of protons but different numbers of neutrons. The “A” inthe notation AX represents the total number of nucleons (protons + neutrons)and since both are iodine, they have the number of protons, but with differentnumber of neutrons in nucleus.

Front 54

144. A flatteningfilter is designed to produce a uniform dose in field at:

(A) dmax.

(B) 5 cm depth.

(C) 10 cm depth.

(D) the surface.

Back 54

Key: C Rationale: Flattening filterspreferentially attenuate the center of the forward-peaked photon beam emergingfrom the target, but create a beam that is softer (containing more low-energyphotons) the farther you get from the central axis. The beam thereforeattenuates at a different rate depending where you are radially in the beam.Flattening filters are tuned to be flat at a depth of 10 cm. Upstream of thisdepth, the edges of the beam will have a higher dose than the center. Downstreamof this depth, the edges of the beam will become rounded and lower than thecentral axis dose.

Front 55

161. In which type ofinteraction is an orbital electron ejected as a result of the absorption of aphoton?

(A) Pair production

(B) Compton scattering

(C) Coherent scattering

(D) Photoelectriceffect

Back 55

Key: D Rationale: During a photoelectricreaction, a photon is absorbed and causes an orbital electron to be ejectedfrom the atom.

Front 56

165. When used inpatient-specific QA to compare planned and measured dose distributions, whatdoes the gamma analysis consider?

(A) Absolute dosedeviation at each point

(B) Dose deviation anddistance to agreement

(C) Maximum dosedeviation in a region of interest

(D) Root mean squaredifference in dose distributions

Back 56

Key: B Rationale: The gamma evaluates notonly the accuracy of the dose agreement at each point but also compared thedistance to agreement for each point in the dose distribution.

Front 57

179. Medicalmisadministration involving a linac is defined by:

(A) FDA.

(B) NRC.

(C) state regulations.

(D) federalregulations.

Back 57

Key: C Rationale: The NRC regulates themedical use of radiation from radioactive isotopes, which does not apply toradiation produced by a linear accelerator. State laws regulate the medical useof radiation produced by a linac.

Front 58

183. A linearaccelerator electron scattering foil:

(A) spreads out thenarrow raw electron beam.

(B) filtersbremsstrahlung x-ray contamination.

(C) collimates the beamand reduces penumbra. (D) converts the high energyphotons to electrons.

Back 58

Key: A Rationale: The purpose of thescattering foil is to spread out the very narrow raw electron beam that emergesfrom the accelerator wave guide. The foil should be thin enough to avoidsignificant bremsstrahlung production but it does not filter thiscontamination.

Front 59

193. For which electronbeam energies is the 80% depth dose invariant as the field size decreases from10 x 10 cm2to smallerfield sizes?

(A) ≤ 6 MeV

(B) ≤ 9 MeV

(C) ≥ 12 MeV

(D) ≥ 18 MeV

Back 59

Key: A Rationale: Rationale has a graphsand this tab does not support it. However, explanation says "as theelectron beam energy increases over ~9MeV, the d80% becomes more sensitive tofield size and will decrease (become shallower) as the field size is reduced.Because of this, potential underdosing to the treatment volume may occur ifhigh energies and small field sizes are used for treatment."

Front 60

195. A 10 MeV photon inwater undergoes Compton scattering through an angle of 90 degrees. The kineticenergy of the recoil electron is approximately:

(A) 8.0 MeV.

(B) 8.5 MeV.

(C) 9.0 MeV.

(D) 9.5 MeV.

Back 60

Key: D Rationale: High energy photonsscattered through 90 degrees have a final energy of 0.511 MeV. This leaves anapproximate total of 9.5 for the recoil electron. This is important to know forshielding considerations. Scatter radiation from patients that is incident onwalls results from scattering at approximately 90 degrees.

Front 61

199. According to ICRU78, the RBE of protons is approximately:

(A) 0.6.

(B) 1.1.

(C) 1.5.

(D) 2.1.

Back 61

Key: B Rationale: The RBE of protons issimilar to that of photons. It is approximately 1.1.

Front 62

201. A rectangular 10 x15 cm2 field is approximatelyequivalent to a square field with a side of:

(A) 11 cm.

(B) 12 cm.

(C) 13 cm.

(D) 14 cm.

Back 62

Key: B Rationale: Using Sterling'sapproximation, a rectangular field is equivalent to a square field if they havethe same area to perimeter ratio. The side(s) of an equivalent square field iscalculated by 4 x (Area/Perimeter.) In this case it is (150/50) x 4 = 12

Front 63

203. In a typicaldiagnostic x-ray tube, what percent of the incident electron energy is emittedas photons?

(A) 0.01

(B) 0.1

(C) 1

(D) 10

Back 63

Key: C Rationale: The efficiency ofphoton generation through the bremsstrahlung process increases with increasingelectron energy. For relatively low energies found in diagnostic x-ray tubes,only 1% of the incident energy goes into photon production. At megavoltageenergies, such as those found in therapeutic linear accelerators, around 30% ofthe energy is radiated as photons. The remaining energy is dispersed as heat.

Front 64

204. The advantage ofCBCT or MVCT over portal images is:

(A) shorter acquisitiontime.

(B) three-dimensionalinformation.

(C) use of thetreatment beam energy.

(D) correction forgantry sag during rotation.

Back 64

Key: B Rationale: CBCT is a threedimensional imaging modality. As such it provides information abouttranslational shifts in three directions and rotations about three axes.Acquisition time is longer for CBCT. Although choice D is correct, this is notan advantage. CBCT usually employs kilovoltage energy.

Front 65

213. The health carecommunication standard for medical images is:

(A) HL7.

(B) 4DCT.

(C) JPEG.

(D) DICOM.

Back 65

Key: D Rationale:DICOMis the common format for digital images. It stands for Digital Imaging andCommunications in Medicine. HL7 is a standard code for information interchangeand is the principle code for patient information used in admissions, dischargesand transfers. JPEG is a compressed file format for digital images. 4DCT is anabbreviation for a 4 dimension CT scan that is often used in radiation therapyplanning

Front 66

224. For a linearaccelerator with FFF mode, how does the average energy of its 6 MV FFF beamcompare to the average energy of its flattened 6 MV beam?

(A) Lower

(B) Equal

(C) Higher

(D) Double

Back 66

Key: A Rationale: The flattening filterpreferentially filters out low energy photons, increasing the average energy ofthe beam. FFF modes do not have a flattening filter, so, even though thenominal accelerating energy is the same, the average beam energy is lower.

Front 67

233. How do protonspredominately interact with matter?

(A) Coulomb forces

(B) Photoelectriceffects

(C) Pair and tripletproduction

(D) Rayleigh andCompton scattering

Back 67

Key: A Rationale: It is important tounderstand that charged particles (such as protons) interact primarily byexcitation and ionization due to Coulomb interactions with orbital electronsand atomic nuclei. Photoelectric, Compton and pair production are allinteractions between photons and matter.

Front 68

236. The dose in thebuildup region in an electron beam will increase due to:

(A) an increase in SSD.

(B) a decrease in beamenergy.

(C) a significantincrease in field size.

(D) a significantreduction in field size.

Back 68

Key: C Rationale: The surface doseincreases. Decreasing beam energy results in lower dose in buildup region.Increasing the SSD will result in lower dose.

Front 69

238. Why does thebuild-up incorporated into diode dosimeters used for routine in-vivo dosimetryconsist of high density materials such as brass or tungsten?

(A) These materialsprovide increased conductivity

(B) To limitbremsstrahlung production in the diode

(C) To compensate forp-n junction energy dependence

(D) So that the diodecan be made reasonably compact in size

Back 69

Key: D Rationale: Tissue density build-upwould require up to 3 – 4 cm thickness, which is impractical. Diodes areusually calibrated to measure the dose at a depth of dmax.

Front 70

239. For IMRT,treatment plan quality is relatively independent of beam energy provided that:

(A) number of beams issufficient.

(B) only coplanar beamsare used.

(C) image guidance isused in delivery.

(D) full heterogeneitycorrections are applied.

Back 70

Key: A Rationale: A low energy beam has ahigher proximal dose and a lower dose distal to the target. When a small numberof low energy beams are used to treat a deep target, the proximal dose must behigh to deliver the required dose at depth. As the number of beams increasesthe proximal dose will be diluted. Heterogeneity corrections, IGRT and beamarrangement should not influence energy choice.

Front 71

240. Prevalence ofwhich interaction in kV imaging is responsible for improved bone/soft-tissuecontrast compared to a MV portal image?

(A) Compton

(B) Photoelectric

(C) Pair production

(D) Bremsstrahlung

Back 71

Key: B Rationale: The photoelectriceffect is proportional to the cube of the atomic number (Z) and inverselyproportional to the cube of the photon energy (E). The high Z dependence makesphotoelectric interactions very sensitive to small changes in materialcomposition, but the strong E dependence means photo electric only makes asignificant contribution at lower photon energies. In tissue, the photoelectriceffect is approximately equal to the Compton effect at about 30 keV, with theprevalence of photoelectric interactions decreasing rapidly for the higherenergies found in megavoltage beams.

Front 72

241. In 3D conformalplanning, beam apertures are designed to frame the PTV with an additionalmargin to account for:

(A) patient motion.

(B) internal scatter.

(C) beam penumbra.

(D) setupuncertainties.

Back 72

Key: C Rationale: The CTV accounts foroption A, while the PTV accounts for options B and C. Due to beam penumbra,beam apertures need an additional margin for PTV coverage

Front 73

251. Withthe addition of volumetric imaging systems placed on linear accelerators (i.e.,CBCT), the recommended testing frequency and tolerance difference between themechanical and imaging isocenters are:

(A) daily, ≤2mm.

(B) daily, ≤4mm.

(C) monthly,≤ 1mm.

(D) monthly,≤ 2mm.

Back 73

Key: ARationale:Since volumetric imaging systems are used to position patients on a dailybasis, the coincidence between the mechanical isocenter and the imagingisocenter must be validated each day prior to patient treatment. Therecommended tolerance for linear accelerators, not used for stereotacticdeliveries, is ≤ 2mm.

Front 74

256. According toICRU-62, an ITV is created by adding a margin to:

(A) GTV.

(B) CTV.

(C) PTV.

(D) PIV.

Back 74

Key: B Rationale: ICRU report 62recommends that an internal margin be added to the CTV to compensate forinternal physiological movements.

Front 75

263. The angle ofphoton scattering that results in the largest energy transfer to a Comptonrecoil electron is:

(A) 0 degrees.

(B) 45 degrees.

(C) 90 degrees.

(D) 180 degrees.

Back 75

Key: D Rationale: In Compton scattering,when a photon is backscattered, it transfers the maximum fraction of its energyto the recoil electron.

Front 76

265. Compared toIodine-125 brachytherapy seeds, Palladium-103 seeds are characterized by:

(A) longer half-lifeand lower average energy.

(B) longer half-lifeand higher average energy.

(C) shorter half-lifeand lower average energy.

(D) shorter half-lifeand higher average energy.

Back 76

Key: C Rationale: Compared to I-125,Pd-103 has a shorter half-life (17 days vs. 60 days) and a lower average energy(21 keV vs. 28 keV). Pd-103 seeds are used in many of the same applications asI-125, including prostate seed implant and eye plaque therapy.

Front 77

267. In stereotacticradiotherapy, the Winston-Lutz test is used to verify the:

(A) constancy of beamenergy.

(B) constancy ofmachine output.

(C) accuracy of the MLCleaf motion with respect to isocenter.

(D) alignment ofpatient positioning lasers with the radiation isocenter.

Back 77

Key: D Rationale: In the Winston Lutz, apointer is set up using the lasers and the pointer is irradiated to verify thealignment of the lasers to the radiation field.disease.

Front 78

269. Which of the following brachytherapy dose calculation factorsis often approximated by either a point or line source?

(A) Geometry factor

(B) Air kerma strength

(C) Dose rate constant

(D) Radial dosefunction

Back 78

Key: A Rationale:Geometryfactory accounts for dose variation due to spatial distribution within thesource (ignoring scatter and attenuation in the source.) It is oftenapproximated by either a point or line source. The air kerma strength is ameasure of source strength. The dose rate constant converts the strength todose in water at a distance of 1 cm on the transverse axis. The radial dosefunction accounts for effects of absorption and scatter in medium along thetransverse axis of the source

Front 79

270. Localizationdigitally reconstructed radiographs (DRRs) are principally generated to mimic thespatial geometry of a(n):

(A) CT simulator.

(B) diagnostic CT.

(C) treatmentaccelerator.

(D) traditional (kV)simulator.

Back 79

Key: C Rationale: DRRs are generated in avirtual environment by creating a planar radiograph using the patientvolumetric information (CT) and a ray tracing algorithm that accounts for beamattenuation (ignoring scatter) through the CT of the patient. The geometry ofthe virtual source and detector pair that creates the DRR is that of the linearaccelerator since the DRR is compared to the actual MV EPID or kV planarverification image for positioning purposes.

Front 80

300. What is anadvantage of an active scanning versus a passive scattering delivery system forproton beam radiotherapy?

(A) Reduced deliverytime

(B) Reduced neutrondose to patient

(C) Reduced sensitivityto patient motion

(D) Reduced maximumdepth of proton Bragg peak

Back 80

Key: B Rationale: Reference: In an activescanning system, the proton beam is steered using a magnetic field-basedsystem. Because of this, there is no need to have scattering foils orcompensators in the beamline thereby reducing the neutron dose production inthese components. Additionally, because the proton beam is not attenuated in ascanning system, the depth of the Bragg peak is not modified prior to patiententrance. However, since most active scanning approaches utilize a spotscanning technique, there is an increase in sensitivity to patient motion.