RAPHEX VIII

Question 1

order of components that photon beam passes through in linac

Answer 1

target, FF, ion chamber, x and y jaws

Question 2

difference between magnetron and klystron

Answer 2

Magnetrons generate RF, while klystrons require an RF source (RF driver), which they then amplify. A thyratron is a switch.

Question 3

As the linac is switched from 10 MV to 15 MV photon delivery, the number of neutrons produced _____ and the mean neutron energy _____. A. increases; significantly increases B. increases; significantly decreases C. increases; slightly increases D. decreases; slightly increases E. decreases; slightly decreases

Answer 3

The number of neutrons produced per MU increases rapidly with beam energy, but the energy spectrum of the neutrons does not have a strong dependence on the beam energy, although the mean neutron energy does slightly increase

Question 4

Why do linacs that deliver >10 MV require bending magnets, but lower-energy (e.g., 6 MV only) linacs do not?

Answer 4

The accelerating tube for >10 MV linacs is too long

The length of the accelerating tube needed to produce 6 MV x-rays is short, only a couple of feet. Therefore, it can be mounted in the head, in line with the beam. The long accelerating tube required to produce higher energies means that a 100 cm SAD machine could only be placed in a treatment room with both a very high ceiling and also a treatment couch placed very high above the ground. Both requirements are rather impractical. Thus, the need for a bending magnet.

Question 5

Automatic brightness control in a gantry-mounted kV imaging system’s fluoroscopy mode can modify the following parameters:

Answer 5

kV and mAs The purpose of the automatic brightness control is to obtain the best possible quality of the fluoroscopy image by changing kV and mA

Question 6

A superficial x-ray treatment using 100 kV x-rays is prescribed to the skin surface. A 4 cm circular cone with a calibrated output is used. A lead mask is fabricated to expose only the desired treatment area, which is smaller than the cone. What additional information is needed to compute the beam-on time? A. the linear attenuation coefficient for lead B. the mass attenuation coefficient for lead C. the room temperature and pressure at the time of treatment D. the backscatter factors for the cone and lead cutout E. the percent depth dose at 10 mm

Answer 6

the backscatter factors for the cone and lead cutout For the calculation, the output will be reduced by: BSF(cutout) / BSF(cone).

Question 7

A radiopharmaceutical has a physical half-life (Tp) and a biological half-life (Tb). Which of the following is true of the relationship between the effective half-life (Teff), Tp, and Tb? A. Teff = Tp – Tb B. Tp > Teff and Tb > Teff C. Teff > Tp > Tb D. Teff > Tp and Teff > Tb E. Teff = Tp + Tb

Answer 7

Tp > Teff and Tb > Teff Of the three half-lives, Teff will be the shortest. The effective half-life is calculated:

Question 8

Why are there no naturally occurring isotopes with an atomic number greater than 92? A. They cannot be produced by any means. B. They would require too many orbital electrons to be stable. C. They would be chemically unstable. D. They exist, but their half-lives are too short to exist naturally. E. The Coulomb attractive force between protons is too strong

Answer 8

They exist, but their half-lives are too short to exist naturally. There are many artificially produced isotopes with an atomic number greater than 92, but because of the large number of protons in the nucleus and the large repulsive force between the nuclear protons, they are all radioactive with rather short half lives.

Question 9

what is an amu?

Answer 9

1/12 of the mass of a 6 12 C atom amu is defined as 1/12 of the mass of a 6 12 C atom, so 1 amu = 1.66 × 10–27 kg.

Question 10

Monte Carlo-based dose calculation algorithms are the most advantageous over convolution/ superposition algorithms under which of the following conditions? A. small fields in the center of a material B. large fields in the center of a material C. small fields at the interface between two materials D. large fields at the interface between two materials E. All of the above situations are advantageous for Monte Carlo.

Answer 10

small fields at the interface between two materials Small fields at the interface between two heterogeneities is the situation where model-based algorithms, such as convolution/superposition, are likely to break down and Monte Carlo will be beneficia

Question 11

In the photon beam buildup region, there is _____ charged particle equilibrium and the size of the buildup region _____ as the beam energy increases. A transient; increases B. transient; decreases C. a lack of; increases D. a lack of; decreases E. full; decreases

Answer 11

a lack of; increases The size of the buildup region is related to the range of the secondary charged particles released by the incident photons. As the photon energy increases, so does the secondary charged particle range and, hence, the length of the buildup region. This region is characterized by a lack of charged particle equilibrium.

Question 12

Two adjacent symmetric fields of equal size are used to cover a spinal cord that is 60 cm long. What is the gap on the patient surface between the two fields if they meet at depth of 5 cm? Assume the same value of SSD = 95 cm for both fields.

Answer 12

1.50 cm Skin gap = 0.5 × 30 × (d / SAD) + 0.5 × 30 × (d / SAD) = 30 × 5 / 100 = 1.5 cm.

Question 13

A single field delivers a dose D1 at a reference depth of d1. The TMR values at the depths d1 and d2 are TMR (d1) and TMR (d2), respectively. The dose at the depth of d2, D2, can be calculated from _____. A. D1 × TMR(d2) / TMR(d1) B. D1 × TMR(d1) / TMR(d2) C. D1 × TMR(d2) / TMR(d1) × [(SSD+d2)2 / (SSD+d1)2] D. D1 × TMR(d2) / TMR(d1) × [(SSD+d1)2 / (SSD+d2)2] E. D1 × TMR(d1) / TMR(d2) × [(SSD+d1)2 / (SSD+d2)2]

Answer 13

D1 × TMR(d2) / TMR(d1) × [(SSD+d1)2 / (SSD+d2)2] TMR is the ratio of the dose rate at a given point in phantom to the dose rate at the same source-point distance and at the reference depth of maximum dose. It takes into account the scattering contribution at the depth d relative to that at dm, and the primary beam attenuation for the thickness (d – dm). The inverse square law has to be applied when calculating the dose at d2 from the dose at d1

Question 14

Radiation meters that can determine the presence of Radium-226 in a radiation safe without opening the safe are based on the detection of _____. A. alpha radiation B. beta radiation C. gamma radiation D. All of the above are true. E. None of the above is true

Answer 14

gamma radiation Each radioactive isotope has its own decay scheme. In the decay scheme of Radium-226, alpha and beta radiation cannot be detected outside of the safe. However, high-energy photons—such as the unique gamma radiation of Po-214 at 609 keV—can be measured by a sensitive spectrometer, even at low intensities outside of the safe.

Question 15

An incident photon has an energy of 5 MeV and undergoes a pair production interaction, producing a positron-electron pair. The combined kinetic energy of this pair will be _____ MeV. A. 0 B. 0.511 C. 1.022 D. 3.978 E. 4.489

Answer 15

3.978 An incident photon cannot undergo a pair production interaction unless its energy is at least the sum of the rest masses of the two particles that are produced. The rest mass of an electron (and positron) is 0.511 MeV, so the threshold for the interaction is 1.022 MeV. All of the incident energy, above and beyond the interaction threshold, will be converted into kinetic energy.

Question 16

Given two filters, one aluminum (Al) and the other copper (Cu), each of 1 HVL thickness for a 125 kVp beam, which of the following is true? A. The Cu filter attenuates more photons. B. The Cu filter is thicker. C. The quality of the attenuated 125 kVp beam will be the same for both filters. D. Both filters will attenuate the same intensity of photons. E. None of the above is tru

Answer 16

Both filters will attenuate the same intensity of photons. Since these two filters are both 1 HVL for this beam, they will each attenuate the same intensity of photons. However, the quality of the remaining photons will be different.

Question 17

The best electron energy to treat a target that is between 1.5 cm and 2.5 cm below the surface while sparing other tissue is _____. A. B.
6 MeV 9 MeV
ANSWER
C. 12 MeV D. 15 MeV E. 20 MeV

Answer 17

9 MeV The depth of the distal side of the 90% isodose line can be approximated as E/3.2 of the electron energy. So, a 9 MeV beam will treat to approximately 2.8 cm. Use of a higher energy will unnecessarily expose more tissue.

Question 18

A 9 MeV electron field has a 7 × 8 cm2 cutout in a 10 × 10 cm2 cone. If the cutout size is changed to 5 × 6 cm2, the required number of monitor units (MU) is _____. A. increased by 10% B. increased by 5% C. about the same D. decreased by 5% E. decreased by 10%

Answer 18

about the same
There is little change in the output of an electron beam if the cutout size is larger than or equal to the practical range. The practical range of a 9 MeV beam is about 4.5 cm (E/2). The output will have significant changes only when the minimum cutout dimension is less than 4.5 cm.

Question 19

As electron energy increases, the distance between PDD(80) and PDD(20) _____ and the photon contamination _____. A. increases; increases B. decreases; increases C. increases; decreases D. decreases; decreases E. There is not enough information to answer the questio

Answer 19

increases; increases The dose fall-off gradient becomes less steep with increasing energy, as does the bremsstrahlung production (radiative collisions) from the higher-energy electrons.

Question 20

PACS allows you to _____ digital medical images. A. store B. view C. transfer D. All of the above are true. E. None of the above is true.

Answer 20

All of the above are true. PACS stands for Picture Archiving and Communication System. Its advantages are less physical space necessary for storage, images can be reviewed remotely or simultaneously at different locations, and the data can be easily transferred to other digital systems by way of the DICOM standard

Question 21

According to HIPPA regulations, which of the following types of data should not be stored on personal computers or storage devices? A. linac calibration records B. beam data files used for treatment planning C. QA committee minutes D. All of the above are true. E. None of the above is true.

Answer 21

QA committee minutes QA committee minutes may contain private patient information and, therefore, they can only be stored on secure devices.

Question 22

In order to establish DICOM connectivity, which of the following are required: A. IP address, port number, AE title B. IP address, MAC address, port number C. MAC address, AE title, port number D. domain name, IP address, port number E. AE title, domain name, IP address

Answer 22

IP address, port number, AE title DICOM is a standard used so that applications can export and accept data from other applications. In order for other DICOM applications to connect to your application, you would need the IP address of the computer hosting the application, the port number, and the AE title. “AE” stands for application entity, and it is the unique name used to identify your application to the other DICOM applications that wish to connect to it.

Question 23

In order to legally prescribe Gamma Knife treatments, a physician must be _____. A. A full-time employee of the hospital B. board certified in radiation oncology, radiology, or nuclear medicine C. listed as an authorized user for Gamma Knife on the hospital’s radioactive materials license
D. All of the above are true. E. None of the above is true

Answer 23

listed as an authorized user for Gamma Knife on the hospital’s radioactive materials license As long as the physician is approved as an authorized user, specific for the Gamma Knife by either the NRC or the appropriate state regulatory agency, any licensed physician may prescribe radiation treatments. Neurosurgeons, for example, may be approved as an authorized user provided they have the proper training. In practice, however, it is difficult for anyone other than a board certified radiation oncologist to obtain designation as an authorized user

Question 24

A radiation survey of an uncontrolled area with an ionization chamber with the beam aimed directly at a barrier wall reports 25 mR hr–1. If the use (U) factor is 0.5 for that barrier and the beam is on for a total of 5 minutes per hour, what is the estimated mR in any hour at this location? A. B.
1 mR in any hr 2 mR in any hr
ANSWER
C. 12 mR in any hr D. 20 mR in any hr E. 25 mR in any hr

Answer 24

1 mR in any hr By applying the fraction of time the beam is on and the use factor, one determines that 25 mR/hr × 0.5 × 5 min/hr / 60 min/hr = 1.0 mR in any 1 hour. Although the circumstance that the location is an uncontrolled area is not paramount to the answer, note that the resultant estimated dose to this location is less than 2 mR in any hr, a public dose lim

Question 25

What is RO-ILS

Answer 25

a radiation oncology incident learning system sponsored by ASTRO and AAPM RO-ILS stands for “radiation oncology incident learning system.” For more details see Evans and Ford, “Radiation Oncology Incident Learning System: A Call to Participation.” IJROBP 90:249–50, 2014.

Question 26

Deformable image registration is the least reliable when registering which pair of images? A. an old CT to a new CT for the purpose of dose summation B. the CT portion of a PET/CT to a simulation CT and applying the transformation to the PET
ANSWER
C. a diagnostic contrast CT to a simulation CT D. a T1-weighted MRI to a simulation CT E. a simulation CT to a cone-beam CT

Answer 26

a T1-weighted MRI to a simulation CT Compared to performing a deformable image registration between image sets of the same modality, inter-modality deformable image registration, such as CT to MR, is less reliable.

Question 27

Diagnostic x-ray tubes usually have both a large and a small focal spot mode. One would use the larger focal spot when _____. A. a large field size is needed B. high mAs is required C. sharper field penumbra is needed D. high contrast is needed E. All of the above are true

Answer 27

high mAs is required A larger focal spot facilitates better heat dissipation, which is required when high mAs (milliamp seconds) exposures are required. Otherwise the tube would overheat. In all other respects, however, a large focal spot image is inferior in image quality to a small focal spot.

Question 28

When the image fusion of a diagnostic PET/CT and a simulation CT is performed, which scans should be registered to each other? A. sim CT to CT from the diagnostic PET/CT B. sim CT to PET from the diagnostic PET/CT C. CT from the diagnostic PET/CT to PET from the diagnostic PET/CT D. All of the above are true. E. This type of registration is not possible

Answer 28

sim CT to CT from the diagnostic PET/CT The CT and PET portions of the diagnostic PET/CT share DICOM coordinates. In other words, the images are acquired and automatically aligned based on the couch positions, under the assumption that no patient motion occurred between scans. To correctly fuse the simu-lation CT and the diagnostic PET/CT, the CT from the diagnostic PET/CT scan is registered to the simulation CT. That registration matrix is then applied to the PET from the diagnostic PET/CT so that the PET information can be shown overlaid upon the simulation CT

Question 29

Which image has the wider CT window

Answer 29

b
The width of the CT window is a measure of the range of CT number values displayed. When the window is narrow, as in picture a, the visual transition from dark to light structures in the image will happen quickly. Narrow windows are used when trying to differentiate between tissues with similar attenuation properties, whereas wide windows are used in areas where tissues with very different attenuation properties are adjacent to one

Question 30

In a CT scan, as the _____ increases, the signal-to-noise ratio (SNR) decreases. A. mAs B. slice thickness C. patient thickness D. All of the above are true. E. None of the above is true

Answer 30

patient thickness The SNR increases as the number of photons absorbed by the detector increases. Increasing the mAs and the slice thickness will both increase the number of photons incident upon a particular detector. However, increasing patient thickness will result in greater attenuation of the x-ray beam, resulting in fewer photons reaching the detector

Question 31

A lung V20 of 25% means that _____% of the lung receives _____ Gy. A. 75; at most 20 B. 75; exactly20 C. 75; at least 20 D. 25; at most 20 E. 25; exactly 20

Answer 31

75; at most 20 A V20 of 25% means 25% of the volume receives at least 20 Gy, or 75% of the volume receives at most 20 Gy

Question 32

Patient motion during treatment delivery results in errors in delivered dose and in target conformality. Rank the impact of patient motion errors for the treatment modalities below from smallest to largest. A. protons, 3D conformal x-rays, IMRT x-rays, brachytherapy B. brachytherapy, 3D conformal x-rays, IMRT x-rays, protons C. 3D conformal x-rays, protons, IMRT x-rays, brachytherapy D. brachytherapy, protons, IMRT x-rays, 3D conformal x-rays E. Errors are approximately the same for all of these treatment modalities.

Answer 32

brachytherapy, 3D conformal x-rays, IMRT x-rays, protons Since brachytherapy sources are implanted into the patient, the patient motion errors are smallest. The proton range depends critically on radiological path length, and changes in patient position could have a large effect on the types of tissues traversed by the beam. Although photons are less sensitive to radiological path length than protons, IMRT x-rays result in larger errors than 3D conformal x-rays because of the concave dose distributions created by IMRT.

Question 33

A 2 mm misalignment between the radiation and imaging isocenters would result in a _____ error, and its contribution to the average setup error would be _____ 2 mm. A. random; less than B. random; greater than C. random; exactly D. systematic; less than E. systematic; exactly

Answer 33

systematic; exactly The misalignment between the radiation and imaging isocenters is a systematic error that cannot be corrected with image guidance.

Question 34

Which IGRT technique utilizing orthogonal kilovoltage imaging would be the most accurate method of setup verification for a respiratory-gated treatment of a liver-lesion? A. acquire at end-inspiration with alignment to soft tissue B. acquire at end-exhalation with alignment to soft tissue C. acquire during the respiratory gate with alignment to soft tissue D. acquire during the respiratory gate with alignment to bony anatomy E. acquire during the respiratory gate with alignment to fiducial markers implanted in the tumor

Answer 34

acquire during the respiratory gate with alignment to fiducial markers implanted in the tumor Images should be acquired during the respiratory gate in order to validate the correlation between the external gating signal and the internal anatomy. The setup alignment should be based on fiducial markers, high-contrast objects that move with the tumor. Orthogonal kilovoltage images cannot reliably resolve soft tissue.

Question 35

It is desired to have the intensity across a “step and shoot” IMRT treatment field decrease by 3% /cm. If the width of the field in the direction of the intensity gradient is 10 cm, what is the minimum number of steps needed to achieve this intensity pattern? A. B. C.
1 3 5
D. 10 E. 30

Answer 35

10
Such a treatment would require at least 10 steps with field widths for each step of 10, 9, 8, …., 2, and 1 cm, respectively. Even with 10 steps, the dose intensity variation would not be smooth across the field, but rather would have discrete steps of 3% every cm. Thus, more than 10 steps would result in a smoother dose gradient.

Question 36

The effects of “interplay” between dynamic multi-leaf collimator motion and tumor motion due to respiration becomes less important as you _____ the number of fractions and _____ the number of treatment fields or arcs. A. decrease; decrease B. decrease; increase C. increase; decrease D. increase; incre

Answer 36

increase; increase Increasing the number of fractions and the number of treatment fields or arcs will decrease the likelihood of synchronization between the MLC motion and the target motion. Even if it is synchronized for a particular fraction or beam, the effect would get washed out over the course of many fractions or beam

Question 37

For the same anatomy, a VMAT plan generally has _____ total monitor units than a static-gantry IMRT plan delivered with a sliding window technique. A. more B. fewer C. about the same number of D. The answer depends on the photon energy. E. No relation can be establish

Answer 37

fewer The MU for a VMAT plan is generally lower than that for a static IMRT plan for the same patient target and anatomy because VMAT treatments usually have larger beam apertures and, thus, higher MU efficiency.

Question 38

Which treatment modality will most likely deliver the lowest dose to the heart when treating a left-sided breast cancer? A. parallel opposed tangents using “hard” physical wedges B. parallel opposed tangents using virtual or dynamic wedge C. parallel opposed tangents using field-in-field technique D. multi-beam IMRT photons E. IMPT protons

Answer 38

E. IMPT protons Assuming the treatment plan is properly calculated, the range of a proton beam can be precisely controlled, thus minimizing the heart dose for proton therapy

Question 39

Regarding TBI treatments, which of the following is true? A. When used, a spoiler needs to be placed close to the patient. B. Dose homogeneity is better for lateral beams than for anterio-posterior beams. C. The bowel is the dose-limiting organ. D. The radiation and light fields are congruent at extended SSDs. E. All of the above are true.

Answer 39

A. When used, a spoiler needs to be placed close to the patient. The spoiler needs to be placed close to the patient in order to increase the surface dose. If it is placed too far away, an insufficient number of electrons will reach the patient. Lungs are at the greatest risk for complication, and they are usually blocked. Patient separations are generally smaller in the anterio-posterior direction, so the dose homogeneity will be better with that beam arrangement. Although the light and radiation fields may be congruent at 100 cm SAD, this is not the case for extended distances.

Question 40

Which of the following is used to calculate the administered activity for 223Ra treatment of castration-resistant prostate cancer metastatic to bone? A. weight B. height C. body-mass index D. PSA level E. Gleason scor

Answer 40

A. weight Currently, the administered activity is 1.49 μCi/kg

Question 41

decay of Y-90

Answer 41

beta–Y-90 undergoes beta–decay with a decay energy of 2.28 MeV. 0.01% of the decays produce 1.7 MeV photons.

Question 42

Radium-223 is used in the treatment of castration-resistant prostate cancer metastatic to bone. Which particle is responsible for delivering almost all of the dose? A. alpha B. beta+ C. beta–D. gamma E. neutron

Answer 42

alpha Radium-223 goes through a decay chain yielding 4 alphas, 3 betas, and numerous gammas. About 95% of the dose to the target cells is from alpha particles.

Question 43

The treatment time to deliver 100 Gy using a SRS technique with multiple (~200) Co-60 sources is 25 minutes. How many minutes would it take to deliver a prescribed dose of 60 Gy with the same plan and the exact same Co-60 sources five years later? A. 20 B. 25 C. 30 D. 50 E. 60

Answer 43

Radium-223 is used in the treatment of castration-resistant prostate cancer metastatic to bone. Which particle is responsible for delivering almost all of the dose? A. alpha B. beta+ C. beta–D. gamma E. neutron

Question 44

According to the AAPM Practice Guidelines for SRS/SBRT (MPPG 9a), end-to-end dosimetric evaluation using the SRS frame or IGRT system should be performed at initial commissioning and _____. A. at no other time B. daily C. weekly D. annually E. bi-annually

Answer 44

annually The AAPM Practice Guideline for SRS/SBRT describes the minimum level of medical physics support necessary for SRS and SBRT. Table 1 in the guideline summarizes equipment QA and associated tolerances. End-to-end testing, both for localization assessment and dosimetric evaluation, is listed as an annual test

Question 45

According to the AAPM Practice Guidelines for SRS/SBRT (MPPG 9a), the tolerance of the monthly radiation isocentricity test should be within _____ mm for SRS and _____ mm for SBRT. A. 0.5; 0.5 B. 1.0; 2.0 C. 1.5; 1.5 D. 0.5; 1.0 E. 1.0; 1.5

Answer 45

1.0; 1.5
The monthly radiation isocentricity test should cover the range of gantry, collimator, and couch positions used clinically.

Question 46

Which of the following is a unique consideration when planning with a single, rather than multiple, isocenter for treatment of multiple cranial metastases? A. dose spillage between targets B. dose conformality C. image guidance technique D. distance from each target to the skull E. distance from each target to the isocente

Answer 46

distance from each target to the isocenter When using separate isocenters for each target, each isocenter is usually placed within the centroid of the target. However, when using a single isocenter for multiple metastases, the isocenter is usually at some point between the targets. The farther a particular target is from the isocenter, the larger the effect of rotational setup erro

Question 47

For the same dose level, which of the following types of radiation would most likely cause the greatest harm to human tissue? A. 10 keV neutrons B. 2 MeV neutrons C. 12 MeV electrons D. 1.33 MeV gamma rays E. 70 MeV protons

Answer 47

2 MeV neutrons Relative biological effectiveness (RBE) per unit mass tissue can be used to compare the amount of harm caused by each interaction with the various masses of human tissue. RBE values are based on ICRP 60’s 1990 Recommendations of the International Commission on Radiological Protection. Neutrons have a range of RBE from 5 to 20 depending on energy, with the most damaging neutrons in the range of 100 keV to 2 MeV

Question 48

A brachytherapy source which emits a single gamma ray per decay has a half-life (t1/2), average gamma energy (E), and activity (A). The dose rate in air at 5 cm distance from this source depends on _____. A. only the half-life B. only the activity C. only the energy D. both the half-life and the activity, but not the energy E. both the activity and the energy, but not the half-life

Answer 48

both the activity and the energy, but not the half-life The half-life is irrelevant. The activity specifies how many gammas are emitted per second, and the energy allows calculation of the dose per decay

Question 49

According to the ACR-AAPM Technical Standard for the Performance of High-Dose-Rate Brachytherapy Physics, after a HDR source exchange, the activity of the source must be verified to fall within _____ of the manufacturer’s certificate. A. 0.1% B. 0.5% C. 1% D. 3%

Answer 49

3% The 2015 document describes the entire HDR process, including details on the appropriate quality assurance program, of which, the calibration of the sealed HDR source is part.

Question 50

According to the ACR and AAPM, what is the maximum allowable deviation between the measured and intended dwell positions in the applicator and the step-size spacing between dwell positions for HDR? A. 0.1 mm B. 0.5 mm C. 1.0 mm D. 2.0 mm E. 5.0 mm

Answer 50

1.0 mm
According to the joint ACR-AAPM technical standards, a qualified medical physicist can use an autoradiograph or other suitable method to make this measurement before the first use of the remote afterloader on each given day.

Question 51

Radioactive seeds for a prostate implant should be calibrated using _____. A. a well chamber B. a Farmer chamber at 10 cm in air C. a Farmer chamber at 1 cm in water D. radiochromic film E. radiographic film

Answer 51

a well chamber Well chambers calibrated by an Accredited Dosimetry Calibration Laboratory (ADCL) can be used to measure LDR, HDR, and beta sources.

Question 52

According to the NRC and AAPM Task Group 59, daily checks for remote afterloader high-dose-rate brachytherapy include all of the following except _____. A. temporal accuracy B. source positioning accuracy C. door interlocks D. source output measurement E. All of these choices are daily tests for high-dose-rate brachytherapy.

Answer 52

source output measurement Well chamber measurements of source output are required annually or when a source is exchanged. The source output should be verified daily in the treatment planning system versus a decay table

Question 53

Two patients are being treated with a high-dose-rate afterloader using vaginal cylinders. The prescribed dose, treatment length (i.e., the number of dwell positions), and target depth of 0.5 cm from the cylinder surface are all the same for both. Patient A is treated using a 2.5 cm diameter cylinder, and patient B is treated using a 3.5 cm diameter cylinder. When compared to patient A, the treatment time for patient B will be _____ and the dose at the cylinder surface will be _____. A. shorter; lower B. shorter; higher C. shorter; unchanged D. longer; lower E. longer; higher

Answer 53

longer; lower For a line source, the dose rate at a point near the source is approximately inversely proportional to the distance from the source. The prescription point for patient B is farther from the source than the prescription point for patient A, hence it requires a longer treatment time. Since, for patient B, the distance from the prescription point to the source is larger, the dose falloff from the cylinder surface to the prescription point is less (i.e. dose fall from 1.75 cm to 2.25 cm for patient B is less than the dose falloff from 1.25 cm to 1.75 cm for patient A). Therefore, for the same prescribed dose at 0.5 cm depth from the cylinder surface, the dose at the cylinder surface for patient B is lower

Question 54

A permanent seed implant is performed using an isotope with half-life of 20 days. What percent of the total dose will the patient have received 60 days after the implant was performed? A. 50.0% B. 69.3% C. 75.0% D. 87.5% E. 95.2%

Answer 54

87.5% For this isotope, 60 days is 3 half-lives. The remaining activity after 3 half-lives is: (0.5)3 = 12.5% or e-0.693(60/20) = 12.5%. This means that 87.5% of the initial radioactive material has decayed, so the patient has already received 87.5% of the total dose.

Question 55

During an HDR treatment, which of the following personnel must be present? A. prescribing radiation oncologist B. radiation therapist C. radiation oncology resident D. qualified medical physicist E. oncology nurse

Answer 55

D. qualified medical physicist A radiation oncologist must be present, but not necessarily be the prescriber. The qualified medical physicist must be there as well. Depending on the institution, a radiation therapist may be there, but a radiation oncologist can operate the afterloader

Question 56

For the first 5 cm, the dose falloff with distance in water from a point source follows the inverse square law (1/r2) for all of the following isotopes except _____. A. 137Cs B. 192Ir C. 198Au D. 60Co E. 103Pd

Answer 56

103Pd For 137Cs,192Ir, 198Au, and 60Co, over a distance of about 5 cm from the source, the attenuation of the primary photons is very much compensated for by the contribution of scattered photons. For 103Pd, and 125I, the attenuation component is larger than the scattering due to their low photon energies.

Question 57

In AAPM Task Group 43, the anisotropy factor, F(r, θ), accounts for the angular dependence of _____. A. only photon absorption in the source encapsulation and the medium B. only photon scattering in the source encapsulation and the medium C. both photon absorption and scattering in the source encapsulation and the medium D. only photon absorption in the source encapsulation E. only photon scattering in the source encapsulation

Answer 57

C. both photon absorption and scattering in the source encapsulation and the medium In TG-43, the anisotropy factor accounts for both photon absorption and scattering in the source encapsulation and the medium, with the inverse square correction factored out.

Question 58

Use of a dual-energy CT scanner for proton beam treatment planning has the advantage(s) over a single-energy CT scanner of _____. A. more accurate determination of proton beam range in the patient B. more accurate segmentation of soft tissues C. better information about setup uncertainties D. All of the above are true. E. None of the above is true.

Answer 58

A. more accurate determination of proton beam range in the patient Proton range depends on both the density and atomic number (Z ) of the tissue. Single-energy CT scanning does not allow accurate determination of atomic number, but dual-energy CT scanning has the clear potential advantage of improving proton beam range predictions. Thus, single-energy scanning is adequate for photon treatment planning, but may lead to errors in proton beam planning. Dual-energy scanners can acquire scans at both energies simultaneously, so they won’t add information about setup uncertainties. Soft tissues have similar atomic numbers, so dual-energy CT won’t enhance the ability to segment them over single-energy CT.

Question 59

Pencil beam scanning protons have _____ than double scattering protons. A. a lower entrance dose B. a higher RBE C. a sharper penumbra D. All of the above are true. E. None of the above is true

Answer 59

A. a lower entrance dose Typically pencil beam scanning has lower entrance dose since the plan can be inverse planned for proximal dose. Double scattering beams are optimized for distal dose. RBE is about the same as double scattering (1.1) and, typically, the penumbra is less sharp than double scattering because of the absence of an aperture near the patient. Apertures are slowly being introduced into PBS.

Question 60

The proton beam RBE at the end of the spread out Bragg Peak is _____ the RBE at the center of the spread out Bragg Peak. A. always higher than B. always lower than C. always the same as D. higher for passive scattered but lower for pencil beam scanning E. lower for passive scattered but higher for pencil beam scanning

Answer 60

A. always higher than End-of-range effects may be responsible for higher RBE effects at the end of the spread out Bragg Peak. Some proton centers attempt to mitigate these effects by not having all of the beams stop on the same critical structure

Question 61

The primary advantage of protons over x-rays for therapy is _____. A. less cost B. sharper penumbra C. higher RBE D. no exit dose E. insensitivity to organ motion

Answer 61

D. no exit dose The absence of exit dose allows proton beams to spare organs at risk downstream. Protons are more expensive than x-rays and are more sensitive to uncertainties, such as those introduced by organ motion. The clinical RBE is higher, 1.1, but this is incorporated in the prescription doses used, so the equivalent dose to the target is similar to that for photons. The lateral penumbra for protons is sharper at shallow depths, but larger at deeper depths

Question 62

The penumbra of a proton beam is a function of _____. A. depth B. energy C. air gap between the patient and the aperture D. All of the above are true. E. None of the above is true

Answer 62

All of the above are true. Proton scatter causes lateral spread of the beam beyond the field edges, and the amount of scatter is dependent upon the depth in the patient, the energy of the beam, and the air gap between the aperture and the patient.