RAPHEX XII

Question 1

The ____ is the strongest force in the nucleus. A. electromagnetic force B. weak nuclear force C. gravitational force D. strong nuclear force E. None of above is true

Answer 1

strong nuclear force
There are four different forces in the nucleus. The gravitational force involved in the nucleus is very weak and can be ignored. The electromagnetic force between protons is quite strong, but it is repulsive and tends to disrupt the nucleus. The strong nuclear force is much stronger than the electromagnetic force and is responsible for holding the nucleons together in the nucleus. The weak nuclear force is very weak and appears in certain types of radioactive decay.

Question 2

Alpha particle emission usually in occurs in atoms with _____. A. substantially more protons than neutrons in the nucleus B. substantially more protons than electrons in the atom C. nuclei with an atomic number larger than 82 D. nuclei with an atomic number less than 82 E. None of the above is true.

Answer 2

C. nuclei with an atomic number larger than 82
As the atomic number increases beyond 82, the coulomb forces of repulsion between the protons become large enough to overcome the nuclear forces that bind the nucleons. The unstable nucleus emits an alpha particle composed of 2 protons and 2 neutrons.

Question 3

A. A 0.25-cm MLC can produce better conformality for small brain lesions. B. A 0.25-cm MLC may limit the use of monoisocentric plans for large fields (e.g., breast tangents with supraclav field).
ANSWER
C. A 0.25-cm MLC will require a separate treatment planning system. D. A 0.25-cm MLC can be used for IMRT and VMAT deliveries. E. Dynamic wedges can be used regardless of MLC width.

Answer 3

A 0.25-cm MLC will require a separate treatment planning system.
The smaller leaf width will result in more dose conformity around small lesions. Often the MLC containing smaller leaves will not cover the entire field length, so while the 0.25-cm MLC can be used for IMRT, VMAT, and dynamic wedge deliveries, large field techniques—such as single isocenter breast and supraclav treatments—may not be possible due to restrictions on the maximum field size that can be shaped with the MLC

Question 4

The ion chambers of a linac monitor _____. A. output B. flatness C. symmetry D. All of the above are true. E. None of the above is true.

Answer 4

D. All of the above are true.
The ionization chambers in a linac are usually sealed and pressurized so that environmental variations do not affect machine output. They are located before the secondary collimator. They are designed such that they can measure output, beam flatness, and beam symmetry.

Question 5

As the collimator setting decreases, photon backscatter into the monitor chamber _____. A. increases B. decreases C. remains the same D. There is not enough information given to answer this question.

Answer 5

increases

Question 6

An 80-keV photon will most likely undergo _____ in water. A. coherent scattering B. the photoelectric effect C. Compton scattering D. pair production E. We are unable to determine thi

Answer 6

C. Compton scattering
Photon interactions are probabilistic; it is impossible to predict which interaction an individual photon will undergo. However, the relative importance of various types of interactions in water can be found in Table 5.2 of Khan’s The Physics of Radiation Therapy. A 10-keV monoenergetic photon beam has 95% photoelectric effects and 5% Compton effects. At 26 keV the relative percentage of interactions in water is 50% photoelectric effects and 50% Compton effects. At 60 keV the relative percentage of interactions in water is 7% photoelectric effect and 93% Compton effects.

Question 7

Charged particle equilibrium occurs when charged particles leaving a volume and charged particles entering the same volume have _____. A. the same number of each type of particle at each specific energy B. the same number of each type of particle, regardless of energy C. the same number of particles, although the particle types may differ D. the same total energy, regardless of whether the particle types are the same E. the same total energy, regardless of whether the particle quantities are the same

Answer 7

A. the same number of each type of particle at each specific energy
In order for charged particle equilibrium to exist, the number of charged particles of each particular type and energy leaving a volume is the same as the number of charged particles of the same type and energy entering the same volume.

Question 8

Of the following, _____ would be the most suitable for measuring CBCT dose on the linac. A. a Farmer-type ionization chamber B. a free air ionization chamber C. optically stimulated luminescent dosimeters (OSLDs) D. thermoluminescence dosimeters (TLDs) E. radiochromic film

Answer 8

A. a Farmer-type ionization chamber
Of the options listed, a Farmer-type ionization chamber would be the most suitable choice for measuring CBCT dose. A free-air ionization chamber is most often used by standards laboratories for measurements of exposure. OSLDs and TLDs have low sensitivity, and great care would need to be taken to ensure an accurate reading. Radiochromic film also would be difficult to use.

Question 9

If an ion chamber is not sealed, the chamber reading for a given exposure will increase as the temperature _____ or the pressure _____. A. increases; increases B. increases; decreases C. decreases; increases D. decreases; decreases

Answer 9

C. decreases; increases
The density of air in the ion chamber cavity depends on the temperature and pressure, in accordance with the gas law. The density or mass of air in the chamber volume will increase as the temperature decreases or pressure increases. Exposure is defined as the ionization charge collected per unit mass of air. Therefore, the chamber reading for a given exposure will increase as the temperature decreases or as the pressure increases

Question 10

An ion chamber scanning vertically in a water tank measures which of the following parameters for an electron treatment beam? A. backscatter factor B. tissue phantom ratio C. tissue maximum ratio D. percent depth ionization E. percent depth dose

Answer 10

D. percent depth ionization
The ion chamber actually measures the ionization by the radiation beam, which does not exactly coincide with the radiation dose. The stopping power ratio, which is energy-dependent and changes with depth, can be used to convert percent depth ionization (PDI) to percent depth dose (PDD).

Question 11

What is physically happening when an optically stimulated luminescent dosimeter (OSLD) emits light?
ANSWER
A. An electron falls into an electron trap. B. An electron escapes from an electron trap. C. An electron travels freely through the valence band. D. An electron travels freely through the conduction band. E. None of the above is true

Answer 11

B. An electron escapes from an electron trap.
When an OSLD chip is irradiated, an electron from the valence band can get sufficient energy to move into the conduction band. As it falls back down from the conduction band it may get trapped by an electron trap. These traps are caused by imperfections in the lattice structure of the material. Visible light of a particular wavelength is used when reading out an OSLD chip, which frees the trapped electron. The energy difference between the trapped state and the valence band is emitted as visible light.

Question 12

Of the following, a _____ is an absolute dosimeter. A. free-air ionization chamber B. calorimeter C. ferrous sulfate (Fricke) dosimeter D. All of the above are true. E. None of the above is true

Answer 12

D. All of the above are true.
Absolute dosimeters can determine radiation dose without reference to another dosimeter. A free-air ionization chamber is often used by standards laboratories and measures electric charge resulting from ionization of air in a radiation field. Calorimeters measure the heat resulting from the deposition of radiation energy. A Fricke dosimeter is a chemical dosimeter, where the oxidation of ferrous ions to ferric ions in a radiation field is measured.

Question 13

The average energy of a linac’s 15-MV photon beam at the surface of the patient is _____. A. 3 MeV B. 5 MeV C. 7.5 MeV D. 10 MeV E. 15 MeV

Answer 13

5 MeV
The average energy of a bremsstrahlung x-ray energy spectrum is approximately Emax / 3, where Emax is equal to the ionizing potential of the linac for that beam, in this case 15 MV.

Question 14

A patient is being treated with a four-field box with 6-MV x-ray beams equally weighted to the prescription point. At setup it is noted that the AP beam’s depth to the prescription point is 2 cm less than expected, and the depths of the other 3 beams are unchanged. What is the approximate dose difference at the prescription point? A. 10% less than planned B. 5% less than planned C. 2% less than planned D. E.
2% more than planned 5% more than planned

Answer 14

D. 2% more than planned
A general approximation is that each centimeter of missing tissue results in 3.5% less attenuation for a 6-MV x-ray beam; 2 cm × 3.5%/cm / 4 is approximately 2% more dose than expected.

Question 15

For an MV photon treatment on a patient with a bolus placed on top of an ill-fitting thermoplastic immobilization mask, as an air gap between the bolus on the mask and the patient surface increases, the depth of maximum dose will _____ and the surface dose will _____. A. decrease; decrease B. increase; decrease C. decrease; increase D. increase; increase E. remain the same; decrease

Answer 15

B. increase; decrease
The effect of bolus is reduced as the air gap increases, resulting in the depth of maximum dose increasing and surface dose decreasing.

It’s like having an air gap in the PDD. You once had electronic equilibrium, then you removed it. So the buildup region exists again, and dmax gets deeper. The further you move away, the more the electrons you’ve created disperse and scatter away, so surface dose decreases.

Question 16

A patient is treated with an isocentric AP/PA setup on a linac. The anterior SSD is 86 cm, and the posterior SSD is 92 cm. The patient’s AP/PA separation is _____. A. 14 cm B. 16 cm C. 22 cm D. 28 cm E. This cannot be determined from the information given.

Answer 16

C. 22 cm
The isocenter is 100 cm – 86 cm = 14 cm from the anterior surface and 100 cm – 92 cm = 8 cm from the posterior surface. Therefore, the patient separation is 14 cm + 8 cm = 22 cm.

Question 17

The collimator setting is the field length and width as defined at the level of the _____. A. patient entrance surface B. patient exit surface C. jaws in the linac head D. multileaf collimator in the linac head E. isocenter

Answer 17

E. isocenter
The collimator setting reported by the linac is the size of the projection of the jaws at the isocenter of the machine. In other words, the size of the physical jaw opening that one would measure with a ruler in the head of the linac would be much smaller than the reported collimator setting. The collimator setting is independent of the position of the patient, although surely the patient could be placed such that the isocenter is at their entrance or exit surface.

Question 18

A patient is being treated to a dose of 10 Gy to a depth of 10 cm with a 6-MV posterior photon beam in an SAD setup. The spinal cord is located upstream of the target at a 4-cm depth. The TMRs are 0.770 and 0.946 for 10 cm and 4 cm depths, respectively. What is the dose to the spinal cord?

A. 8.0 Gy B. 10.8 Gy C. 12.2 Gy D. 13.9 Gy E. This cannot be determined using the information given.

Answer 18

D. 13.9 Gy
Given that the SPD is the source to point of interest distance, D(d1) / D(d2) = TMR(d1) / TMR(d2) × (SPD2 / SPD1)2 10 Gy / D(4 cm) = (0.770 / 0.946) × (94 / 100)2 D(4 cm) = 13.9 Gy

Question 19

External beam prostate treatments should use photon energies less than or equal to 10 MV for patients with a pacemaker because with higher energies _____. A. the fields will be too large and will include the pacemaker B. photons scattered from the treatment fields would give more than 5 Gy to the pacemaker C. photons scattered from the treatment head would give more than 5 Gy to the pacemaker D. stray neutrons that penetrate the linac shielding can trigger pacemaker irregularities E. prostate cancer is always treated with lower-energy photons or brachytherapy

Answer 19

D. stray neutrons that penetrate the linac shielding can trigger pacemaker irregularities
It has been observed that stray neutrons can trigger pacemaker irregularities. Significant numbers of neutrons are only produced for photon beams of energies above 10 MeV. (See the Report of AAPM Task Group 203, Management of Radiotherapy Patients with Implanted Cardiac Pacemakers and Defibrillato

Question 20

Tungsten eye shields effectively protect the lens and eye for _____. A. Co-60 teletherapy B. megavoltage x-ray teletherapy C. megavoltage electron teletherapy D. eye plaque brachytherapy E. All of the above are true.

Answer 20

C. megavoltage electron teletherapy
A tungsten eye shield can be used to stop external beam electrons, but it would not be thick enough to stop Co-60 gamma rays or MV x-rays.

Question 21

what does raphex use for 90% isodoe line for electrons?

Answer 21

E/3.2

Question 22

When the electron field size is smaller than the practical range of the electron beam, all of the following are true EXCEPT for which statement? A. The dose rate decreases. B. Dmax moves closer to the surface. C. The PDD curve changes. D. Dose rate remains constant with the field shape.

Answer 22

D. Dose rate remains constant with the field shape.
For electron field sizes larger than the practical range of the electron beam, the PDD curve remains relatively constant with increasing field size, since the electrons from the periphery of the field are not scattered sufficiently to contribute to the central axis depth dose. When the field is reduced below that required for lateral scatter equilibrium, the dose rate decreases, Dmax moves closer to the surface, and the PDD curve changes. In addition, when the electron field size (e–insert) is smaller than the electron practical range, the output can be sensitive to the field shape

Question 23

Order the following materials from LEAST to MOST production of backscattered electrons for a 9-MeV electron beam. A. bolus, aluminum, lead B. lead, bolus, aluminum C. aluminum, lead, bolus D. lead, aluminum, bolus E. bolus, lead, aluminum

Answer 23

A. bolus, aluminum, lead
Backscatter increases with increasing atomic number (Z). Bolus has an effective atomic number similar to water (~8), followed by aluminum (Z=13), and lead (Z=82). When designing internal shielding for therapy electron beams, the backscatter properties of the materials are considered. Often composite materials, e.g., lead and bolus, are used in the shield in order reduce the shield’s thickness. In such a case, the highest-Z material should be at the end of the electron range, i.e., adjacent to the OAR, as it will backscatter into the lower-Z material

Question 24

The cumulative dose for an implanted cardiac device (ICD), according to AAPM Task Group 203, should be kept to less than or equal to _____ or the manufacturer’s recommended dose threshold if higher. A. B. C. D.
10 cGy 100 cGy 200 cGy 500 cGy
E. 1000 cGy

Answer 24

D. 500 cGy
AAPM TG-203 recommends that the cumulative dose for ICDs should be kept to less than or equal to 500 cGy, or the manufacturer’s recommended dose threshold, if highe

pretty sure this is 2 Gy now..

Question 25

According to AAPM Task Group 142, the tolerance for the daily check of the linac optical distance indicator (ODI) is _____. A. 0.5 mm B. 1 mm C. 2 mm D. 3 mm E. 5 mm

Answer 25

C. 2 mm
Table 1 in AAPM TG-142 specifies that the daily check of the ODI is within 2 mm for linacs, regardless of the intended types of treatments for that machine.

Question 26

According to AAPM Task Group 142, the tolerance for the monthly check of the jaw position indicators is _____ mm for symmetric and _____ mm for asymmetric jaw settings. A. 0.5; 0.5 B. 1; 1 C. 1; 2 D. 2; 1 E. 2; 2

Answer 26

2; 1
The tolerance for each individual jaw is 1 mm, which is why the tolerance for the total symmetric jaw setting is twice the tolerance for each individual jaw, i.e., 2 mm

Question 27

How is a patient’s name associated with a series of CT slices in the DICOM standard? A. Patient demographic information is contained within a text file in the same folder as the CT image file.

B. Patient demographic information is contained within a header inside the CT image file. C. The name of the CT image file must contain the patient’s name. D. The name of the CT image file must contain a unique code that can get traced back to the patient demographic information on the CT scanner.
E. None of the above is true.

Answer 27

B. Patient demographic information is contained within a header inside the CT image file.
In the DICOM standard, each DICOM file contains a header with patient demographic information. For a CT scan DICOM object, the header would also contain technical information about the scan

Question 28

The concrete shielding of an existing linear accelerator vault is calculated to be insufficient for a new high-energy machine and will require placement of 2” of lead. Where should the lead be positioned relative to the concrete, given in order from the first material the beam interacts with to the last? A. 2” lead then concrete B. concrete then 2” lead C. 1” lead, then concrete, then 1” lead D. All of the above are equivalent. E. We are unable to determine this from the information given.

Answer 28

A. 2” lead then concrete
The lead must be affixed to a sturdy and stable surface, such as concrete or steel. High-energy photons when interacting with the lead can generate photo-neutrons, which can then produce capture photons with a maximum energy of approximately 2 MeV. These generated photons will also need to be shielded. Therefore, the lead should be affixed inside the vault against the concrete barrier

Question 29

Positron Emission Tomography (PET) images are formed by detecting two _____ emitted in colinear directions. A. positrons B. electrons C. protons D. photons E. None of the above is true.

Answer 29

photons
PET uses positron-emitting isotopes incorporated into a biologically active compound (e.g., fluorodeoxyglucose, 18F-FDG) which is injected into the patient. 18F decays by positron emission (half-life ~110 min). The positrons travel about 1–2 mm before annihilating with an electron and emitting two 511-keV gammas approximately in opposite directions (180° ±0.25°

Question 30

On a T1-weighted MR image, _____ appears brightest. A. air B. blood C. cerebrospinal fluid (CSF) D. cortical bone E. fat

Answer 30

fat

Question 31

Regarding CT dose index (CTDI), which of the following statements is FALSE? A. CTDI100 is the measured dose in one scan rotation over a longitudinal length of 100 mm.
ANSWER
B. CTDI100 is the measured dose at 100 cm from the source in one scan rotation. C. AAPM TG-75 recommends the reporting of CT dose in the units of CTDI. D. There are challenges with CTDI100 in a cone-beam geometry that is wider than 100 mm. E. CTDIw is the weighted average of CTDI100 measurements at two different locations in a phantom, one central and one peripheral.

Answer 31

B. CTDI100 is the measured dose at 100 cm from the source in one scan rotation.
AAPM TG-75 recommends the reporting of CT dose in units of CT dose index (CTDI). Important parameters are: *CTDI100, the measured dose in one scan rotation over a longitudinal length of 100 mm. *CTDIw, the weighted average of CTDI100 measurements at two locations in a specified phantom (one central and one peripheral).
*CTDIvol, the “volume CTDI” includes the effect of scan pitch and is defined as CTDIw/pitch.
* Dose length product (DLP), defined as CTDIvol multiplied by scan length.

Question 32

According to the report of AAPM Task Group 142, the test of cone-beam CT constancy of Hounsfield Units should be performed _____. A. daily B. weekly C. monthly D. semi-annually E. annually

Answer 32

C. monthly
This monthly requirement is different from the corresponding guidance for diagnostic CT and CT-sim, where HU in water is checked daily

Question 33

ich statement regarding the dose statistics table below can be definitively stated?

A. CTV includes volumes not contained in PTV. B. PTV contains erroneous voxels. C. CTV contains holes. D. Maximum dose is not located in CTV expansion. E. This lung lesion is peripherally located.

Answer 33

A. CTV includes volumes not contained in PTV.
Following ICRU 62, the PTV is an expansion of the CTV and, therefore, must include all voxels within the CTV. If the CTV minimum dose is less than the PTV minimum dose, the PTV must not be including these CTV voxels of minimum dose, indicating that the volumes were defined incorrectly

Question 34

_____ cGy is the typical MV-CBCT dose to isocenter for a pelvic case. A. 0.01 B. 0.1 C. 1 D. 10 E. 100

Answer 34

D. 10
According to the report of AAPM Task Group 180 (Image Guidance Doses Delivered During Radiotherapy: Quantification, Management, and Reduction), MV-CBCT can result in a dose greater than 10 cGy depending on the imaging site and the protocol

Question 35

Which of the following is a DISADVANTAGE of the use of optical surface imaging (OSI) for frameless intracranial stereotactic radiosurgery alignment? A. It can be used by RTTs while inside the treatment room. B. It has sub-millimeter accuracy for patient motion detection. C. It uses the face as a surrogate for the lesion position. D. All of the above are true. E. None of the above is true.

Answer 35

C. It uses the face as a surrogate for the lesion position.
It is true that OSI can be used by RTTs while inside the room, unlike x-ray-based imaging, although that is an advantage. OSI has good accuracy for motion detection. However, OSI uses the face as a surrogate for the lesion position for cranial SRS, which can be disadvantageous in that you are not directly monitoring the lesion itself. Motion of the face, or aspects of the face, don’t necessarily directly correlate to the position of the lesion.

Question 36

Which of the following techniques will deliver 21 Gy to a round brain lesion away from organs at risk in the shortest amount of time while providing similar coverage at the same beam energy and available maximum dose rate? A. VMAT B. step and shoot C. sliding window D. conformal arcs E. All of the above would take the same amount of time.

Answer 36

D. conformal arcs
3D conformal therapy excludes fluence modulation and will, therefore, deliver the plan in the shortest amount of time.

Question 37

A radionuclide injection used to treat bone metastasis from prostate cancer is _____. A. Ir-192 B. Pd-103 C. Ra-223 D. I-125 E. I-131

Answer 37

C. Ra-223
Ra-223 dichloride is available as a commercially produced injection that has been approved by the FDA to treat bone metastasis from prostate cancer. Ra-223 is an alpha emitter and can be used to treat bone cancers. Ir-192, I-125, and Pd-103 are gamma emitters typically used as sealed sources for brachytherapy. I-131 is a gamma and beta emitter ingested for thyroid treatment.

Question 38

Which of the following intraoperative radiotherapy (IORT) delivery methods will typically result in the greatest depth of penetration and best dose homogeneity in the treatment area? A. electrons B. HDR afterloader C. kilovoltage photons D. All of the above are similar.

Answer 38

A. electrons
Electrons used for IORT usually have energies in the 4 to 12 MeV range, which will result in greater penetration and dose homogeneity than one can get from an HDR afterloader or kilovoltage photon techniques. MV photons are not typically used for IORT.

Question 39

A cyclotron bombards stable nuclei with protons, typically producing _____ radioisotopes that decay via _____. A. short-lived; β–decay B. short-lived; positron emission C. long-lived; β–decay D. long-lived; positron emission E. long-lived; α decay

Answer 39

B. short-lived; positron emission
Cyclotrons bombard stable nuclei with protons, placing the resulting radioisotope below the line of stability. Therefore, the typical decay mechanism is via positron emission. Many of the short-lived radioisotopes used in PET imaging are produced by cyclotrons.

Question 40

For SRS, the Conformity Index was defined by the RTOG as a ratio of the _____, and the desired ratio is _____. A. volume covered by the prescribed dose to the target volume; less than 1 B. volume covered by the prescribed dose to the target volume; between 1 and 1.5 C. volume covered by the prescribed dose to the target volume; between 2 and 2.5 D. target volume to the volume covered by the prescribed dose; between 1 and 2 E. target volume to the volume covered by the prescribed dose; between 2 and 2.5

Answer 40

B. volume covered by the prescribed dose to the target volume; between 1 and 1.5

Question 41

An HDR plan was created using a 3-cm-diameter vaginal cylinder. A plan using a 3.5-cm-diameter cylinder but normalized to deliver the same dose to the reference point 5 mm from the cylinder surface would deliver _____. A. the same dose to the cylinder surface B. less dose to the cylinder surface C. more dose to the cylinder surface D. It depends on the length of the cylinder. E. There is insufficient information given to determine this.

Answer 41

B. less dose to the cylinder surface
An HDR vaginal cylinder acts as a line source, so the dose decreases proportional to the inverse of the distance from the source. This way the dose fall-off will be reduced when using a cylinder with a larger diameter. If the point at 5 mm depth is the same, the dose to the surface of the cylinder should be less for a larger cylinder.

Question 42

The American Brachytherapy Society guidelines for 3D planning for HDR cervical brachytherapy describe that the dose should be specified in terms of _____. A. maximum tolerance of the ICRU rectum point B. maximum tolerance of the ICRU bladder point C. D90 of the target volume D. point A E. point B

Answer 42

C. D90 of the target volume
According to the ABS recommendations made in 2011, “A consensus decision was made to integrate strategies utilizing 3D image-guidance when possible.” Furthermore, in those cases where 3D imaging is integrated, “though the dose to point A should be recorded, the goal should be good coverage (i.e., a D90) of the involved region.”

Question 43

All of the following isotopes are used for permanent prostate implants EXCEPT _____. A. Pd-103 B. I-125 C. Cs-131 D. Ir-192 E. All of the above are used for permanent prostate implants.

Answer 43

D. Ir-192
Due to the higher photon energy, Ir-192 is used for manual loading of temporary implants and remote afterloaders, but not for permanent prostate implan

Question 44

_____ is the unintended dose to a staff member who is 100 cm from the 10-Ci 192Ir source for 10 minutes (192Ir source exposure rate constant is 4.69 R-cm2 /mCi-h, f Factor = 0.962 cGy/R). A. 7.5 mGy B. 15 mGy C. 30 mGy D. 50 mGy E. 75 mGy

Answer 44

A. 7.5 mGy
The dose can be calculated: 4.69 R-cm2 / mCi-h × 1/(100 cm)2 × 10,000 mCi × 10 minutes × 1h/60 minutes × 0.962 cGy/R = 0.75 cGy = 7.5 mGy, where 0.962 cGy/R is the f Factor (roentgen-to-rad conversion factor for 340 keV photon energy and muscl

Question 45

The advantage of single-catheter, balloon-based HDR techniques over external beam radiotherapy techniques for partial breast irradiation is _____. A. one can treat irregularly shaped tumor beds B. one can sculpt the dose from chest wall or skin C. high dose gradients and rapid dose fall-off D. All of the above are true. E. None of the above is true.

Answer 45

C. high dose gradients and rapid dose fall-off
Single-catheter, balloon-based HDR techniques typically use an Iridium-192 source, resulting in higher dose gradients and more rapid dose fall-off than can be achieved using an external beam radiotherapy technique. However, the use of a single catheter in a balloon means that this technique cannot sculpt the dose to avoid organs at risk or to conform to an irregularly shaped target

Question 46

A brachytherapy point source delivers 1 Gy/min to a point 1 cm away in a water medium. Given that the radial dose function of the source g(r) = 0.9 when r = 3 cm, the same source delivers _____ to a point 3 cm away from the source in a water medium. A. 1.9 Gy/min B. 1.3 Gy/min C. 0.9 Gy/min D. 0.3 Gy/min E. 0.1 Gy/min

Answer 46

E. 0.1 Gy/min
The radial dose function, g(r) accounts for dose fall-off due to photon scattering and attenuation. In this case, given 1Gy/min at 1 cm, the dose rate at 3 cm would be Ḋ(r = 3cm) = [Ḋ(r = 1cm)] × (ISL) × g(r = 3) = 1 Gy/min × (1 cm / 3 cm)2 × 0.9 = 0.1 Gy/min.

Question 47

For the same prescribed dose in a permanent implant, selecting a radioisotope with a shorter half-life will necessitate using _____ initial dose rate. A. a lower B. a higher C. the same D. The relationship between half-life and initial dose rate is isotope dependent.

Answer 47

B. a higher
For any radioisotope, for the same prescribed dose, as the half-life decreases, the initial dose rate would need to increase. For example, when comparing Pd-103 (T1/2 = 17 days) and I-125 (T1/2 = 60 days) for prostate interstitial implants, Pd-103 will deliver the total prescribed dose in fewer days

think of formula D = Do1.44T1/2

Question 48

Rank the following in terms of typical skin sparing from BEST TO WORST, given pencil-beam scanning protons = PBS, uniform scanning protons = US, double scattering protons = DS, and 6 MV x-rays = 6 MVx A. PBS; US; DS; 6 MVx B. 6 MVx; DS; US; PBS C. US; PBS; 6 MVx; DS D. 6 MVx; PBS; US; D

Answer 48

D. 6 MVx; PBS; US; DS
Photons give better skin sparing than protons used for therapy. Pencil-beam scanning permits control of the proximal (entrance) dose so that it is possible to sculpt dose out of the skin. Uniform scanning and pencil-beam scanning give comparable skin dose to one another in general

Question 49

The uncertainty of the distal range for proton beams arises from _____. A. SOBP width B. beam energy C. CT calibration D. field size E. accuracy of the on-board imaging system

Answer 49

C. CT calibration
The uncertainty in the CT calibration contributes to the uncertainty in the proton beam range. Uncertainty in the distal range also arises due to patient setup and motion uncertainties.

Question 50

The RBE for protons is highest at the _____ of the spread-out Bragg peak (SOBP). A. entrance region B. one third point C. center D. two thirds point E. distal edge

Answer 50

E. distal edge
The proton RBE increases with increasing linear energy transfer (LET) and thus with depth in an SOBP from ~1.1 in the entrance region to ~1.15 in the center, ~1.35 at the distal edge, and ~1.7 in the distal fall-off. (See Phys. Med. Biol. 2014 Nov 21; 59(22):R419–72. doi:10.1088/0031-9155/59/22/R419)

Question 51

The penumbra for proton beams _____ as the depth increases. A. increases B. decreases C. remains the same D. One is unable to determine this using the information given.

Answer 51

A. increases
Protons scatter within the patient. With increasing depth the protons have had more opportunity to scatter, increasing the penumbra as a function of depth.

Question 52

Failure modes and effects analysis (FMEA) uses occurrence, severity, and detectability indices to quantify risk of failure modes. These indices are roughly _____. A. linear B. logarithmic C. quadratic D. sigmoidal E. None of the above is true.

Answer 52

B. logarithmic As per AAPM Task Group 100, risk indices are roughly logarithmic.

Question 53

In a workplace with an established safety culture, what is expected of an employee who discovers a radiotherapy near miss, i.e., an issue that if it had been propagated to the patient it might have caused harm? A. Report it in the departmental incident learning system. B. Notify the patient. C. Notify the institution’s radiation safety officer. D. Notify the relevant city, state, or national regulatory body. E. All of the above are true.

Answer 53

A. Report it in the departmental incident learning system.
In patient safety and error reporting, a near miss represents an event that if propagated to the patient might cause harm. Tracking of near misses in an incident learning system is important because it enables a department to identify and resolve problems or problematic processes before they become medical events, which are events that actually impact patient treatment and might cause harm. It is typically not necessary to notify a patient, the radiation safety officer, or a regulatory body about a near miss. However, there are regulatory reporting requirements for actual medical events if the incorrectly delivered dose exceeds criteria set by those regulatory bodies.