Radiation Therapy Physics Final

Describe 3 functions of the monitor ionization chamber in the treatment head of a linac.

Measure the integral dose Measure the dose rate

From a practical standpoint, which will make the best target material for a standard diagnostic X-ray tube given the following characteristics?

Metal #2: Z = 77, melting point = 3370 deg C

List 2 advantages of cerrobend over MLC's blocks.

More conformal Match divergence

The stability of a nucleus is highly dependent on the ______ of the nucleus.

N - Z ratio

force

Newton (N)

In the electron mode of a linac, the spreading of the electron beam to produce a more uniform electron fluence is accomplished with the use of a ____

Scattering Foil

What is used in a diagnostic x-ray tube circuit to change the current supplied by the hospital to the necessary current in the filament circuit?

Step-down transformer

What is used in a diagnostic x-ray tube circuit to change the voltage supplied by the hospital to the necessary voltage between the anode and the cathode?

Step-up transformer

Which is smaller, the charge on one electron or 1 Coulomb of charge?

The charge on 1 electron

Which of the following is true concerning the K-shell electrons of two different atoms?

The electron binding energy is the same if they have the same number of protons.

In stable isotopes of heavy nuclei:

The number of protons is less than the number of neutrons.

Which of these statements about the voltage waveform in a diagnostic x-ray tube is false?

The transformer in the high-voltage circuit converts the AC input power to DC.

Rectifier

To allow current to flow in only one direction

Transformer Oil

To help dissipate heat in the tube

Tube housing

To protect the tube from damage

List the 2 purposes of the cooling water for the accelerating structure.

To provide controlled water to establish the optimal operating temperature of sensitive components. To remove residual heat dissipated in components

Because of the angular distribution of x-rays in megavoltage units, the most common target for these units is a:

Transmission type target

A microtron is a cyclic accelerator

True

A microtron is a cyclic accelerator.

True

Charged particles can be maintained in a circular orbit by a magnetic field.

True

Charged particles can be maintained in a circular orbit by a magnetic field.

True

Collisions are considered elastic when kinetic energy is conserved.

True

Collisions that eject inner shell electrons are inelastic.

True

Decreasing the tube current will not change the average energy of the x-ray spectrum.

True

Electrons are not accelerated in cyclotrons.

True

In an orthovoltage unit, the electrons are accelerated by a nearly constant or pulsating positive voltage.

True

Photons have no net charge.

True

The efficiency of bremsstrahlung production increases with increasing atomic number of the target.

True

True or False: When a moving charged particle enters a magnetic field, it travels in a circular path.

True

X-rays that exit the tube housing but are not part of the useful beam are called leakage radiation.

True

Which of the following are true about diagnostic energy X-ray tubes?I. The cathode is the source of electrons. II. X-rays are emitted by thermionic emission. III. Typically, the cathode is made of stainless steel. IV. The anode produces an electron beam by thermionic emission. V. Typical X-ray production efficiency is only 1%.

V and I

Which part of a linac requires a dielectric gas for proper operation?

Waveguide

Which of the following statements about electron capture is false? a. In electron capture, a neutron is emitted. b. Electron capture can compete with positron emission in isotopes with an excess of protons. c. Characteristic x-rays and Auger electrons can be emitted as a result of electron capture. d. In electron capture, the parent nuclide transmutes into a different element.

a

Which of the following statements are true? a. Neutrons are produced in high energy linacs in photodisintegration reactions. b. Most of the neutrons produced by a high energy linac are produced in the treatment head. c. The neutrons produced by a high energy linac do not contribute dose to patients. d. Neutrons are produced by cobalt teletherapy units.

a and b

Work function, W

average energy expended to produce an ion pair

LET

average energy lost per unit path length

The anode of a diagnostic x-ray unit _____.

conducts heat away from the target

After 6 half-lives, the fraction of activity remaining in a source is ___.

d. (1/2)^6

Of the following changes, which will not result in a decrease in the average energy of an x-ray spectrum?

decrease atomic number of target decrease kVp decrease tube current

The purpose of the electronics in the modulator cabinet is to ______________________ ______________________________ that must be delivered simultaneously to both the ______________________ and the ____________________.

form, high-voltage pulses, microwave power source, electron gun

Range

he straight line distance traversed by a particle before it is completely stopped

A stable atom is an atom _______________________________.

in its lowest energy state.

Of the following changes, which will not result in an increase in the average energy of an x-ray spectrum?

increase filtration increase atomic number of target INCREASE TUBE CURRENT increase kVp

Describe the easiest way that a technologist could increase the average energy of an x-ray beam from a tube that already exists in the workplace.

increase kVp on the control panel

Describe the easiest way that a technologist can increase the output (exposure rate) of an x-ray tube that already exists in the workplace.

increase mA

Which type of interaction results in the production of characteristic radiation?

inelastic collisions - interaction with target nucleus

c. Which type of interaction results in the production of bremsstrahlung radiation?

inelastic interaction of incident electron with target nucleus

Which of the following parts of a linac requires a vacuum for proper operation?

klystron

Specific Ionization

number of ion pairs produced per unit path length of incident radiation

Describe two design modifications that have been made to target anodes to spread heat out over a larger area, but keep a small focal spot?

rotate and angle the anode

In the electron mode of a linac, the spreading of the electron beam to produce a more uniform electron fluence is accomplished with the use of a ________.

scattering foil

Which kind of radioactive equilibrium can occur when a very long-lived radionuclide decays to a short-lived daughter?

secular equilibrium

Will a linac have a larger or smaller penumbra than a Cobalt unit?

smaller

With regard to the production of electron beams by linear accelerators, which of the following is true?

the beam current is much lower in the "electron mode" compared with the "photon mode"

The minimum photon energy in an x-ray beam is determined by _____.

the filtration of the beam

Define tube current.

the flow of thermionically emitted electrons from the cathode to the anode

The maximum photon energy in an x-ray beam is determined by:

the maximum kV across the tube

hich of these waveforms produces the best x-ray beam? (circle the best answer)

three-phase rectified

Step-down transformer

to supply the correct voltage to the filament circuit

What is the most important factor controlling the output of a diagnostic x-ray beam?

tube current

The efficiency of bremsstrahlung production: 1. Increases with increasing Z 2. Decreases with increasing Z 3. Increases with increasing tube potential 4. Decreases with increasing tube potential

1 and 3

Which of the following is not true?

1 cm = 2.54 in

n a 100 kVp diagnostic x-ray tube, _____ % of the incident electron energy goes into x-ray production and _____ % of the incident electron energy goes into heat loss

1% and 99%

To produce and monitor a photon beam from a linear accelerator, in what order are the following components arranged in the head? 1. flattening filter 2. scattering foil 3. target 4. monitor chamber

1,3,4

The geometric penumbra for a 2.0 cm diameter Co-60 source at an SSD of 80 cm, a depth of 7 cm, and a source-to-diaphragm distance of 45 cm is:

1.87cm

Which of the following is true?

10 MHz = 10E7 cycles/sec

In a typical diagnostic x-ray beam from a tungsten target, characteristic radiation amounts to approximately _____ percent of the total beam.

10%

How many interactions do electrons have?

1000's

What is the energy carried by a photon with a wavelength of 9.85 nm? h = 4.14 x 10-15 eVs

126 eV 126 keV

A radioactive source of strength 25 mCi with a decay constant of 2.43 E-3 / d will decay to _____ mCi after 0.73 yr

13.1

To produce and monitor an electron beam from a linear accelerator, in what order are the following components arranged in the head? 1. flattening filter 2. scattering foil 3. target 4. monitor chamber

2,4

The geometric penumbra for a 2.5 cm diameter Co-60 source at an SSD of 80 cm, a depth of 5 cm, and a source-to-diaphragm distance of 45 cm is:

2.22cm

The xray intensity at 120 cm is 480 mR. What is the intensity at a distance of 1.5 m?

307 mR

Approximately how many half-lives are required before a quantity of radioactive material has decayed to around 3% of its original value?

5

In a typical MV x-ray beam from a tungsten target, about ____% of the incident electron energy is converted into x-rays.

50%

What will the output of a Cobalt-60 unit be at a distance of 100 cm if its output at 80 cm is 85.6 cGy/min?

54.8cGy/min

If the decay constant of a radionuclide is 0.012 / day, the half-life is ____.

57.8 days

In a typical diagnostic x-ray beam from a tungsten target, bremmstrahlung radiation amounts to approximately ___ percent of the total beam.

90

Which of the following will reduce the output of a Co-60 teletherapy unit?

A. Insufficient warmup time C. Low electron current B. Low klystron energy D. None of the above D

Which of the following statements are true regarding the mass-energy equivalence theory?

A. It was proposed by Bohr. B. Energy and mass are related by the equation E = moc2. C. The mass of a particle is independent of its velocity. D. THE SPEED OF LIGHT IS CONSTANT IN A VACCUUM E. THE REST ENERGY OF A PARTICLE IS MoC2

Any error in the reproducible positioning of the flattening filter will affect

A. beam flatness B. beam symmetry C. beam output D. all of the above D

Which of the following statements are false? I. Characteristic x-rays are emitted with a continuous spectrum of energies. II. Collisions that eject inner shell electrons are inelastic. III. If the binding energy of an atom's K shell is 20.0 keV and the binding energy of an L shell is 2.7 keV, then a 17.3 keV incident electron can create a Kalpha x-ray.

All 3 are false.

current

Ampere (A)

Which of the following statements are true regarding the mass-energy equivalence theory? A. It was proposed by Bohr. B. Energy and mass are related by the equation E = mc2. C. The mass of a particle is dependent on its velocity. D. The speed of light is 1.6 x 10-19 m/s in a vacuum. E. The rest energy of a particle is moc2.

B, C and E

During an isomeric transition, all of the following may be emitted except:

Beta particles

How can you determine the output of a diagnostic x-ray beam from the energy spectrum?

By calculating the area under the curve

Which of the following design modifications have been made to Diagnostic x-ray tubes to spread heat out over a larger area, but keep a small focal spot? A. Employ a focusing cup B. Rectify the tube voltage C. Rotate the anode D. Use two filaments E. Angle the anode

C and E

From the following list of nuclides, identify all of the isotones: 133Cs55 133Xe54 131I53 134Ba56 133I53

Cs-133, I-131, and Ba-134

From the following list of nuclides, identify all of the isobars: 133 Cs 55 - 133 Xe 54 - 131 I 53 - 134 Ba 56 - 133 I 53

Cs-133, Xe-133, and I-133

With regard to the production of photon beams by linear accelerators, which of the following is true? a. the beam current is much higher in the "photon mode" compared with the "electron mode" b. photon beam flatness depends on the design of the flattening filter c. targets are made thick enough to allow for sufficient x-ray production but not too thick as to attenuate too many photons d. all of the above

D. All of the above

Which type of radiation are electrons?

Directly ionizing radiation

Which of the following are true about diagnostic energy X-ray tubes? A. The cathode is the source of electrons B. X-rays are emitted by thermionic emission. C. Typically, the cathode is made of stainless steel. D. The anode produces an electron beam by thermionic emission. E. Typical X-ray production efficiency is only 1%.

E and A

Which of the following statements is true? All of the answers are true. Electromagnetic waves are waves of "fields", not matter. A photon is a bundle of energy in the visible light part of the electromagnetic spectrum. Electromagnetic waves require a medium in which to travel.

Electromagnetic waves are waves of "fields", not matter

Which of the following statements is true?

Electromagnetic waves do not require a medium in which to travel

How is the collimation of an electron beam different from the collimation of a photon beam? Why are they different?

Electron beams require collimation closer to the skin of the patient to produce flat beams with small penumbras. Photon beams can use different types of collimators that may be fixed, can move independently and across the center line and allow any beam shape to be produced subject to the width of the leaves.

Which of the following statements is true regarding x-ray production in diagnostic style x-ray tubes?

Electrons need to accelerate to gain large amounts of energy

Briefly describe 4 safety features either on a linac itself or in the room in which it is located.

Emergency off systems: cut power to the linac and shut off the beam. Timer: a timer keeps track of the amount of time that the beam is on and can act as a backup to the monitor IC in terminating the beam. Door Interlocks: to terminate the beam if the door is opened during beam-on and to prevent the beam from being turned on if the door is open. Audio/Video Systems: to provide 2-way contact with the patient during treatment.

Which change in environmental conditions will increase a radionuclide's activity?

Environmental conditions do not affect activity

The term applied to the raising of an electron to a higher energy orbit within a given atom is called:

Excitation

A betatron accelerates protons via a changing magnetic field.

False

Acceleration does not take place in the gap between the dees of the cyclotron; it only occurs in the dees themselves.

False

Acceleration does not take place in the gap between the dees of the cyclotron; it only occurs in the dees themselves

False

Characteristic x-rays are emitted with a continuous spectrum of energies.

False

Charge, mass, energy, and momentum are four quantities that must be conserved in nature.

False

Electrons are accelerated in cyclotrons.

False

If the binding energy of an atoms K shell is 69.5 keV and the binding energy of an L shell is 11 keV, then a 58.5 keV incident electron can create a Kα x-ray.

False

In a linear accelerator, the electrons are accelerated by an alternating a.c. voltage. True or false?

False

Protons have no net charge.

False

The K x-rays from a 90 kVp x-ray beam (tube has tungsten target) are not shown in the energy spectrum due to inherent filtration.

False

The process by which a filament is cooled to a sufficient temperature to emit electrons is called thermionic emission.

False

The purpose of having 2 filaments in the x-ray tube is to double the number of heat units that the anode can accept.

False

As the energy of electromagnetic radiation increases, the _________.

Freuquency Increases

frequency

Hertz (Hz)

Some important advantages of a linac unit over a Co-60 are:

I. higher beam output III. sharper beam II. less maintenance cost IV. less complex in design I & III

Some important advantages of a linac over a Co-60 unit are: I. higher beam output III. sharper beam II. less maintenance cost IV. less complex in design

II and IV

Describe/explain the "fail-safe" design of a cobalt source transport mechanism.

If power is interrupted when the beam is on, the mechanism will automatically retract the source to a safe position

How do electrons travel?

In a straight-line path when interacting in material In a path that has many changes in direction

List 2 dosimetric limitations of radiation therapy treatment with orthovoltage x-rays.

Increase Scatter Increase dose to bone

Which electron interaction results in the production of characteristic x-rays?

Inelastic interaction with atomic electron

If I increase the activity of a radioactive source by a factor of 2, its half-life ______________.

Is unchanged

mass

Kilogram (kg)

Will a Cobalt unit have a larger or smaller penumbra than a linac? Explain:

Larger Ex: We want a small source size to maintain a high output in a small sized source...Cobalt has a larger pen than linac since it has a larger source size than a linac.