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RSO Exam PHYSICIST INDIA > Radiation Generator > Flashcards

Radiation Generator Flashcards

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1
Q

What voltages were primarily used in external beam radiotherapy up to about 1950?

A

Up to 300 kVp

This refers to kilovoltage peak, a measure of the maximum voltage applied across the x-ray tube.

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2
Q

What is Grenz ray therapy?

A

Treatment with beams of very soft (low-energy) x-rays produced at potentials below 20 kV

Grenz rays have very low depth of penetration and are no longer used in radiation therapy.

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3
Q

What are the typical operating potentials for contact therapy machines?

A

40 to 50 kV

These machines are used for irradiating accessible lesions at very short source to surface distances.

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4
Q

What is the half-value layer (HVL)?

A

The thickness of a specified material that reduces the exposure rate by one-half

HVL is used to express the degree of hardening or beam quality.

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5
Q

What is the depth of maximum dose for superficial therapy beams?

A

Approximately 5 mm

Beyond this depth, the dose drop-off is too severe to deliver adequate depth dose.

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6
Q

What is orthovoltage therapy used for?

A

Treatment with x-rays produced at potentials ranging from 150 to 500 kV

Most orthovoltage equipment operates at 200 to 300 kV.

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7
Q

What is a major limitation of orthovoltage therapy?

A

High skin dose when adequate doses are delivered to deep-seated tumors

This became a concern leading to the use of megavoltage beams.

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8
Q

What is supervoltage therapy?

A

X-ray therapy in the range of 500 to 1,000 kV

This therapy was developed in the 1950s and 1960s as a quest for higher-energy x-ray beams.

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9
Q

What is the function of a resonant transformer in x-ray machines?

A

To generate x-rays from 300 to 2,000 kV efficiently

The transformer operates at its resonant frequency to achieve high amplitude voltage.

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10
Q

What are megavoltage beams classified as?

A

X-ray beams of energy 1 MV or greater

This classification includes γ-ray beams produced by radionuclides.

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11
Q

What does the Van de Graaff generator do in radiotherapy?

A

Accelerates electrons to produce high-energy x-rays, typically at 2 MV

The generator operates by collecting negative charges on a spherical dome.

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12
Q

What is the main operational principle of a linear accelerator (linac)?

A

Uses high-frequency electromagnetic waves to accelerate charged particles

Electrons can be used directly for superficial tumors or to produce x-rays for deeper tumors.

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13
Q

What are the two types of wave structures used in linear accelerators?

A

Traveling wave and standing wave structures

Each type has a different design for accelerating particles.

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14
Q

How does the standing wave structure in a linear accelerator function?

A

Provides maximum reflection of waves at both ends to create stationary waves

This design allows for more efficient particle acceleration.

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15
Q

What is the typical initial energy of electrons injected into the accelerator structure of a linac?

A

About 50 keV

Electrons gain energy from the microwave electromagnetic field during acceleration.

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16
Q

What is a significant design feature of high-energy linear accelerators?

A

The accelerator structure can be placed horizontally or at an angle

This design allows for bending the electron beam before it strikes the target.

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17
Q

What is the main function of the magnetron?

A

To produce microwaves as a high-power oscillator generating microwave pulses.

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18
Q

What is the frequency of the microwaves generated by a magnetron?

A

About 3,000 MHz.

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19
Q

Describe the construction of a magnetron.

A

It has a cylindrical construction with a central cathode and an outer anode containing resonant cavities machined from copper.

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20
Q

How are electrons generated in a magnetron?

A

Through thermionic emission from a heated cathode.

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21
Q

What is the typical peak power output of a magnetron used in low-energy linacs?

A

2 MW.

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22
Q

True or False: The klystron generates microwaves.

A

False.

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23
Q

What is the role of the buncher cavity in a klystron?

A

To modulate the velocity of electrons using low-power microwaves.

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24
Q

What phenomenon occurs when electrons are incident on a high-Z material target in a linac?

A

Bremsstrahlung x-rays are produced.

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25
What is the average photon energy of the x-ray beam in a linac?
Approximately one-third of the maximum energy.
26
What is the purpose of the electron scattering foil in a linac?
To spread the electron beam and achieve uniform electron fluence.
27
Fill in the blank: The treatment head consists of a thick shell of high-density shielding material such as _______.
[lead, tungsten, or lead-tungsten alloy]
28
What does the flattening filter do in a linac?
It makes the beam intensity uniform across the field.
29
What materials are commonly used for the flattening filter?
Lead, tungsten, uranium, steel, aluminum.
30
What is the function of the ion chambers in the treatment head?
To monitor the dose rate, integrated dose, and field symmetry.
31
What is the significance of the isocenter in a linac?
It is the point where the collimator axis intersects the axis of rotation of the gantry.
32
What principle does the operation of a betatron rely on?
An electron in a changing magnetic field experiences acceleration in a circular orbit.
33
What are the advantages of the microtron compared to a linear accelerator?
Simplicity, easy energy selection, small beam energy spread, and smaller size.
34
What is the typical energy range for betatrons?
Less than 6 to more than 40 MeV.
35
What is the primary use of the klystron in a linac?
As a microwave amplifier driven by a low-power microwave oscillator.
36
What type of beam does the treatment head produce?
X-ray or electron beams.
37
True or False: The monitor chambers in the treatment head are usually sealed.
True.
38
What is the purpose of the light localizing system in the treatment head?
To ensure the light field is congruent with the radiation field.
39
What happens to the electron beam as it exits the window of the accelerator tube?
It is a narrow pencil about 3 mm in diameter.
40
What is the role of the secondary low-Z foil in a linac?
To flatten the electron beam and minimize additional bremsstrahlung radiation.
41
What is the principal advantage of a microtron over a linear accelerator?
Simplicity, easy energy selection, small beam energy spread, and smaller machine size. ## Footnote These advantages lead to simplified beam transport systems and the ability to supply a beam to multiple treatment rooms.
42
Who proposed the method of accelerating electrons used in the microtron and when?
Veksler in 1944. ## Footnote This method laid the groundwork for the development of microtrons in radiotherapy.
43
What was the first microtron for radiotherapy described, and when?
A 10-MeV unit in 1972 by Reistad and Brahme. ## Footnote This marked a significant development in the field of radiotherapy.
44
What is the energy range of x-ray beams produced by the 22-MeV microtron developed by AB Scanditronix?
6 or 10 and 21 MV. ## Footnote It also produced electron beams of various energies: 2, 5, 7, 9, 11, 13, 16, 18, 20, and 22 MeV.
45
What is a racetrack microtron?
A microtron that uses a standing wave linac structure to accelerate electrons. ## Footnote This design alleviates constraints associated with circular microtrons.
46
What is the main use of a cyclotron in radiation therapy?
As a source of high-energy protons for proton beam therapy. ## Footnote It has also been adopted for generating neutron beams.
47
What nuclear reaction occurs when deuterons strike a low atomic number target in a cyclotron?
Neutrons are produced by stripping. ## Footnote This reaction is typically performed with targets like beryllium.
48
What is the basic structure of a cyclotron?
A short metallic cylinder divided into two sections called Ds. ## Footnote These sections are placed between the poles of a DC magnet.
49
How are particles accelerated in a cyclotron?
An alternating potential is applied between the Ds, allowing particles to travel in circular orbits and gain energy with each pass. ## Footnote Positively charged particles such as protons or deuterons are injected into the chamber.
50
What is the energy limit of particles in a cyclotron due to relativistic effects?
Particles gain mass at high velocities, affecting their synchronization with the alternating potential. ## Footnote This limitation is addressed in synchrotrons by adjusting the frequency of the potential.
51
52
What are some examples of radionuclides used in teletherapy?
Radium-226, Cesium-137, Cobalt-60 ## Footnote These radionuclides emit γ-rays during radioactive disintegration.
53
Which radionuclide is considered most suitable for external beam radiotherapy?
Cobalt-60 ## Footnote It is preferred due to higher specific activity, greater radiation output, and higher average photon energy.
54
List the reasons why cobalt-60 is chosen over radium and cesium.
* Higher specific activity (curies per gram) * Greater radiation output per curie * Higher average photon energy * Lower cost and less self-absorption than radium
55
What is the half-life of Radium-226?
1,622 years
56
What is the average γ-ray energy of Cobalt-60?
1.17 and 1.33 MeV
57
How is the Cobalt-60 source produced?
By irradiating ordinary stable 59Co with neutrons in a reactor.
58
What is the decay product of Cobalt-60?
Cobalt-60 decays to Nickel-60.
59
What type of radiation is emitted during the decay of Cobalt-60?
* β particles (Emax = 0.32 MeV) * Two photons (1.17 and 1.33 MeV)
60
What is the purpose of the double-welded seal in the Cobalt-60 source?
To prevent leakage of the radioactive material.
61
Define geometric penumbra.
The region at the edge of a radiation beam where the dose rate changes rapidly as a function of distance from the beam axis.
62
What is a typical diameter range for a teletherapy Cobalt-60 source?
1.0 to 2.0 cm
63
What is the purpose of the source housing in a Cobalt-60 unit?
To provide shielding and a mechanism for moving the source to the treatment position.
64
List the four methods for moving the source from the off position to the on position.
* Rotating wheel mechanism * Heavy metal drawer mechanism * Mercury flow mechanism * Fixed source with shutter mechanism
65
What is the role of the collimator system in a Cobalt-60 unit?
To vary the size and shape of the beam for individual treatment requirements.
66
What is transmission penumbra?
The region irradiated by photons that are transmitted through the edge of the collimator block.
67
What effect does increasing the source diameter have on penumbra width?
It increases the penumbra width.
68
What is the impact of extending the source to diaphragm distance (SDD) on penumbra?
It decreases the penumbra width.
69
What is the term for the lateral distance between two specified isodose curves at a specified depth?
Physical penumbra width.
70
True or False: The geometric penumbra is dependent on field size.
False
71
Fill in the blank: The combined effect of the transmission and geometric penumbras creates a region of dose variation at the field _______.
edges
72
What are the main radiations used in radiotherapy?
X-rays and electrons ## Footnote Heavy particle beams offer advantages in dose localization and therapeutic gain.
73
What are the types of heavy particles used in radiation therapy?
* Neutrons * Protons * Deuterons * α particles * Negative pions * Heavy ions ## Footnote Their use in radiation therapy is experimental due to high costs.
74
What is the primary advantage of heavy particle beams in radiotherapy?
Greater effect on tumor than on normal tissue ## Footnote This is referred to as therapeutic gain.
75
How are high-energy neutron beams produced for radiotherapy?
By deuterium–tritium generators, cyclotrons, or linear accelerators ## Footnote The bombarding particles can be deuterons or protons.
76
What is the reaction that produces neutrons in a D–T generator?
Low-energy deuteron beam incident on a tritium target ## Footnote The disintegration energy of 17.6 MeV is shared between the helium nucleus and the neutron.
77
What is a major problem with D–T generators in neutron therapy?
Lack of sufficient dose rate at the treatment distance ## Footnote The highest dose rate achieved is about 15 cGy/min at 1 m.
78
What is the typical energy range for deuterons used in cyclotrons?
~15 to 50 MeV ## Footnote Deuterons bombard a low-atomic-number target like beryllium to produce neutrons.
79
What is the Bragg peak?
The region of high dose at the end of the particle range ## Footnote It is characterized by a rapid falloff to zero dose after the peak.
80
What is the range-energy relationship for protons in water?
α = 0.0022 cm, p = 1.77 ## Footnote This applies for protons of energy <200 MeV.
81
What does the term 'specific kinetic energy' refer to?
Kinetic energy per nucleon (MeV/u) ## Footnote Particles with the same MeV/u have approximately the same velocity.
82
What is the mass of a pi meson compared to an electron?
273 times that of an electron ## Footnote Pions can have positive, negative, or neutral charges.
83
Which type of pions have been used for radiation therapy?
Negative pions ## Footnote They are produced in nuclear reactions using protons.
84
What are the challenges associated with pion beams in radiation therapy?
* Low dose rates * Beam contamination * High cost ## Footnote Despite their attractive radiobiologic properties, these challenges limit their use.
85
What has largely replaced kilovoltage and cobalt-60 units in radiotherapy?
Linear accelerators ## Footnote Some older machines are still in use for specific applications.
86
What frequency energizes a linear accelerator?
~3,000 MHz ## Footnote This frequency is used in the microwave technology of linear accelerators.
87
What is the function of the flattening filter in a linear accelerator?
To make the x-ray beam intensity uniform across the field ## Footnote It is used in the x-ray mode of the treatment head.
88
What is a penumbra in radiotherapy?
A dose transition region near the borders of the field ## Footnote There are three kinds: geometric, transmission, and physical penumbra.
89
What is the difference between geometric penumbra and physical penumbra?
* Geometric penumbra: due to source dimensions * Physical penumbra: spread of dose distribution near field borders ## Footnote Geometric penumbra increases with SSD and depth.

RSO Exam PHYSICIST INDIA (5 decks)

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