1. Principles and Practice of Radiation Therapy
Chapter 7
Treatment Delivery Equipment
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Objectives
Discuss the historical development of radiation therapy treatment delivery equipment
Compare and contrast the clinical applications of kilovoltage equipment, including Grenz-ray therapy, contact therapy, superficial treatments, and orthovoltage therapy
Describe the four major components of the linear accelerator stand, including the klystron, waveguide, circulator, and cooling system
Explain how x-rays are produced in the linear accelerator
Describe the major components located in the gantry of the linear accelerator, including the electron gun, accelerator structure (guide), and treatment head
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Objectives
Explain the concept of indexing as it relates to patient immobilization and positioning
Identify where multileaf collimators (MLCs) are located in the treatment head and explain how they operate
Compare and contrast the use of older equipment such as the betatron, Van de Graaff generator, and cobalt unit
Discuss the characteristics of cobalt-60
Describe emergency procedures related to a cobalt-60 source that fails to retract
Discuss medical accelerator safety considerations
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Historical Overview
1895 – Discovery of x-rays by Wilhelm Conrad Roentgen
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5. Equipment Development
Kilovoltage units used since the turn of 20th century
Units include
Grenz
Contact
Superficial
Orthovoltage
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6. Limitations of Kilovoltage Units
Cannot reach deep-seated malignancy
No skin sparing
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7. Equipment Development
Megavoltage equipment
1937 Van de Graaff
1941 Betatron
1951 Cobalt units
1952 Linear accelerators
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Grenz Ray
1923
Gustav Bucky constructed tube
Less than 20 kV
X-ray tube with a lithium borate window
Transmitted long wavelength
Grenz German for “border”
Rays lie in gray zone between x-ray and ultraviolet
Inherent filtration 0.1 mm aluminum
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9. Grenz Ray and Superficial Similarities
Envelope is glass
Window beryllium
Inherent filtration approximately 0.1 mm
Quality measure in half-value layer expressed in millimeters of aluminum
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10. Grenz Ray Applications
200 R per session
Effective for the treatment of
Inflammatory disorders
Bowen’s disease
Herpes simplex
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Contact Therapy
40-50 kV
Half-value layer of 1 or 2 mm
Tube designed so that surface is in contact with housing only 2 cm from target
Low energy, short distance, decrease in depth dose
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Contact Therapy
Suitable for treatment on surface lesions only
Papillon criteria for rectal lesions
3 x 5 cm mobile lesion
4 treatments of 3000 cGy each at 2-week intervals
Desirable treatment to preserve anal sphincter
Chaoul used contact for hemangiomas R
Total dose R
Use decreased in 1975
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13. Superficial Therapy UnitskV
Half-value layers range from 1 to 6 μm of aluminum to harden the beam
Cones used to collimate the beam
Surface in contact with cone
Source-skin distance (SSD) cm
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Orthovoltage Unit kV
kV most commonly used
For many years provided the most penetrating beams
Called deep therapy
SSD 50 cm
Still used for skin lesions
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Megavoltage
1 MV or higher
Van de Graaff
Betatron
Cyclotron
Linear accelerator
Called direct accelerator
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Linear Accelerator
Accelerator structure allows electrons to accelerate in straight path
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Early Accelerators
Early linear accelerators were large and bulky compared with today’s accelerators
First linear accelerator installed in 1952 in Hammersmith Hospital in London
First linear accelerator installed in US at Stanford University
Treated first patient in 1956
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18. Second-Generation Accelerators
The older 360-degree rotational units (isocentric)
Resulting in improved accuracy and dose delivery
Less sophisticated than today’s units
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19. First Clinical Linear Accelerator
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20. Third-Generation Accelerators
Computer driven
Wide variety of options
Dual photon energies
Multileaf collimators
Several electron energies
Electronic portal verification systems
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21. Linear Accelerator Components
Drive stand
Gantry
Treatment couch
Console electronic cabinet
Some have a modulator cabinet
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Drive Stand
The gantry rotates on a horizontal axis on bearings within the drive stand
Four major components are housed in the drive stand:
Klystron
Circulator
Waveguide
Water cooling system
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Klystron
Source of microwave power
Used to accelerate electrons
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Circulator
Acts like a valve
Receives microwave power from klystron
Directs radiofrequency energy into the waveguide
Prevents microwaves from returning to the klystron
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Waveguide
Hollow, tubelike structure
Conveys the microwave power to the accelerator structure in the gantry
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Water Cooling System
Maintains constant temperature
Cools various components
Accelerator structure
Klystron
Circulator
Target
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Gantry
Directs the beam at a patient’s tumor
The electron gun
Produces electrons
Injects them into the
accelerator structure
Cathode made of tungsten
The accelerator guide
Mounted horizontally (high-energy machines)
Mounted vertically (low-energy machines)
Receives the microwave power and combines that power with the injected electrons to accelerate the electrons to nearly the speed of light
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Treatment Head
For photon therapy
Bending magnet
X-ray target
Primary collimator
Beam-flattening filter
Ion chamber
Secondary collimators
Accessory tray for blocks and wedges
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Bending Magnet
May bend the electron group through a net angle of approximately degrees and onto the x-ray target or scattering foil
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30. Beam-Flattening Filter
Located on the carousel with the scattering foil
Shapes the x-ray beam in its cross-sectional dimension
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Scattering Foils
Located on the carousel with the flattening filter
Spreads out the small, pencil-like beam of electrons and provides a flat, uniform electron treatment field
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Dual Ion Chambers
Monitors/samples the beam for its symmetry in the right-left and inferior-superior direction
Produces electrical signals that terminate the beam when the prescribed dose is given
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33. Secondary Collimation
Control by using the setting knobs at the collimator head or pendant to adjust the upper and lower collimator jaws
MLCs or Cerrobend blocking may further define field
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Field Light
Located in the treatment head
Simulates the field by illuminating the area
Light from a quartz-iodine bulb
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Control Console
Located outside the treatment room
Controls linear accelerator
Digital display, push-button panel, or video display terminal
Prescribed dose (monitor units), mechanical beam parameters, and other status messages
Interlock system
Patient-protection interlocks, beam energy, beam symmetry, dose, radiation, and mechanical hazards
Emergency off buttons
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Treatment Couch
The treatment couch may also be referred to as the “tabletop” or “patient support assembly” (PSA)
Many tabletops support up to 300 pounds and range in width from 45 to 50 cm
Indexed carbon fiber couch
Patient is indexed and locked into place on treatment table
Ensures patient is in same position on table
Carbon fiber reduces scatter
Helpful in intensity modulated radiation therapy (IMRT)
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Modulator Cabinet
Contains three major components
Fan control (to cool the power distribution compartment)
Auxiliary power-distribution system
Primary power-distribution system
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38. State-of-the-Art Technologies
Allow better beam shaping and dose escalation
Types
Conformal radiation therapy
Dynamic wedging
Independent jaws
MLCs
Electron portal imaging
Image-guided radiation therapy (IGRT)
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Dynamic Wedge
Modifies and shapes desired isodose distribution
Replaces hard wedges
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40. Multileaf Collimator (MLC)
leaves
Located below secondary collimation
The computer-driven, heavy, metal collimator rods slide into place to form field shape
MLCs can move during treatment
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41. Multileaf Collimator (MLC)
Two concerns
Penumbra at the end of leaves
Interleaf transmission leakage
2.5% to 4%
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42. Electron Portal Imaging Device (EPID)
Amorphous silicon imaging technology
Images reviewed immediately online
Reduce image-receptor distance for better image quality
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43. Image-Guided Radiation Therapy (IGRT)
Image the patient just before treatment and compare the position of the patient to the plan
Different forms
EPID
In-room computed tomography (CT) scanner
KV cone-beam CT
MV cone-beam CT
Ultrasound
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44. Verification and Recording Devices
Allow verification of setup parameters
Computer-assisted setup
Recording of patient data
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45. Medical Accelerator Safety Considerations
Emergency procedures
Written polices should be at or near treatment control console
Radiation therapists should be familiar with policies
Knowing location of emergency stop buttons is critical
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46. Medical Accelerator Safety Considerations
Electrical
Mechanical
Radiation safety considerations
Misadministration
Incorrect application or delivery of prescribed dose
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47. Medical Accelerator Hazards
Type
Cause
Consequences
Incorrect dose delivered
Electrical, software, and therapist
Serious injury, increased complications, genetic effects, second primary, and compromised tumor control
Dose delivered to the wrong area
Mechanical, software, patient motion, and therapist
Machine
Mechanical, collision software, patient motion, and therapist
Significant injury and death
Incorrect beam
Serious injury, increased complications, genetic effects, second primary, and compromised tumor control
General hazards
Electrical and mechanical
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Betatron
Developed by Kerst in 1941
Produces x-rays of 2 MV
Circular orbit (doughnut shape)
Electron in a changing magnetic field experiences acceleration
{COMP: Insert Figure 7-13}
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Van de Graaff
1937
RJ Van de Graaff developed first electrostatic generator
Produce high-energy x-rays (2 MV)
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Cobalt Unit
Mainstay of departments until the linear accelerator
First practical radiation therapy treatment unit to provide Dmax below the skin surface
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Cobalt-60 Production
Artificially produced isotope
Cobalt-59 is made radioactive in a particle accelerator
Source placed in steel capsule with double weld
This absorbs beta particles produced during the decay process
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52. Cobalt Characteristics
Emits beta particle with energy of 0.31 MeV
Absorbed by steel capsule
Nickel-60 results
Two gamma rays emitted
1.17 MeV
1.33 MeV
Average energy 1.25 MeV
Half-life
5.26 years
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53. Cobalt Characteristics
Correction factor of 1% per month must be applied to the output
Replace source every 5.3 years
Penumbra
Area where dose rate changes rapidly
Edge of the field
Large penumbra on cobalt because of its large source size
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Cobalt Machine Design
Source positioning
Most common is air-pressure piston
Air pressure pushes cobalt source into the on position
Travel time
Time added to treatment dose to allow for source to travel into position
0.2 second
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Cobalt Shielding
Source head contains the source
Steel shell filled with lead
Housing may be up to 2 feet in diameter
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56. Cobalt Machine Components
Interleaf collimator is part of source head
Collimator assists in reducing penumbra
Trimmers or satellite collimators may further sharpen the beam
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57. Cobalt Calibration and Leakage
Qualified radiation physicist must perform full calibration testing
Perform
When source is replaced
If 5% deviation is noticed during spot check
After a major repair is done
Monthly
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58. Cobalt Calibration and Leakage
Full calibration may include:
Radiation and light field coincidence
Timer accuracy
Exposure rate or dose rate
Accuracy of distance-measuring devices
Wipe test, which must be done twice a year
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59. Cobalt Radiation Monitoring and Light System
Light system to show when machine is on and off
On console
At head of the machine
At entrance to the room
If the beam is “on,” a red light must be lit
If the beam is “off,” a green light must be lit
Monitoring system to detect radiation in treatment room
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Emergency Procedures
Refer to Box 7-1 on page 155 of the textbook
for the procedure for cobalt-60.
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