Chapter 19
Simulator Design
Simulator
• Purpose:
– Assist the physician and other team members
of the radiation therapy team in the treatment
planning process.
– Reproducibility of treatment is a critical factor
for all patients.
Simulation process
• Conventional simulation:
– Patient data is obtained using fluoroscopy,
radiographs and physical measurements of patient.
• Computed Tomography simulation:
– Patient data gathered using detailed CT images in the
transverse plane.
– DRRs document process before data transferred to
treatment planning computer.
– Fused process of patient scanning, tumor and target
localization, treatment planning, and treatment field
verification into single operation.
History
• Treatment planning occurred on the cobalt
unit, betatron, or linear accelerator
– Took time away from treating patients
• Time consuming to estimate target and provide
parameters to ensure reproducibility
– High energy x-rays produce poor quality
images
• Diagnostic range (70-120 kVp) improve contrast
and detail
Simulator Justification
• Patients initially treated for relief of
symptoms
– Obstruction, bleeding, and pain
– 1960’s: 1 in 3 patients cured
– Today: over 50% cured
• Cost effective
– increase # treated on treatment machine
• Increased efficiency and accuracy
Responsibilities
• Radiation Oncologist:
– Patients clinical progress
– Proper treatment
• Radiation Therapist, clinical physicist,
dosimetrist:
– Simulation
– Dose delivery
– Computerized treatment planning
Radiation Therapy Process
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Diagnosis
Consultation
Simulation and Treatment Planning
Treatment
Patient Follow-up
Diagnosis & Consultation
• Diagnosis:
– Complete clinical workup
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Histology
Staging
Grading
Various studies
– X-rays
– Lab work
• Consultation:
– Discussion with patient and family
• treatment options
• Risks and benefits of treatments
Treatment Planning
• Tumor localization:
– Determining extent of tumor and location of
critical structures
• Treatment verification:
– Using diagnostic quality images of each
treatment field from initial sim procedure
• “masters” to compare port films
Goals of CT Sim Localization
and Field Design
• Acquisition of patient data set
• Target and normal structure localization
• Definition and marking of a patient
coordinate system (triangulation points)
• Transfer of info to treatment planning
system
• Production of image for treatment
verification
Quality Assurance
• Objectively and systematically monitor the
quality and appropriateness of the
simulation process as it relates to patient
care.
• British Standards Institution:
– Published guide for performance values for
RTH simulator
• Based on International Electrotechnical
Commission
Mechanical Components
• Gantry:
– Gantry arm
– Gantry head
– Image intensifier/film holder
• Treatment Couch
• Controls
Conventional Simulator
• Designed to simulate mechanical,
geometrical, and optical conditions of a
variety of treatment units.
Isocenter
• Reference point in space, a fixed distance (80 to
100cm) from the focal spot of the anode, (100130cm) from floor.
– Distance from isocenter to source of x-ray production
same from every gantry angle.
– Three axes of rotation all meet at isocenter:
• Central axis of beam
– Central portion of beam emanating from target, only part of
beam that is not divergent
• The axis of rotation of the gantry
• Treatment couch axis
Gantry Head
• Provides stability for:
– Collimator assembly
– Optical distance
indicator
– X-ray tube
– Field defining wires
– Beam limiting
diaphragm
– Accessory holder
Collimator Assembly
• Most of gantry head
• Provides support for x-ray tube aperture,
field defining wires, light field indicator,
beam-limiting diaphragms, and accessory
holder
• Directs the path of the beam toward the
patient, after it emerges from the x-ray
tube
Optical Distance Indicator (ODI)
• AKA rangefinder
• Projects scale onto
patients skin which
corresponds to the
SSD
• Mounted near
collimator
X-ray Tube
• Mounted onto diaphragm system
• Must have large and small (no greater
than 0.6mm) focal spot
– In order to obtain a sharp image of the 0.5mm
diameter field defining wires.
Field Defining Wires
• Located in collimator assembly
• AKA delineators
• Simulate maximum field size of 40 x 40 cm
at 100cm SSD recommended.
• Wires represent edge of treatment field
within larger image.
Beam-Restricting Diaphragms
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AKA x-ray shutters, blades, or collimators
Located within collimator assembly
Consist of 2-3mm Pb
Defines both size and axis of x-ray beam, limits
area exposed during fluoroscopy or during
radiograph
– Every x-ray should show evidence of collimation by
displaying a 1-2cm clear border of unexposed film.
• Optically indicates coverage of x-ray field
– Restricts light field on patients skin.
Fiducial Plate
• Plexiglas or plastic trays imbedded with lead
markers
• AKA beaded trays or reticule
• Positioned in the head of the gantry between the
field defining wires and the accessory holder
– represent various geometries ranging from 80 to 100
source-axis distance (SAD).
• Protects tungsten wire crosshairs located within
the collimator assembly and mark the center of
the field radiographically.
Accessory Holder
• Block tray holder:
– Simulates shielded areas during the
simulation process.
– Verifies the geometry of individualized
shielding blocks on the simulator before they
are used in actual treatment.
• Electron cone adapter
Image Intensifier System
• Receives and processes the created
image
• Major components:
– Film holder:
• Some provide a slot for an optional grid in front of
cassette
– Grids should be employed both to absorb the scatter
radiation emitted from the thicker body parts and to allow
the use of beam energies needed to maximize differential
absorption between similar tissues.
Major Components cont.
• Image Intensifier:
– Changes the quantity of photons and electrons,
representing the image, at each stage of the process.
– Converts x-ray image into a video (light) image to be
viewed on monitor.
– Amplifies the brightness of an image (500-8000 X’s).
– Glass envelope containing:
• Input screen: Fluorescent screen: absorbs x-ray photons,
emits light photons
• Photocathode: Absorb light photons, convert into electrons
• Electrostatic lenses: Focus and accelerate electrons
• Anode
• Output screen: Converts electrons into light photons
Major Components cont.
– Television camera
– Video monitor
Patient Table top
• Hard flat surface that minimally attenuates the beam
• Constructed out of carbon fiber:
– Made by binding a fabric of pure cotton fiber with a resin
– Extremely supportive with low density and low x-ray absorption
– High tensile strength: resistance in lengthwise stress, measured
in weight per unit area
• Provide support identical to that of treatment unit to
maintain reproducibility
– 400 lb
– 45-50 cm wide
Control Area
• X-ray generator
– Provides radiographic and fluoroscopic control of the simulator
• Television monitor
– Provides ready access to the fluoroscopic image during the
simulation process
• Remote control panel
– Located close enough to the observation window so therapist
can observe patient while simulator is moving
• Observation window
– Allows therapist to view patient and the mechanical motions of
the equipment
Room Design
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Recommended minimum size of 400sq.ft.
Counter with sink, work space and writing area
Storage for immobilization devices
Ventilation
Adjustable lighting for both treatment room and
control area
• Positioning lasers
– Provide therapist several external reference points in
relationship to the position of the isocenter
Shielding
• Lead
– Denser than concrete
• Less needed to stop equal amount of radiation
– More expensive than concrete
– Difficult to support structurally
• Concrete
Shielding Factors
• Use factor (U): time the machine is normally aimed at a wall or
ceiling
– Primary walls and ceilings: ¼
– Floors: 1
• Occupancy factor (T): considers how an area on the other side of
wall is used.
• Workload (W): the current (mA) times the time (min) a department
expects to run machine each week
• Weekly permissible dose (P):
– Occupationally exposed: 1 mSv/week
– General public: 0.02 mSv/week
B = P(d)2/WUT
• d: distance (in meters) from the source to the opposite of the barrier
• B: transmission of radiation through the barrier required to meet the
weekly permissible dose
CT Simulation
• CT provides the most useful information
for treatment planning purposes because
the scans can produce three-dimensional
representation of the patient and external
structure
• Two types:
– Conventional simulator with a CT mode
– Actual CT scanner adapted for simulation
Simulator with a CT mode
• Incorporate the conventional benefits of a simulator with
the added benefits of cross-sectional information.
• Scans can be preformed before, during, or after
simulation process.
• Advantages:
– Cost
– Not restricted by conventional CT aperture opening
– More representative of the treatment unit geometry
• Disadvantages:
– Poorer image quality
– Increased amount of time to simulate patient
– heat limitation of x-ray tube
CT Simulator
• CT hardware configuration
• Controls associated with the virtual
simulator workstation
• Patient marking system
Virtual simulation
• Virtual simulation work station, equipped
with:
– CT scanner
– Software to perform volume definition/
treatment planning dose calculation
– Software to produce DRRs
• The target is defined first, and then the
fields are shaped to conform to the target
during 3DCRT treatment planning
Gantry
• ~2400 scintillation (50%) or gas filed
(45%) detectors
– designed to receive and measure the
attenuated beam from a rotating x-ray tube
• As aperture size increases, so does the
number of detectors, the patient dose, and
time to scan the patient.
Spiral CT
• Use solid state detectors 80% efficient
– Reduce patient dose
– Improves image quality
– Allows for faster scan times
• Reduced motion artifacts
• Disadvantages:
– Increased processing time
– Increased axis resolution
– Increased image noise