Chapter 20 - RadTherapy

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Chapter 20
Simulation Procedures
Simulation
•
The success of treatment is directly related to the
effectiveness of the simulation procedure.
 Helps in determining the location and extent of disease relative
to adjacent critical normal tissues.
•
The precise mockup of a patient treatment; may
include:
1. The selection of immobilization devices, radiographic
documentation of treatment ports, measurement of the patient,
construction of patient contours, and shaping of fields.
2. Artificially duplicates the actual treatment conditions by
confirming measurements, verifying treatment, and confirming
shields
3. A virtual workstation, equipped with a CT scanner, software to
perform target volume definition and treatment planning dose
calculation, and production of DRRs
Definitions
• Localization: geometrical definition of the
position and extent of the tumor or
anatomic structures by reference of
surface marks that can be used for
treatment setup purposes.
• Verification: a final check that each of the
planned treatment beams does cover the
tumor or target volume and does not
irradiate critical normal structures.
Definitions
• Radiopaque marker: a material with a
high atomic number (lead, copper, or
solder wire) used on the surface of a
patient or placed in a body cavity to
delineate special points of interest for
calculation purposes or to mark critical
structures requiring visualization during
treatment planning, often used to mark
specific points on a patient during the CT
acquisition.
Definitions
• Separation (intrafield distance IFD): the
measurement of the thickness of a patient along
the central axis or at any other specified point
within the irradiated volume.
 Helpful in calculating the amount of tissue in front of,
behind, or around a tumor
 Measured with a caliper
• Field size: the dimensions of a treatment field at
the isocenter, represented by width x length
 Determined by the field-defining wires
Specific Target Volumes
• Gross tumor volume (GTV): indicates the
gross palpable or visible tumor
• Clinical target volume (CTV): indicates the
gross palpable or visible tumor and a
surrounding volume of tissue that may contain
subclinical or microscopic disease
• Planning target volume (PTV): indicates the
CTV plus margins for geometric uncertainties,
such as patient motion, beam penumbra, and
treatment setup differences.
Simulator
• Primary function: to localize the tumor
volume in three dimensions
• Should define the anatomic area so that it
is reproducible for daily treatment.
• The location of a treatment field during
simulation must reflect precisely what will
happen in the treatment room.
CT/MRI
• Cross-sectional information provided by CT and
MRI imaging contributes considerable
information to the radiation oncologist:
 Diagnosis
 Tumor and normal tissue localization
 Tissue density data for dose calculation
 Follow-up treatment monitoring
• Conventional CT: provides detailed diagnostic
information used by the radiologist and radiation
oncologist to evaluate the extent of the disease.
CT Simulation
• No image receptor such as film or image intensifier,
• A collimated x-ray beam is directed at the patient, and
the attenuated beam is measured by a detector whose
response is transmitted to a computer.
• The computer analyzes the signal from the detector,
reconstructs the image and then stores and/or displays
the image.
• Components of a CT simulator workstation:
 Target localization routine that allows the target to be defined
and transfers the appropriate mark to the patient skin surface
 Virtual simulation package that generates DRRs used to
evaluate and simulate the case
• the target is defined first, then fields are shaped to conform to the
target
Conventional Simulation
•
•
•
•
The field locations are determined
Target is defined
The fields are shaped to treat the target
May use fluoroscopy to initially view the
area
• Radiographs document what has been
done during the simulation process and
used as “masters” when comparing port
films
Conventional Simulation
Localization Methods
• SSD: positions a fixed treatment distance of 80
or 100 cm on the patient’s skin for each field.
 Requires repositioning the patient for each field
before treatment.
 usually single field, two laterals or an AP/PA treatment
 Requires tumor localization in two dimensions only,
because all tissues within these fields are treated and
the exact depth of the tumor is not critical.
Conventional Simulation
Localization Methods
• SAD (isocentric technique): provides
tumor localization in three dimensions
The isocenter is placed within the target
volume with the aid of fluoroscopy and other
imaging modalities
Orthogonal films taken: two radiographs taken
at right angles to one another.
Contrast
• Visually enhance anatomic structures that would
normally be more difficult to see.
– Barium sulfate:
• not absorbed by the GI tract
• Administered orally or rectally
• Patient should be advised as to the use of a laxative
– Iodinated contrast materials:
• Used for kidneys, bladder, and prostate, GI when barium
contraindicated
• Sterile procedures must be followed
• May be administered intravenously or through bladder
catheterization
– Negative contrast agents:
• Carbon dioxide, oxygen, and air
• Appear as dark areas on a radiograph
Conventional Simulation Procedure
1. Presimulation planning
2. Room Preparation
3. Explanation of procedure
4. Patient positioning and immobilization
5. Operation of simulation controls
6. Setting field size parameters
7. Selecting exposure technique
8. Radiographic exposure
9. Documenting pertinent data
10. Final Procedures
Presimulation Planning
• An assessment of all relevant patient information
and an evaluation of possible treatment
approached before the patient arrives.
• Minimally, the patients history and physical
examination notes should be reviewed, other
available information (CT, X-rays, pathology
reports and operating reports)
• The preparation of specialized immobilization
devices.
Preparing the Room
• Proper room preparation can aid in the
effective use of simulator time.
• Sanitize materials used from previous
patient.
• Clean cloth or paper sheet placed on
simulator couch.
• Anticipated immobilization devices
prepared and ready
Explanation of procedure
• Assessment: assess patient’s needs,
cultural differences, nonverbal
communication, and then attempt to
communicate therapeutically and
effectively with the patient.
– Physical condition and emotional state
• Nervous, withdrawn, fearful
• Require oxygen, medication
• Difficulty standing, sitting, walking
Explanation of procedure
• Communication: therapeutic communication can
establish an environment conducive to communication
– Introduce staff and explain the simulation procedure in detail and
an explanation of what procedures to follow after simulation and
treatment
• How to take care of skin
• Fullness of bladder
• Follow-up instructions for barium
–
–
–
–
–
–
Keep the conversation directed at the patient
Avoid close ended questions
Face the patient and maintain eye contact whenever possible
Check for understanding, restate or repeat
Reduce unwanted noise
Speak clearly, confidently, and at a rate and tone conducive to
listening
Explanation of procedure
• Observation: noting facial expressions, body gestures,
space relations, and contradictions in patients
communication
• Cultural diversity: be aware of cultural differences in
both verbal and nonverbal communication to avoid being
misunderstood, offending someone, or being offended
by someone.
• Educating the patient and family: about the physical
aspects of radiation therapy but also the emotional
aspects
• Simulation is an opportunity to educate the patient and
answer questions concerning the treatment process,
side effects and skin care.
Patient Positioning
• Patient positioning should be communicated along with
an explanation of why that position is needed- facilitates
patient’s cooperation
• Three directional lasers are used for patient alignment
 A persons age, weight, general health, and anatomic area can
affect position
• Ink tattoos, visible skin marks, references to topographic
anatomy used to delineate the area
• If a CT scan is performed on a conventional CT, the
therapist must accompany the patient to ensure the
patient is in the same treatment position when scanned.
Immobilization
• Immobilization is used to achieve true reproducibility and
accuracy.
• Once the threshold dose for tumor response has been reached,
small increases in the absorbed dose may make large
differences in tumor control.
• Once the threshold for normal tissue injury has been reached,
small increases in dose may greatly increase the risk of
complications.
• Effective immobilization devices:
– Aid in daily treatment setup and provide reproducibility
– Ensure that immobilization of the patient or treatment area is done with
a minimum of discomfort
– Achieve the conditions prescribed in the treatment plan
– Enhance precision of treatment with minimal additional setup time
– Are rigid and durable enough to withstand an entire course of treatment
– Take into consideration the patient’s condition and treatment unit
limitations
Immobilization
• Positioning aids: devices designed to place the patient
in a particular position for treatment
– Very little structure, widely available, easy to use, may be used
for more than one patient
• Head holders, pillows, cushions, sandbag, L-shaped arm board
• Simple immobilization: restrict some movement but
usually require the patients voluntary cooperation
• Tape, Velcro, rubber band, arm to foot straps
• Bite block: helps the patient maintain the position of the chin, and
moves the tongue out of the treatment field.
• Complex immobilization: are individualized
immobilizers that restrict patient movement (plaster,
plastic, Styrofoam)
• Vac-loc, foaming agents, aquaplast
Operating Controls
• Mechanical components: gantry rotation,
collimator movements, treatment couch
• Optical components: laser system,
optical distance indicator (ODI), field light
indicator
• Radiographic components: kVp, mAs
Setting Field Parameters
• Field parameters such as width, length,
gantry angle, collimator angle, and
position of the isocenter should be
established for both the SSD and SAD
setup.
– The isocenter is positioned at the CA on the
patients skin for an SSD approach and within
the patient for SAD
• Orthogonal films, which provide three dimensional
information may be used with the SAD
Producing Quality Images
• Selecting exposure technique: vary from one clinical
site to the next and from one simulator to another.
– Body habitus: attenuation of the x-rays will vary, depending on
the patients thickness and , to a lesser degree to the body’s
composition
• Orienting the film: grid use, fast screen?
– Centering the film, reducing the size of the diaphragm opening,
and setting an appropriate source-film distance, collimation
– Phototiming: form of automatic exposure control in which one or
more ionization cells automatically stop the exposure at preselected density
• Processing the film
• Documenting the radiographic images: information on
the film
Documenting Pertinent data
• Essential to accurately reproduce the
geometry of the setup on the treatment
unit
• To maintain accurate medical records
• To aid in the treatment planning and dose
calculation processes
• Includes both
– Marking patient
– Documentation in chart
Documenting Pertinent data
• IFD: directly influences the dose to both the
tumor and other normal tissues
• Using bony landmarks as reference has
advantages:
– Skin marks are highly mobile, especially for obese
patients, whereas the location of the target volume
remains essentially constant with respect to bony
landmarks
– A resimulation is not required if the skin marks are
lost
– The treatment field can be easily reconstructed ling
after the current course of therapy.
Contours
• Contour: a reproduction of an external body
shape, usually taken through the transverse
plane of the CA of the treatment beam
• Provides the therapist and dosimetrist with the
most precise replica of the patients body shape
so that accurate information may e gathered
concerning the dose distribution within the
patient.
– The treatment volume and internal structures are
transposed within the contour using data from the
simulation images and/or CT or MRI films.
• Assists in repositioning the patient
Types of Contours
Material/Method
Advantages
Disadvantages
Solder Wire
Reusability, pliability
Pliability (distortions)
Plaster strips
Inexpensive, transferability of
surface ink markings
Drying time, messy, not
reusable
Aquaplast contour
tubes
Inexpensive, reusable, shapes
well
Drying time, not well suited
for intricate areas
Pantograph contouring
device
Time-saving operation,
reproduces detail well
Cost, size, storage space
required
CT
Accurate transverse views
Cost of interface
MRI
Accurate transverse, coronal,
and sagittal contours
Cost of interface
Ultrasound
Discernable transverse
correlation of internal structures
Poor quality of imaged deep
structures
Record and verify systems
• Tolerances may be set on many of the
treatment units positions, such as couch
height and couch positions in the left/right
and inferior/superior direction
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