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 tumorMeasured with a caliper

• Field size: the dimensions of a treatment field at the isocenter, represented by width x lengthDetermined 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:DiagnosisTumor and normal tissue localizationTissue density data for dose calculationFollow-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

treatmentRequires 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 dimensionsThe 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 planning2. Room Preparation3. Explanation of procedure4. Patient positioning and immobilization5. Operation of simulation controls6. Setting field size parameters7. Selecting exposure technique8. Radiographic exposure9. Documenting pertinent data10.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 pre-selected 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 ContoursMaterial/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