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HSC 340 99-16-10
• Conventional Simulation procedures
• Computed Tomography Simulation procedures
Radiation Therapy process
• Not all patients will need all steps, nor will they always occur in sequence
Diagnosis
• Screening
• Cancer Imaging
• Pathology
• Staging
Therapeutic Decisions
• Cure
• Palliation
• Benign
• Surgery-radiation-chemotherapy
• Patient Interview
Simulation
• Fluoroscopy based
• CT simulation
• Patient positioning
• Immobilization devices
• DRRs
Treatment Planning
• Identifying PTV
• Identifying critical structures
• Selection of treatment technique
• Isodose distribution
• Calculation of treatment beams
• Optimization
Treatment
• Treatment verification & imaging
• Dosimetry checks
• Treatment delivery and monitoring
• Patient assessment
• Record keeping
Patient follow-up
• Patient assessment
• Normal tissue response
• Tumor control
Conventional Simulation Procedures
• Synonymous with fluoroscopy-based simulation
• Nomenclature– Localization
– Verification
– Radiopaque marker
– Contrast media
– Seperation
– Field Size
ICRU Reports
• GTV- gross tumor volume
• CTV- clinical tumor volume
• ITV- Internal target volume
• PTV- planning target volume
• TV- treated volume
• IV- Irradiated volume
• OAR- Organs at risk
Acronymspg. 446 image
• CAX
• IFD
• ISO
• SAD
• SFD
• SSD
• TT
Body PlanesSagittal- L and RCoronal- Ant and PostTransverse- Inf and Sup
Localization Methods
• SAD and SSD
• SSD method
– Requires repositioning the patient for each field before treatment
– ODI reading on skin will always be 100cm
• SAD method
– aka: isocentric technique
– ODI reading on skin will vary according to thickness or seperation of the patient, also depending on the depth of the tumor from the skin
Both define the field size at 100cm, only difference is where that distance is located (on skin or within patient)
Contrast Media
• Used to visually enhance structures
• Positive- Barium, iodinated contrast
• Negative- air
• Careful workup
Procedure
*****Must be well documented*****• Presim planning
• Room preparation
• Explanation (Consent?)
• Patient positioning
• Immobilization
• Setting parameters
• Radiographic exposure
• Documentation
Presim planning
• Assessment of patient
• Extent of disease
• Concerns
• Therapist-Physician consult
• Chart thoroughly read
Room preparation
• Usually busy, make the most of it
• H & N– Water bath temp checked– Mask ready– Headrest chosen (A to F)– Stent, bite block, mouthpiece– Wires for surgical scar delineation– Straps to pull shoulders down
• Thorax– Headrest– Vac-lok– Wingboard– BBs, wire
• Pelvis– Alpha cradle
– T-bar with strap
– Headrest
– Ring for hands to hold
– Contrast agents
– Catheter
Explanation
• Assessment
• Communication
• Education
Patient Positioning
• If they are not comfortable, then not effective
• If it is not reproducible then it may cause a geographic miss or irradiation of critical structures
• Supine or Prone
• Emergencies may lead to semi-sitting position
Patient Immobilization
• Effective immobilization devices restrict a patient from moving
• Can range from simple to complex– Tape
– Rubberbands
– Alpha cradleYouTube - Alpha Cradle Mold Maker Instruction
– Vac-lok YouTube - SecureVac Vacuum Cushions from Bionix
– Head mask
Setting Parameters
• Field parameters:– Width, length, gantry angle, collimator angle,
position of isocenter
Radiographic Exposure
• Radiographic images taken in sim are referred to as “masters”
• kVp, mA, time and distance
• Note patients body type and condition
• Source to film distance recorded to show mag factor
Documentation
• This is a medical record
• Accuracy is key
• Involves both patient and chart
• Tattoos
• Landmarks
Contouring
• Contour is a reproduction of external body shape
• Solder wire, thermoplastic tube, plaster strip
• Must be accurate!!
CT Simulation ProceduresHounsfield Units- attenuation rates displayed as pixels of different shades of gray. Correspond to different e- densities
-1000 Air
0 water
1000 dense bone
Figure 23-8
X-ray tube---collimation---360 rotation---transmission measurements recorded by detectors
• Conventional CT- slice by slice, one image, table moves
• Helical/spiral CT- volume, table moves while scanning
Major steps in CT Simulation
• Presim planning• Room preparation• Explanation of procedure• Patient positioning and immobilization• CT data acquisition• Target and normal tissue localization• Virtual sim of treatment fields• Generation of dose distribution• Documentation
Benefits of CT Simulation
• Outline and view critical structures
• Optimal beam placement
• Cone down or boost can be done w/o patient
• BEV capability
• Electronic field shaping
Contrast
• Intravascular
• Oral
• Intrathecal
• Intraarticular
• Must get medical history first
IV Contrast agents & site specificities
• H & N- injected seconds before scan– Highlights vessels and distinguish from LN
• Liver- scan needs to be started 20 to 90 sec after injection– Visualize blood supply from portal vein and hepatic
artery
• Pelvis- IV contrast 15 minutes after injection– Bladder
• Brain- 10 to 30 minutes prior to scan– Highlight tumor due to vasculature
Other contrast agents and the specificities
• GI tumors- barium paste 30 min prior for small bowel
• Rectal marker, vaginal marker, radiopaque catheters….
***High contrast materials may alter dose calcs***
Reactions to contrast
• Mild
• Moderate
• Severe
Room preparation
• Same as conventional, be aware of bore size and patient positioning
• Flat table top if regular couch is curved
CT Artifacts
• Unwanted image abnormalities that can be caused by patient motion, anatomy, design of scanner, or system failure
• Beam hardening• Partial volume effect• Star artifact• Ring artifact• Motion and helical artifact
Fusion/Registration
• Starts with 2 sets of images
• Locate similar image features (landmarks)
• Manually move one image on top of another
Respiratory Gating
• Used to have patients hold breath
• Now we track the patients range of motion while breathing
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