Radiation Therapy Simulation and Planning Vocabulary

General Simulation Practices

Isocenter Determination: During the simulation process, the location of the isocenter is determined.
Patient Positioning: The patient must be positioned in a manner suitable for the treatment area and accurately reproducible on the treatment table.
Aids in Reproducibility: Properly placed marks, photographs, and detailed descriptions are essential for reproducibility.
Marking Tools: Patient marks can be applied using: * Felt pens. * Paint pens. * Carfusion. * Tattoos.
Margins in Advanced Therapy: When administering conformal 3D or Intensity-Modulated Radiation Therapy (IMRT), margins are extremely tight, leaving minimal room for error. Setup stability cannot rely solely on patient marks.
Setup Verification: Patient setup should be verified radiographically as often as feasibly possible.
Leveling: To ensure the patient maintains a level position, marks should be placed using all laser lines.
Sagittal Alignment: Alignment along the sagittal plane can be maintained by placing two marks $10\,cm$ to $20\,cm$ apart along the sagittal laser lines.
Simulator Graticules: If a treatment machine is used for initial simulation, the crosshairs provided by the graticule serve the same purpose as sagittal lasers.
Portal Imaging: Portal images verify the beam's path, shape, and central axis relative to patient anatomy. Tools include: * Film. * Electronic Portal Imaging Devices (EPIDs).
Image-Guided Radiation Therapy (IGRT): Linear accelerators with on-board kV imagers allow for Cone-Beam CT (CBCT) technology. CBCT images are compared to planning images to determine setup accuracy or necessary shifts on the treatment machine.

The XYZ Coordinate System

Localization Process: Isocenter localization during simulation is accomplished using a three-dimensional XYZ coordinate system. Localization is defined as the delineation of the treatment target and the placement of the isocenter relative to that target.
Axis Definitions: * x: The transverse axis extending right to left in the patient. * y: The longitudinal axis extending head to foot. * z: The axis extending upward from the table top.
Reference Points: XYZ coordinates are fixed relative to the simulator, CT, treatment planning computer, or linear accelerator table top.
Standard Orientations: 1. Supine and Head First: Positive $x$ is on the patient’s left; positive $y$ is cephalic; positive $z$ is anterior. 2. Prone and Head First: Positive $x$ is at the patient’s right; $y$ is unchanged (cephalic); positive $z$ is posterior. 3. Supine and Feet First: Positive $x$ is on the patient’s right; positive $y$ is caudal; $z$ is unchanged (anterior).

Lasers Used for Localization

Mounting: Lasers are mounted on walls and ceilings in simulation and treatment rooms.
Convergence: Wall-mounted lateral lasers and sagittal ceiling or wall-mounted "toe" lasers are designed to converge at the point of SAD (Source-to-Axis Distance) or the isocenter.
Marking: Laser beams are very narrow; patient marks are placed on the laser light to indicate isocenter location.
Implementation: * On treatment machines: Localization is achieved through table movement. * On CT simulators: Localization is done via laser movement or in the planning computer after simulation.

Positioning, Positioning Aids, and Immobilization Devices

Consistency: Soft/pliable items like head holders, pillows, sponges, and table mats foster inconsistency and should be avoided.
Positioning Aids: Firm, commercial positioning aids facilitate consistent positions but do not immobilize. Examples include: * Commercial/custom head holders. * Knee sponges (reduce lordotic curvature of the spine for comfort). * Wing boards, prone pillows, Duncan masks, chin straps, and shoulder pulls.
Knee Support Caveat: Knee supports should be avoided for treatment below the diaphragm because leg elevation is rarely consistent and can cause variation in internal abdominal/pelvis anatomy.
Immobilization Devices: These help limit patient movement during treatment and significantly decrease setup and targeting errors.

Anatomical Site Specific Positioning

Head and Neck: * Usually treated supine for comfort and to facilitate lateral beam arrangements. * Allows clinical verification that the eye lens is excluded from fields. * Immobilization often uses rigid plastic head holders or Thermoplastic molding (e.g., Aquaplast). * Aquaplast Application: Become pliable in warm water, contours to patient, and dries rigid. Marks can be applied to the mask instead of the skin. Glabella, chin, and auditory meatus should be molded using fingers. * Shoulder Management: Shoulder pulls/straps are controversial due to variability. Alternatives include computer contouring of shoulders with dose constraints or setting gantry angles to avoid shoulders. * Bite Blocks: Used to ensure consistent chin extension and to position/immobilize the tongue and jaws.
Breast: * Typically treated supine, but prone position (using a breast board) is useful for large breasts. * Legs/ankles must not be crossed to avoid torso rotation. * Wing Boards: Allow arms to be raised symmetrically above the head; can be used with a breast board at a $5 - 10$ degree incline to reduce chest slope. * Immobilizers: Allowing the breast to find its natural position is often more reproducible than using tissue immobilizers. * Skin Folds: Prone to erythema and desquamation; manageable by adjusting arm position or using computer planning. * Custom Bags: Vac-Loc bags (Styrofoam beads/vacuum) or Alpha Cradle (thermal reaction chemicals) provide custom support.
Chest, Abdomen, Pelvis, and Extremities: * Wing boards are appropriate for lateral or posterior oblique chest/abdomen fields. * Pelvic Treatment: Prone position reduces small bowel volume in the field and decreases gluteal folds. Belly boards move small bowel anteriorly. * Extremity Planning: Tailored to avoid opposite limbs/neighboring tissues during high-dose therapy (e.g., sarcoma).

CT Simulation and Virtual Simulation

Process: Images contain 3D anatomical information used for virtual patient simulation after acquisition.
Surface Fiducials: Small radiopaque wires (that don't cause artifacts) are placed at XYZ coordinates. They allow the preliminary isocenter to be viewed on CT images.
Scan Parameters: * Marginal scanning above and below the area of interest. * Slice Thickness: $2 - 3\,mm$ is used for IMRT for detail and density differentiation. $5\,mm$ may be used for non-IMRT to reduce contouring workload. * Helical Scanning: Preferred to lessen the chance of skipped data.
Virtual Tools: * Beam’s-Eye View (BEV): Computer-generated radiograph to visualize anatomy with respect to the treatment beam. * Digitally Reconstructed Radiograph (DRR): Reconstructed image from CT data; resolution declines as slice thickness increases.

4D and MRI Simulation

4D CT Simulation: Combines CT simulation with respiratory motion tracking. * Gated Imaging: Patient holds breath at maximum inspiration/expiration; requires patient training. * Free Breathing: Images acquired through several respiratory cycles via helical scan.
MRI Simulation: * Advantages: Superior soft tissue contrast; provides metabolic and functional data. * Disadvantages: No electron density data for dose algorithms; image distortion; incompatible with some aids. * Use Cases: Brain tumors (primary/metastatic), prostate, and abdominal tumors.

Computerized Treatment Planning Principles

Multifield Approach: Preferable to provide homogenous dose, reduce high doses in single areas, and minimize side effects.
Image Fusion (Registration): Superimposing two sets of images on the same coordinate system (e.g., CT/MRI or CT/PET). The CT dataset remains the primary set for dose calculations.
Anatomical Contouring: Defined as image segmentation. Critical structures and tumor volumes (GTV) are outlined manually or automatically.

Target Volume Terminology (ICRU)

Gross Tumor Volume (GTV): The tumor visible by imaging (CT, MRI, PET).
Clinical Target Volume (CTV): GTV plus a margin of up to $2.0\,cm$ for unseen disease.
Internal Target Volume (ITV): CTV plus an internal margin to compensate for physiological movement (respiration, heartbeat, bladder filling).
Planning Target Volume (PTV): CTV/ITV plus a margin of $0.5\,cm$ for patient movement and setup error. Formula: $CTV + ITV + \text{Setup Margin} = PTV$ .
Treated Volume: Includes margins for treatment technique limitations.
Irradiated Volume: Tissue receiving > 50\% of the target dose.
Organs at Risk (OAR): Normal tissues whose sensitivity limits the dose.
Planning Organ at Risk Volume (PRV): Margins added to OAR to compensate for movement.

Beam Energy and Modality Selection

Photon Energy: * Higher Energy: Deeper targets (Pelvis/Abdomen). Deeper $D_{max}$ , more penetrating. * Lower Energy: Shallow targets (Head/Neck, Breast). Shallower $D_{max}$ . * Examples: $6\,MV$ photon beam has $D_{max} = 1.5\,cm$ ; $18\,MV$ beam has $D_{max} = 3.5\,cm$ .
Electron Beams: Treat superficial targets. Rapid dose fall-off in tissue. * Rules of Thumb ( $E$ = electron energy): * $\frac{E}{2} = \text{Range of beam in cm}$ * $\frac{E}{2.8} = \text{Depth of the 80\% isodose line}$ * $\frac{E}{3.2} = \text{Depth of the 90\% isodose line}$

Treatment Field Modifiers and Techniques

Gantry Angles: Must avoid dose-limiting organs and ensure mechanical clearance with table/patient.
Wedges: Manipulate dose distribution. Defined by the angle of the $50\%$ isodose line at CAX (low energy) or at $10\,cm$ depth (high energy). * Hinge Angle Formula: $\text{Hinge angle} = 180 - 2 \times (\text{wedge angle})$ * Virtual Wedges: Achieved by dynamic collimators. * Safety: Place wedges at least $15\,cm$ from the patient to avoid electron scatter contamination.
Bolus: Tissue-equivalent material used to fill deficits (homogeneity) or shift $D_{max}$ closer to the skin surface.

Interpreting Treatment Plans and DVHs

Dose-Volume Histogram (DVH): A graph showing the volume of an organ relative to the radiation dose it receives.
Composite Plans: Combinations of multiple treatment phases to show summative doses.
Fluence Maps: Reference tools for dose concentration in IMRT.
Standard Field Arrangements: * Parallel Opposed Pair: Homogenous for central targets (Whole brain, femur). * 4 Field Box: Spares periphery; concentrates dose at intersection (Prostate, bladder). * 90-degree Hinge: For unilateral targets (Maxillary sinus, brain tumors). * Arcs/IMRT: Concentrated dose for small, deep targets.

Managing Treatment Plan Deviations

Machine Malfunction: Document actual MUs given; adjust for missing dose.
Weight Change: Requires recontouring, remeasuring, or MU recalculation.
Incorrect Fractionation: Calculate Biological Effective Dose (BED) and adjust schedule.
Incorrect Accessory: Replan with incorrect accessory to determine actual absorbed dose.

Adjacent Fields and Matching Techniques

Methods to reduce overlap: 1. Skin Gap Formula: $\text{Gap} = \frac{1}{2} A_1 \times (\frac{d}{SSD}) + \frac{1}{2} A_2 \times (\frac{d}{SSD})$ 2. Beam Divergence Formula: $\theta = \tan^{-1} (\frac{A}{2 \, SSD})$ 3. Half-Field Technique: Uses asymmetric jaws to block up to the central axis (where divergence is zero), allowing direct abutting of fields.
Match line changes (Feathering): Periodic adjustments to the boundary of adjacent fields.

Monitor Unit (MU) Calculations

Parameters: * Equivalent Square (Sterling Formula): $\frac{2 \times (\text{field width} \times \text{field length})}{\text{field width} + \text{field length}}$ * SAD Formula: $\text{Target dose} \div (FSOF \times TMR \text{ or } TAR \times WF \times TF) = MU$ * SSD Formula: $\text{Target dose} \div (PDD \times FSOF \times BSF \times \text{Inverse Square Factor} \times \text{Source Output Factor} \times TF \times WF) = \text{Time/MU}$
SSD Disadvantages: Patient movement for each angle; no rotational therapy; higher risk of setup error miss.
Mayneord F Factor: Used at extended distances to correct for Percentage Depth Dose (PDD) changes based on the inverse square law.

Brachytherapy Principles and Isotopes

Delivery: Radioactive sources placed on or near the target.
Dose Rates: * LDR: $0.5 - 2.0\,cGy/min$ . * MDR: $2.0 - 20\,cGy/min$ . * HDR: > 20\,cGy/min.
Decay Types: Alpha (helium nucleus), Beta (positron/negatron), Electron Capture, Internal Conversion.
Sources: Common isotopes include Cobalt, Cesium-137 ( $LDR$ ), Iridium-192 ( $HDR$ ), Palladium, and Gold. Sources are double-sealed in steel to filter alpha/beta particles.
Applicators: * Fletcher Suite: Combination of tandem (uterine tube) and ovoids (distal vagina). * Afterloading: Installing the applicator first and loading sources later to reduce staff exposure.
Implants: * Intracavitary: Uterus. * Interstitial: Prostate. * Intraluminal: Bronchus or MammoSite.
Dosimetry Systems: * Patterson-Parker (Manchester): Nonuniform source distribution for uniform dose (Point A). * Quimby: Uniform source distribution leads to nonuniform dose. * Paris: Used for line sources (plastic ribbons).