Exhaustive Study Notes on Radiation Therapy Simulators and Simulator Technologies

Course Objectives and Introduction to Radiation Therapy Simulators

• RT Studies 1 (E GILES): This module focuses on the identification and use of equipment for planning and delivering radiation therapy.

• Course Objective 1 (CO1): Identify the equipment utilized to plan and deliver radiation therapy treatment, specifically the necessary landmarks and measurements required for setting up patients for treatment.

• Course Objective 2 (CO2): Interpret documentation in patient treatment records, including commonly used terminology within the field of Radiation Therapy.

• Graduate Quality 1: Emphasis on the body of knowledge related to Radiation Therapy.

The Conventional Simulator

• Definition: An isocentrically mounted diagnostic x-ray machine designed to reproduce treatment conditions exactly.

• Screening Facility (Fluoroscopy): Consists of an image intensifier linked to a closed-circuit television (CCTV) system for real-time visualization.

• System Components:

  • Gantry: The movable frame supporting the radiation source and detectors.

  • X-ray Tube and Generator: The source of the diagnostic radiation.

  • Collimator Head: Houses beam-defining components.

  • Image Intensifier: Converts x-rays into visible light images.

  • Film Holder: For capturing static diagnostic images.

  • Treatment Couch: The platform where the patient is positioned.

• Collimator Head Details:

  • Lightbulb Assembly: Used to project a light field onto the patient.

  • Mirror: Reflects the light from the bulb to match the x-ray path.

  • Primary Collimation: Initial beam limiting.

  • Blades/Shutters: Beam restricting diaphragms made of $2-3 \, mm$ of lead ($Pb$). They define the size of the x-ray beam during fluoroscopy or film exposure. By limiting the area, they reduce scatter to the film or image intensifier. These automatically "cone down" in fluoroscopy mode.

  • Reticule (Fiducial Plate): A plastic tray (usually) embedded with lead markings. Markings are spaced to represent field geometry at the treatment distance ($F/SAD$ of $100 \, cm$). This aids in determining film magnification.

  • Field Defining Wires: Help visualize the treatment area on the simulator film.

  • Accessory Holder: For additional tools or filters.

• Manufacturer Specification Example (Varian):

  • Manufacturer: Varian Medical Equipment (Gatwick Road, Crawley, Sussex, England).

  • Model No: TM 56088000.

  • Serial No: 933.

  • Aluminium Equivalent of Collimator Assembly: $2,00 \, mm$.

Conventional Simulation Procedures and Protocols

• General Concept: Patient data is collected using fluoroscopy, films, and physical measurements. This data is then entered into a planning system.

• Step-by-Step Procedure:

  • Communicate and explain the procedure to the patient.

  • Decide on a specific treatment position.

  • Apply stabilization or immobilization aids as required.

  • Employ contrast agents if necessary (e.g., using barium to illuminate esophageal narrowing).

  • Perform "screening" (fluoroscopy) to localize the treatment area.

  • Take an orthogonal set of simulation films (e.g., Lateral Breast Sim film).

  • Record all treatment parameters.

  • Process images and ensure they are of high diagnostic quality.

  • Ensure all images and records are labeled correctly.

  • Take physical measurements and document the patient’s position.

  • Tattoo reference points or the isocentre on the patient’s skin.

  • Advise the patient on the estimated time required to complete the planning phase.

• Primary Functions of Simulation:

  1. Patient alignment.

  2. Localisation.

  3. Reference point definition.

  4. Determination of the Initial Isocentre.

  5. Field/portal design.

  6. Verification of the treatment plan.

The CT Simulator (CT-Sim)

• Overview: A dedicated CT scanner that allows for three-dimensional ($3D$) visualization of the patient and internal structures. It has largely replaced conventional simulators in most modern departments.

• Specific Components:

  • Gantry: Houses the x-ray tube and detectors.

  • Control Console: Operator interface for scan parameters.

  • Couch: Flat top (as opposed to curved diagnostic tops) and often indexed for seamless positioning.

• CT-Sim Features:

  • Wide Bore: Gantry openings up to $90 \, cm$ in diameter to accommodate various treatment positions.

  • Flat Couch Top: Necessary for replicating the linear accelerator (Linac) couch. If a scanner has a curved top, a flat insert must be used.

  • Adjustable Lasers/Patient Markers: Includes external, internal, and movable lasers for patient alignment and "zero-ing."

  • Virtual Simulation: Ability to reconstruct CT data into Digital Reconstructed Radiographs (DRRs).

  • DICOM Link: Connection to planning systems via DICOM RT or DICOM3 standards.

CT Scanning Mechanics and Hardware

• Scanning Types:

  • Axial Scanning: The patient remains stationary while the tube rotates $360^{\circ}$ to collect a single image slice.

  • Spiral (Volumetric/Helical) Scanning: The patient moves slowly through the gantry while the tube rotates continuously. This is facilitated by Slip Rings (metal strips carrying electronic signals and power).

• Detectors:

  • Designed to convert radiation into light.

  • Multislice Detector Arrays: Allow for shorter imaging times and the ability to image more anatomy in the same time frame. They permit varying slice thicknesses (e.g., $4 \times 4 \, mm$ slices, $4 \times 2 \, mm$ slices, or $4 \times 1 \, mm$ slices).

  • DAS: Data Acquisition System.

• Hounsfield Units (HU):

  • Also referred to as CT numbers or pixel values.

  • Measures electron density on a CT scan.

  • Essential for computing the radiation dose to the patient.

  • The HU-CT density conversion is measured annually using a phantom for Quality Assurance (QA).CT Simulation Parameters and Reconstruction

• Reconstruction Time: The duration required for the computer to analyze and process data from the detectors.

• Slice Thickness:

  • Standard slices: $2 \, mm$ to $3 \, mm$.

  • Smaller slices are used for stereotactic techniques.

  • High-quality DRRs require thin slices.

  • Head and Neck protocols typically use $1 \, mm$ slices with $1 \, mm$ spacing.

  • General mapping may use a minimal spacing of $5 \, mm$.

• Field of View (FOV):

  • Standard FOVs may not always image the skin surface; large bores are preferred.

  • Some scanners reconstruct larger FOVs, but HU accuracy must be verified for dose calculation.

• Virtual Simulation (Definition):

  • Beams are computer-planned according to borders rather than strictly conforming to a volume.

  • Utilizes Beam's Eye View (BEV) DRRs to visualize the field from the perspective of the radiation source.

• Scout/Topogram Images:

  • Also known as Surview, Scano, or Pilot images.

  • Typical first images acquired during CT simulation.

  • Usually AP (Antero-Posterior) or Lateral scans taken with a stationary tube and moving table.

  • Purpose: Ensure the patient is straight/centered and plan the start/end positions (superior and inferior) for the actual CT scan.

Documentation and Patient Data Recording

• Electronic and Manual Records Include:

  • Patient details (Name, ID, DOB, Gender).

  • Positioning instructions (e.g., Supine/Prone, Feet first/Head first).

  • Immobilization equipment (Headrest shapes like "B" or sponges, Mattresses, Bolsters).

  • Setup landmarks and measurements (e.g., SSN to Tip of Shoulders, Chin Notch).

  • Consent (Tattoo consent, Site confirmation).

  • Tattoo positions and reference points.

  • Parameters (Slice thickness, spacing, contrast usage).

• Specific CT Documentation Fields:

  • ULID (Upper Limit of Inferior Diaphragm) / JOR (Junction of Ribs).

  • Arms positioning: Abducted, on board, or down.

  • Stretch measurements: e.g., SSN to TOX/JOR in $cm$.

  • Scan Landmark Tattoo: Defined by X (Midline), Y (SN/TOX/JOR), and Z (Couch) coordinates.

  • Treatment Duration estimates: Typically ranges from $10 \text{ to } 60 \, mins$.

Patient Care and Professionalism

• Workflow: Simulation should be a streamlined process requiring advance planning.

• Communication: Continuous communication with the patient is essential throughout the procedure.

Summary Checklist for Students

• Understand the main components of a CT Simulator.

• Detail the steps involved in the simulation process.

• Distinguish between CT Simulators and diagnostic CT scanners (bore size, couch shape, lasers/DRR capabilities).

• Identify all information that must be recorded during a simulation session.

This module focuses on identifying and using equipment for planning and delivering radiation therapy. Key course objectives include identifying necessary equipment and landmarks for radiation therapy treatment, interpreting patient treatment record documentation, and understanding the body of knowledge related to radiation therapy. The conventional simulator is defined as an isocentrically mounted x-ray machine designed to replicate treatment conditions, consisting of components such as the gantry, x-ray tube, collimator head, and treatment couch. Conventional simulation procedures involve collecting patient data, localizing treatment areas, and ensuring correct patient positioning. The CT simulator provides 3D visualization of patients and internal structures, replacing conventional simulators in modern settings. Key CT scanner features include gantry design, couch type, and virtual simulation capabilities. Documentation includes patient details, positioning instructions, and treatment duration estimates, emphasizing the importance of communication and professionalism in patient care.