Computed Tomography (CT) scans provide detailed cross-sectional images of the body, playing a fundamental role in modern healthcare, aiding in diagnosis and precise treatment planning. In radiation oncology, CT simulation is a specialized imaging procedure used to prepare for radiation therapy.
Purpose and Definition
CT simulation is a dedicated imaging process performed specifically for radiation therapy planning, differing significantly from diagnostic CT scans. Its primary objective is to accurately map a patient’s internal anatomy in the exact position they will be in during daily radiation treatments. This process helps identify the tumor, outline its boundaries, and delineate nearby healthy organs that must be protected from radiation exposure. The detailed anatomical information obtained is crucial for designing a precise radiation treatment plan.
The goal is to ensure radiation doses are delivered with high accuracy to the target area while minimizing impact on surrounding healthy tissues. This specialized CT scan serves as a “mock-up” of the actual treatment, allowing the radiation oncology team to visualize and plan therapy in three dimensions. It is the initial step in the radiation therapy journey, setting the foundation for safe and effective treatment delivery.
The Simulation Process
The CT simulation process begins with careful patient positioning to ensure daily treatment reproducibility. Patients are often positioned using specialized immobilization devices, such as molds, casts, or headrests, customized to their body shape and the treatment area. These devices help patients remain still and in the same alignment for both the simulation and daily treatments. Radiation therapists use lasers to align the patient precisely, often performing a preliminary scan to confirm proper positioning.
Once the patient is comfortably and accurately positioned, a CT scan of the area to be treated is performed. In some cases, contrast agents may be administered to help differentiate specific anatomical structures, though this is less common than in diagnostic scans. For tumors affected by breathing, such as those in the lung or abdomen, a 4D CT scan might be used to capture tumor movement throughout the respiratory cycle. This accounts for organ motion, providing a more comprehensive picture for planning. After the scan, small, permanent skin marks, often referred to as tattoos, are placed on the patient’s skin to serve as external reference points for daily treatment setup.
Data Utilization for Treatment Planning
Following CT simulation, acquired images are transferred to a specialized treatment planning system (TPS), a computer software designed for radiation oncology. Radiation oncologists and medical physicists then use these detailed 3D (and sometimes 4D) images to meticulously outline target volumes, which include the tumor (Gross Tumor Volume) and any areas with potential microscopic disease (Clinical Target Volume). Simultaneously, they identify and contour healthy organs nearby that need to be shielded from radiation, known as organs at risk (OARs).
The planning team then calculates the precise radiation doses needed to treat the tumor while adhering to limits for the surrounding healthy tissues. This involves designing the number, shape, and intensity of radiation beams to deliver a customized treatment plan unique to each patient’s anatomy and tumor location. Data from the CT simulation allows for the creation of digitally reconstructed radiographs (DRRs), which are virtual X-ray images that help verify beam coverage and visualize the treatment fields. This digital planning ensures that the radiation is delivered accurately during each treatment session.
Distinguishing Features
CT simulation differs from a standard diagnostic CT scan in several key aspects, primarily due to their distinct purposes. A diagnostic CT aims to detect disease, stage cancer, or monitor treatment progress, focusing on high-quality imaging for visual interpretation. In contrast, a CT simulation’s goal is to capture precise anatomical data and patient positioning for reproducible radiation therapy planning. This means the emphasis shifts from diagnostic clarity to geometric accuracy and setup reproducibility.
CT simulators often feature a larger bore (the opening of the scanner, typically 80 cm or more) and a flat table, mimicking the treatment machine’s couch, which allows for various patient positions and the use of immobilization devices. Unlike diagnostic CTs where contrast is frequently used, CT simulation may use contrast less often, as the primary need is for accurate anatomical mapping for dose calculation. Furthermore, CT simulation often incorporates advanced techniques like 4D CT to account for tumor motion caused by breathing. The data from a CT simulation is directly integrated into treatment planning software, ensuring that the patient’s position and internal anatomy are accurately represented for radiation delivery.