Radiation simulation is the precise preparatory step that takes place before a patient begins radiation therapy (radiotherapy). It is a detailed planning session that maps the patient’s internal anatomy. The goal is to gather the exact information needed to design a safe and effective treatment regimen, ensuring radiation beams are delivered with the highest possible accuracy.
Why Radiation Simulation Is Essential
Simulation establishes the geometric framework for the entire treatment process. This preparatory work is necessary to maximize the radiation dose delivered to the cancerous tumor tissue while minimizing the dose that reaches surrounding healthy organs and structures.
The process creates a detailed three-dimensional map of the patient’s anatomy, including the tumor’s exact location and its relationship to nearby structures. Achieving this level of precision ensures that the patient’s position can be reproduced exactly for every subsequent treatment session. Reproducibility is a fundamental requirement, as small variations in positioning compromise effectiveness and increase risk to normal tissues.
The Simulation Appointment
The simulation appointment focuses on finding and documenting a stable, comfortable position that the patient can maintain for the duration of the daily treatments. Patients lie on a specialized table, and radiation therapists use supportive devices to maintain a fixed posture.
A primary component is the acquisition of an imaging study, usually a dedicated computed tomography (CT) scan. This CT scan provides the high-resolution, three-dimensional data necessary for calculating the radiation beam paths, rather than for diagnostic purposes. The patient must remain completely motionless during the scan, which can take between 30 minutes to over an hour, depending on the complexity of the area.
Following the imaging, therapists use external lasers to mark reference points on the patient’s skin. These marks may be temporary ink lines or very small, permanent tattoos (tiny pin-pricks of ink). These marks act as external coordinates, allowing therapists to align the body precisely with the lasers in the treatment room each day.
Specialized Equipment for Targeting
Precision in radiation delivery relies heavily on specialized equipment designed to eliminate uncertainty caused by patient movement. Immobilization devices are customized tools that help the patient maintain the exact same position throughout the simulation and treatment process. Examples include:
- Thermoplastic masks that mold to the head and neck.
- Custom-shaped foam cradles for the body.
- Vacuum-sealed bags that conform to the patient’s shape.
The imaging systems are specialized CT scanners, often featuring a large bore to accommodate the patient while they are in their immobilized position. These scanners are equipped with external laser systems that project crosshairs onto the patient’s skin, which therapists use to apply the reference marks. For tumors affected by breathing, such as those in the lung or liver, a four-dimensional CT (4D CT) scan may be performed. This technique captures images over the breathing cycle, providing a map of the tumor’s movement.
The simulation software must also be able to fuse the CT images with other high-contrast studies, such as Magnetic Resonance Imaging (MRI) or Positron Emission Tomography (PET) scans. This image fusion allows the radiation oncologist to define the tumor and surrounding healthy organs more clearly than with CT alone. This integration enables the high degree of targeting accuracy in modern radiation therapy.
Next Steps: Treatment Planning
Once the simulation concludes, the generated CT and fused images are transferred to the Treatment Planning System (TPS). The radiation oncologist and a medical dosimetrist collaborate within this software environment to define the boundaries of the tumor target and all nearby organs that need to be avoided.
The dosimetrist uses the TPS to calculate the precise distribution of the radiation dose within the patient’s body. This involves determining the optimal number, shape, angle, and intensity of the radiation beams to deliver the prescribed dose while minimizing exposure to healthy tissues. This process, called dosimetry, is a complex mathematical optimization that can take several days.
The final treatment plan is a detailed set of instructions and parameters for the linear accelerator, the machine that delivers the radiation. The radiation oncologist reviews and formally approves the plan before the first treatment is scheduled. This planning phase ensures the map created during the simulation is translated into a safe, efficient, and highly personalized treatment course.