The CT simulation is a critical step in the radiation treatment process, serving as a planning session rather than a diagnostic exam. This procedure uses a specialized computed tomography (CT) scanner to create a three-dimensional map of the patient’s internal anatomy, including the tumor and surrounding healthy organs. The primary purpose is to precisely define the target area and determine the exact position the patient must maintain for every future treatment session. The acquired images provide the data needed by the radiation oncology team to calculate the precise angle, shape, and dose of the radiation beams.
Total Appointment Time Versus Scan Time
The time a patient spends at the facility for a CT simulation is substantially longer than the actual time the scan takes. A total CT simulation appointment typically lasts between 45 and 90 minutes, depending on the complexity of the case, and this duration includes administrative tasks, changing clothes, initial consultation, patient positioning, and marking.
The actual acquisition of the CT image is brief, lasting only 5 to 15 minutes. The majority of the appointment time is dedicated to ensuring the patient is positioned exactly as they will be for their daily treatments. The variance in total time is directly related to the treatment site; a simulation for a complex head and neck case, which requires custom immobilization and specific alignment, will take longer than a simpler case involving a limb.
Detailed Steps of the Simulation Procedure
The most time-intensive part of the appointment involves achieving and verifying sub-millimeter positioning accuracy. Once the patient is brought into the simulation room, the first step is transferring them onto the CT table and setting up the treatment position. Radiation therapists work to ensure the position is comfortable enough to be held consistently throughout the daily treatments.
Immobilization devices are then created or used to prevent movement and ensure the position is repeatable for every session. This might involve molding a thermoplastic mask over the face and neck, or shaping a vacuum-sealable cushion, known as a Vac-Lok bag, around the torso or pelvis. The process of creating and securing these custom molds can take a considerable amount of time, but it is essential for precision.
Lasers mounted on the walls and ceiling of the room are then used to align external markers on the patient and the immobilization device, setting a coordinate system for the scan. Only after this meticulous setup is complete does the actual CT scan begin, where the table moves slowly through the scanner to acquire the 3D data. Sometimes, verification scans or adjustments are necessary after the initial image acquisition, extending the time spent on the table.
Preparation Requirements That Impact Duration
Adherence to specific preparatory instructions significantly influences the speed and success of the simulation appointment. For example, treatments targeting the pelvic region often require the patient to have a full bladder and/or an empty rectum. This is because the volume of the bladder and bowel can shift the position of nearby organs, like the prostate, which impacts the accuracy of the radiation map.
If the simulation requires the use of intravenous (IV) contrast dye to better visualize blood vessels or specific soft tissues, additional time must be factored in for IV insertion and the contrast injection. Patients may also need to follow fasting instructions before the scan, especially if IV contrast is used. Furthermore, patients must remove all jewelry and clothing containing metal near the area being scanned to avoid image distortion; non-compliance with these instructions can lead to delays or rescheduling.
Post-Simulation Activities
After the CT imaging is complete, the final crucial steps are performed by the clinical staff while the patient is still on the table. The radiation oncologist reviews the initial scans to finalize the exact center point for the treatment beams, followed by the application of permanent alignment marks.
These marks are typically tiny, freckle-sized tattoos placed on the skin, which serve as reference points for daily setup during the weeks of treatment. They allow the therapists to reliably reproduce the simulated position every time the patient comes in for therapy. Temporary ink marks are also drawn on the body or the immobilization device to supplement the tattoos.
Once the setup marks are verified and recorded, the patient is helped off the table and changes back into their clothes. The final minutes of the appointment are spent confirming the next steps, including discussing the timeframe for treatment planning and scheduling the first treatment session. The patient is also often given final instructions regarding skin care or any ongoing preparation requirements for the start of therapy.