A CT simulation is a specialized computed tomography scan that serves as the blueprint for your entire course of radiation therapy. Unlike a diagnostic CT, which is designed to find or characterize disease, a simulation CT is built to map your body in treatment position so that radiation oncologists and medical physicists can design beams that hit the tumor while sparing healthy tissue. The scan itself typically takes fifteen to forty-five minutes, but the data it produces will be used every single day of your treatment. Getting the details right at this stage is what makes precise, modern radiation therapy possible.
What Happens During the Appointment
When you arrive for CT simulation, the radiation therapy team will position you on a flat table top that matches the one on the treatment machine. This is different from the curved, padded table you lie on during a regular CT scan. You will be placed in the exact posture you will hold during every treatment session, and depending on the tumor location, the team may ask you to raise your arms overhead, bend your knees, or tilt your head in a particular direction. The goal is to find a position you can comfortably reproduce, day after day, for anywhere from one to seven weeks of treatment.
Once the position is set, immobilization devices are formed to your body. For head and neck cancers, that usually means a thermoplastic mask: a sheet of warm mesh is draped over your face and shoulders, and as it cools it hardens into a custom shell that gently holds your head in place. A study comparing thermoplastic masks with vacuum cushions in stereotactic body radiotherapy found that masks produced significantly smaller positional shifts across all treatment sessions, roughly half a centimeter on average compared with about one centimeter for vacuum cushions.1PubMed Central. Comparative analysis of thermoplastic masks versus vacuum cushions in stereotactic body radiotherapy For pelvic treatments, thermoplastic molds shaped to the pelvis have been shown to perform at least as well as vacuum-lock bags, with one comparative study concluding that the mold improved overall reproducibility.2International Journal of Clinical and Biomedical Research. Vacuum and Thermoplastic Mould-Based Immobilization Systems Used in Patient Undergoing Pelvic Radiation Therapy: A Comparative Study Breast treatments often involve a tilted board or arm cradle, while lung and abdominal cancers may use a vacuum bag that conforms to the torso.
After immobilization, the CT scanner acquires images, typically in slices a few millimeters thick, covering the treatment region plus a margin above and below. The scan settings tend to be more tightly controlled than a diagnostic scan, because image quality feeds directly into how accurately the planning software can calculate radiation dose.
Tattoos, Lasers, and Surface-Guided Setup
During or immediately after the scan, the team places reference marks on your skin. Traditionally this means three to five tiny permanent tattoos, each no bigger than a freckle, made with a drop of ink and a single needle poke. These dots align with a set of room lasers that are calibrated to a known point inside the CT scanner. When you return for treatment, the therapists line up your tattoos with the lasers on the treatment machine so that your body is in the same position as it was during simulation.
An increasingly common alternative is surface-guided radiation therapy, which uses ceiling-mounted cameras to track thousands of points on your skin surface in real time. This technique can achieve submillimeter positioning accuracy, which is better than what conventional laser-and-tattoo alignment delivers, and it avoids the permanent marks that some patients find psychologically distressing.3PubMed Central. Surface-Guided Radiotherapy: Can We Move on from the Era of Three-Point Markers to the New Era of Thousands of Points? In a study applying surface-guided setup to lung cancer patients treated without a rigid frame, residual setup errors were predominantly within one millimeter, and the agreement between planned and actual tumor volumes during treatment was excellent.4PubMed Central. Scaling Surface-Guided Radiation Therapy to Larger Lung Cancer Cohorts: Frameless Immobilization and Enhanced Setup Accuracy Not every center has the cameras yet, but the technology is spreading quickly.
How the Images Become a Radiation Plan
Once the simulation CT is complete, the images are transferred to a treatment planning computer. The first thing that happens is contouring: a radiation oncologist draws the outlines of the tumor (or the region where tumor cells are most likely to be) and the nearby organs that need protection. These volumes follow standardized definitions maintained by the International Commission on Radiation Units and Measurements, which has refined its guidelines over the decades to keep pace with advancing technology.5PubMed Central. The Practicality of ICRU and Considerations for Future ICRU Definitions
The planning software then uses the CT image data to figure out how radiation will travel through your body. Every pixel in a CT image carries a number that reflects how dense the tissue is at that point. The software converts those numbers into a map of how easily radiation passes through each region, allowing it to calculate how much dose arrives at the tumor and how much spills into surrounding structures. Research into improving these conversions is ongoing; one approach uses images taken at two different X-ray energies to produce a more direct measurement of tissue density, which can improve accuracy for certain tissue types.6PubMed. Initial implementation of the conversion from the energy-subtracted CT number to electron density in tissue inhomogeneity corrections: an anthropomorphic phantom study of radiotherapy treatment planning
The reason a dedicated simulation scan matters, rather than reusing a diagnostic CT you already had, comes down to accuracy. A study comparing treatment plans built on simulation CT versus plans built on prior diagnostic CT found no meaningful difference for straightforward targets like the whole brain, but for the lung and stomach, plans based on diagnostic scans delivered substantially less dose to the target. Lung coverage dropped from about 95% to roughly 86%, and stomach coverage fell from about 95% to 80%, largely because soft tissue positions and organ shapes had shifted between the two scans.7International Journal of Radiation Oncology, Biology, Physics. Dosimetric Evaluation of Diagnostic CT Versus CT Simulation for Palliative Radiation Therapy Planning A simulation scan taken in your treatment position, with your immobilization devices in place, eliminates that mismatch.
When Breathing Moves the Target
Tumors in the chest and upper abdomen move with every breath, sometimes by a centimeter or more. A standard CT scan captures a snapshot of the anatomy at one moment, which could show the tumor at the top, bottom, or middle of its breathing arc. If the treatment plan is built on that single snapshot, the beam may miss parts of the tumor during the breathing phases it did not capture.
The solution is four-dimensional CT, or 4DCT. During a 4DCT scan, images are acquired continuously while a sensor tracks your breathing cycle. The data is then sorted into roughly ten breathing phases, producing a set of CT volumes that show exactly where the tumor goes as you inhale and exhale. From these volumes, planners can outline the tumor in each phase and combine those outlines into a single envelope called an internal target volume, which ensures the beam covers the tumor throughout its full range of motion.8PubMed. Use of maximum intensity projections (MIP) for target volume generation in 4DCT scans for lung cancer
Different strategies for handling 4DCT data produce different target sizes. One analysis found that the internal target volume approach yielded a target about 6% larger than a conventional plan, while gating, which turns the beam on only during a specific portion of the breathing cycle, and a mid-position strategy both produced smaller targets, roughly 10% smaller.9PubMed. Comparison of different strategies to use four-dimensional computed tomography in treatment planning for lung cancer patients Smaller targets mean less healthy tissue gets irradiated, which matters for reducing side effects, but the technique needs to be reliable enough that the tumor does not escape the beam.
4DCT is also used for tumors in the esophagus, where breathing and swallowing create motion. Researchers found that using just three or four carefully chosen breathing phases, rather than all ten, captured most of the tumor’s movement for patients with regular breathing patterns. Patients with irregular breathing needed either an extra phase or a small added margin to achieve the same coverage.10PubMed. Determination of internal target volume for radiation treatment planning of esophageal cancer by using 4-dimensional computed tomography (4DCT)
Contrast Agents and Metal Implants
You might wonder whether you will receive intravenous contrast during your simulation scan, the same kind of dye used during many diagnostic CTs to make blood vessels and organs stand out. The answer depends on the clinical situation. Contrast can help the oncologist see the tumor boundaries more clearly, but the dense dye changes the CT numbers in the image, which could theoretically throw off dose calculations. In practice, a study of pelvic cancers found that dose differences between plans with and without contrast were less than about 1% and clinically tolerable.11Iranian Journal of Medical Physics. The effect of CT contrast agents on treatment planning and dose calculation in radiation therapy of pelvis cancers However, at higher contrast concentrations in the chest, differences above 3% were observed in a phantom study, which starts to approach clinically relevant territory.12International Journal of Cancer Management. The Effect of Contrast-enhanced Computed Tomography (CT) Scans on the Calculated Dose of Radiotherapy in a Thorax Phantom Most centers either avoid contrast or, when they use it, apply corrections in the planning software to account for its presence.
Metal implants create a different problem. Hip replacements, spinal hardware, and even dental fillings produce bright streaks on CT images that obscure nearby anatomy and corrupt the density data the planning software relies on. These artifacts can make it difficult to outline the tumor and can reduce dose calculation accuracy.13PubMed. Metal artifacts in computed tomography for radiation therapy planning: dosimetric effects and impact of metal artifact reduction Software-based metal artifact reduction algorithms help, and in some cases the team will fuse the CT images with an MRI scan, which is not affected by metal in the same way, to get a clearer picture of the anatomy near the implant.
Fusing CT With Other Imaging
CT simulation provides the density information needed for dose calculation, but it is not always the best way to see the tumor itself. MRI offers superior soft-tissue contrast for brain, prostate, and head-and-neck cancers. PET scans show metabolic activity, which helps distinguish active tumor from scar tissue. These images can be registered, or fused, with the simulation CT so the oncologist draws tumor contours guided by the MRI or PET while the planning software still uses CT density data for its dose math. For example, one group used a fusion of PET, CT, and MRI to plan re-irradiation of recurrent brain tumors, taking advantage of the metabolic information from PET and the anatomic detail from MRI to define the target on a CT-based plan.14Radiation Oncology. Re-irradiation of recurrent glioblastoma multiforme using 11C-methionine PET/CT/MRI image fusion for hypofractionated stereotactic radiotherapy by intensity modulated radiation therapy
A more radical shift is the development of MRI-only simulation workflows, which eliminate the CT scan entirely. In these workflows, MRI images are acquired in the treatment position, and a synthetic CT is generated from them using software algorithms, providing the density information the planning system needs without a separate CT scan. A recent study demonstrated the feasibility of this approach for prostate cancer patients treated on an MRI-equipped treatment machine, validating that dose calculations based on synthetic CT matched those from real CT closely enough for clinical use.15PubMed Central. Development and implementation of an MRI-only simulation, planning, and treatment workflow for prostate radiotherapy using synthetic CT on MR-linac For now, MRI-only workflows are limited to specific tumor sites and centers with the right equipment, but they represent a glimpse of where the field is heading.
How AI Is Changing the Process
One of the most time-consuming parts of the planning process is contouring: manually drawing the tumor and every organ at risk on each CT slice. For a head-and-neck plan with dozens of structures, this can take a physician an hour or more. Deep learning models are now being trained to auto-segment these structures, and the technology has matured rapidly.16PubMed Central. Deep learning for autosegmentation for radiotherapy treatment planning: State-of-the-art and novel perspectives
For prostate cancer, a deep learning model trained on both CT and indirect MRI information produced automatic contours that matched physician-drawn outlines closely, with a qualitative review finding that the contours needed minimal or no manual correction in 96% of cases.17PubMed. Incorporating indirect MRI information in a CT-based deep learning model for prostate auto-segmentation Other models are tackling finer-grained problems. One framework automatically segments individual teeth and jawbone sub-volumes from CT images, which helps clinicians estimate radiation doses to dental structures and assess the risk of a serious late side effect called osteoradionecrosis.18PubMed Central. Image-based mandibular and maxillary parcellation and annotation using computed tomography (IMPACT): a deep learning-based clinical tool for orodental dose estimation and osteoradionecrosis assessment
These tools do not replace the physician; they generate a first draft of the contours that the oncologist reviews and edits. But by handling the repetitive work, they free up time for the physician to focus on the cases where clinical judgment matters most.
The Radiation Dose From the Simulation Scan Itself
Because a simulation CT is optimized for planning accuracy rather than diagnostic efficiency, it tends to deliver more radiation than a comparable diagnostic scan. A study of thoracic scans found that simulation CTs delivered about four times the dose of diagnostic chest CTs, with the gap being largest in thinner patients, largely because simulation protocols did not adjust for patient size the way diagnostic protocols do.19Radiography. Radiation dose differences between thoracic radiotherapy planning CT and thoracic diagnostic CT scans A separate analysis of abdominopelvic scans confirmed the same pattern: cancer incidence risk estimates were higher for simulation scans than for diagnostic scans, although the absolute numbers remain small compared with the therapeutic doses delivered during treatment itself.20PubMed. Radiation dose and cancer risks from radiation exposure during abdominopelvic computed tomography (CT) scans: comparison of diagnostic and radiotherapy treatment planning CT scans
Centers are working to close this gap. Establishing reference dose levels specific to simulation scans, rather than borrowing thresholds from diagnostic radiology, allows departments to optimize their protocols. One center that adopted this approach reported dose values lower than those published at other sites, without sacrificing the image quality needed for accurate planning.21PubMed Central. Advanced Computational Methods for Radiation Dose Optimization in CT The broader point: the imaging dose from simulation is not zero, but it is a tiny fraction of the treatment dose, and it is being actively managed.
Quality Assurance Behind the Scenes
The accuracy of every simulation depends on hardware that behaves predictably. The flat table top must move in a straight line and stay level; the lasers must point to the right spot; the CT scanner must produce consistent density values. Verifying these details is a routine part of the medical physics team’s quality assurance program.
Even small mechanical imperfections can matter. A study using a precision coordinate-measuring device found that a CT couch top shifted by up to about five millimeters vertically under a patient-representative weight load and about two millimeters laterally, with tilt angles that grew slightly as the table traveled.22PubMed Central. High-accuracy quality control method of CT system couch tops for treatment planning via an advanced 3D coordinate measuring machine Other teams have developed jig-based methods for measuring the angular alignment between the table, the lasers, and the imaging plane so that any drift can be caught and corrected.23PubMed Central. An approach for measuring the spatial orientations of a computed-tomography simulation system These measurements are invisible to you as a patient, but they are what keeps the chain of precision from simulation through delivery intact.
Dual-Energy CT and Proton Therapy
For patients receiving proton therapy rather than conventional photon radiation, CT simulation carries an extra layer of complexity. Protons stop inside the body at a depth that depends sensitively on the tissue they pass through, which means even small errors in tissue characterization can shift where the dose lands. Conventional single-energy CT provides a workable but imperfect estimate of how much each tissue type slows a proton beam.
Dual-energy CT, which scans at two different X-ray energies, offers a more physics-based approach to this problem. By combining the two energy datasets, planners can extract tissue properties more directly and reduce the uncertainty that comes from translating a single CT number into a proton stopping power. One clinical implementation showed that dual-energy images reduced noise and improved stopping-power predictions compared with conventional single-energy scans, making the approach ready for routine use.24PubMed. Clinical Implementation of Dual-energy CT for Proton Treatment Planning on Pseudo-monoenergetic CT scans A learning-based method applied to dual-energy data achieved average prediction errors under 3% across the whole body volume, and held up well even when extra image noise was introduced, whereas a traditional physics-based method degraded under the same noisy conditions.25PubMed Central. Learning-Based Stopping Power Mapping on Dual-Energy CT for Proton Radiation Therapy As proton therapy centers multiply, dual-energy simulation scans are likely to become standard.
CT Simulation for Children
Pediatric radiation therapy brings a distinct set of challenges to simulation. Children are smaller, making immobilization trickier. Young children may not be able to hold still for the scan or for daily treatments, which sometimes means sedation or general anesthesia is needed. And because children have decades of life ahead of them, the imaging dose from simulation, small as it is, matters more in terms of long-term risk. Guidelines from the Children’s Oncology Group emphasize that heightened attention to imaging doses is necessary to minimize toxicity in survivors, and that many of these challenges must be addressed specifically during simulation.26PubMed Central. Guidelines for Pediatric Radiotherapy Simulation: A Report From the Children’s Oncology Group Radiation Oncology Discipline
Some centers have developed creative approaches to help children cooperate without sedation. One program redesigned the simulation and treatment experience as a space-themed adventure, complete with ambient lighting and narrative storytelling, to reduce fear and build familiarity. Among the most vulnerable group of four- to six-year-olds, a fifth of the children who went through the program were able to transition to fully conscious, un-sedated treatment courses without compromising positioning accuracy.27PubMed Central. The Mission of Brave: a person-centred educational program to transform paediatric radiotherapy into an empowering adventure Avoiding sedation is not just about convenience; it removes the risks of anesthesia and simplifies scheduling for families who may be traveling for treatment.
What to Expect Emotionally
CT simulation is often the first time a cancer patient enters the radiation department, and it can be anxiety-provoking. You are lying on an unfamiliar table, sometimes wearing a mask that restricts movement, surrounded by equipment that looks intimidating. A prospective trial tested whether giving patients a detailed, one-on-one procedure announcement before simulation would reduce anxiety compared with standard information. Patients who received the extra briefing were significantly better informed about things like positioning and skin marks, yet their anxiety scores were not meaningfully lower than those of the control group.28PubMed Central. Anxiety during Radiation Therapy: A Prospective Randomized Controlled Trial Evaluating a Specific One-on-One Procedure Announcement Provided by a Radiation Therapist The encouraging finding from that study was that anxiety dropped substantially for everyone over the course of treatment, regardless of how much information they were given up front. The unfamiliarity of the first visit is the hard part; by the second or third session, the routine feels ordinary.
If you are claustrophobic, mention it ahead of time. The team can often adjust immobilization choices, provide calming strategies, or in some cases prescribe a mild anxiolytic for the simulation appointment. Knowing what the appointment involves, that it is not painful and typically lasts well under an hour, can itself take some of the edge off.