How Long Does a CT Simulation Take for Radiation?

Most CT simulation appointments for radiation therapy last somewhere between 15 and 45 minutes, though straightforward cases can be shorter and complex setups can push past an hour. The CT scan itself, where you lie still while the machine captures images, takes only a few minutes. Everything else that fills the appointment, positioning your body, fitting or making immobilization devices, applying skin marks, and sometimes practicing breathing techniques, is what determines whether you’re in and out quickly or spending the better part of an hour on the table.

What Actually Takes Up the Time

A CT simulation is not just a scan. It’s a rehearsal for every treatment session that follows, so your radiation therapy team needs to get everything precisely right the first time. The appointment breaks down into several distinct steps, and each one takes a chunk of time depending on where on your body the treatment will be aimed.

When researchers compared different simulation methods for field placement, CT-based simulation averaged about 6 minutes of in-room time, while older fluoroscopy-based simulation took roughly 15 minutes and surface-marking-based planning took about 44 minutes.¹ Those numbers reflect the imaging portion, not the full appointment. On top of that imaging time, you have:

• Positioning: The therapist will adjust your body into the exact position you’ll hold for every treatment session. For a head-and-neck case, that might mean snapping a custom mask over your face. For pelvis or abdomen, it could involve knee and foot supports, arm boards, or vacuum-molded cushions.
• Immobilization device fitting: If you need a thermoplastic mask (head and neck) or a body mold, that device is made or adjusted during the simulation. Some devices are pre-made, but many are shaped to your body on the spot.
• Skin marking and tattoos: After the scan, small reference marks or permanent tattoo dots are placed on your skin so therapists can line you up the same way every day. For prostate cancer, for instance, marks typically go on four locations, including lines connecting anatomical landmarks on the pelvis.²
• Verification imaging: The therapist may take extra scout images to confirm the setup before you leave.

External fiducial markers are placed with reference to the treatment center point, called the isocenter, during the simulation scan.³ These markers serve as alignment references throughout your entire course of treatment, which is why the team takes its time getting them right.

How the Treatment Site Changes the Timeline

Not all CT simulations take the same amount of time, and the biggest variable is the part of the body being treated. A brain simulation using a pre-made thermoplastic mask and a quick scan can be done in 15 to 20 minutes. A prostate or gynecologic simulation where the team needs to verify bladder and bowel filling, position you in a body mold, and apply multiple skin marks will run closer to 30 to 45 minutes. Lung and liver simulations often take the longest because they may require 4D-CT imaging or breath-hold training (both discussed below), easily adding 15 to 30 minutes to the session.

Head-and-neck cases have their own time consideration: the mask. Thermoplastic masks are heated in warm water, draped over your face and shoulders, and then left to cool and harden. While the hardening happens quickly, the process of fitting the mask, checking that you can breathe comfortably, adjusting the angle, and letting it set adds time that a chest or pelvis simulation doesn’t require. Some patients find the mask uncomfortable, which can lead to refitting and further delays.

Breathing Techniques and 4D Scanning

If your tumor is in the chest or upper abdomen, it moves every time you breathe. Radiation teams handle this by using a technique called 4D-CT, which captures your breathing cycle in multiple phases so that the tumor’s full range of motion is mapped. In a typical 4D-CT simulation, two sets of images are acquired: first a standard scan under free breathing, and then a second scan while a respiratory monitoring system tracks your breathing cycle.⁴ The images from that second scan are sorted into ten different breathing phases and reconstructed to show where the tumor sits at each point in the cycle.

For left-sided breast cancer, the team may also coach you through a deep inspiration breath-hold technique, where you take a deep breath and hold it so the heart drops away from the chest wall. This protects the heart during treatment but requires practice. A study of patients receiving this technique found that those who received coaching from an oncology nurse at least a week before their CT simulation had shorter setup times throughout their treatment course compared to those who weren’t coached beforehand.⁵ If your center offers pre-simulation coaching for breath hold, it’s worth taking them up on it, as learning the technique at home before your appointment saves real time on the table.

When IV Contrast Is Needed

Some tumors are hard to distinguish from surrounding tissue on a plain CT scan. Liver tumors are a common example. Researchers have developed tumor-specific protocols using timed intravenous contrast injections during 4D-CT to improve the visibility of liver metastases, cholangiocarcinomas, and hepatocellular carcinomas during simulation.⁶ Different tumor types light up best at different phases of contrast enhancement, so the timing of the injection relative to the scan has to be precise.

If your simulation involves IV contrast, expect an extra 10 to 20 minutes for the IV placement, the injection itself, and the waiting period for the contrast to reach the right phase. You’ll also need to follow preparation instructions from your center, which may include fasting or adjusting certain medications. On occasion, both a non-contrast and a contrast-enhanced scan are performed in the same session, as is common in liver or upper-abdominal cases, which doubles the actual scanning time.

Bladder and Bowel Prep for Pelvic Treatments

For prostate, bladder, cervical, and rectal cancers, how full your bladder and bowel are at the time of simulation matters a great deal. The team will give you specific instructions, typically something like drinking a set amount of water a certain time before your appointment and emptying your bowels beforehand. The goal is to have your internal organs in a reproducible state that can be matched at every treatment session.

Getting this wrong doesn’t just affect the quality of the simulation; it can haunt your entire treatment course. In a study of over 200 prostate cancer patients, those who showed up with an overly full bladder at simulation had longer daily treatment sessions, were removed from the treatment table more frequently due to bladder changes, and reported more worry and frustration by the end of their treatment.⁷ The good news from the same study: patients who were coached to correct their bladder filling successfully did so and had shorter treatment times going forward. The simulation sets the template for everything after it, so showing up with the right bladder and bowel state on that first day is surprisingly important.

CT Simulation for Children

Pediatric CT simulations follow the same basic process as adult ones, but with a major added consideration: young children generally cannot lie still for the time required. According to a survey of radiation oncology physicians, the median age at which children no longer needed anesthesia for radiation treatment was about 6 years old, though some centers treated children without anesthesia from as young as 3 or 4.⁸

When anesthesia is used, roughly half of surveyed physicians estimated it added 46 to 60 minutes to each day’s visit to the radiation oncology department, and another quarter estimated it added more than 60 minutes.⁸ That extra time includes induction, monitoring during the scan, and recovery afterward. For the simulation appointment specifically, the team must coordinate the anesthesiologist’s schedule, which can mean the child spends considerably longer in the department than an adult patient would.

Some pediatric centers have worked to reduce anesthesia use through interventions like having a dedicated pediatric nurse prepare the child, providing audiovisual distraction such as a television during the procedure, and allowing two-way audio communication with a caregiver during scans.⁹ Even so, if your child needs general anesthesia, plan for the simulation day to take significantly longer than the numbers quoted for adults. A review of anesthesia during pediatric radiation at one center found no complications among 80 CT simulations, so the procedure itself is considered very safe even when anesthesia is required.¹⁰

Patient Factors That Can Extend the Appointment

Even for the same treatment site, some patients need more time than others. A systematic review on accelerated radiotherapy pathways flagged several patient-related factors that affect how smoothly a simulation goes: physical performance status, adequacy of pain control, psychological factors like anxiety or claustrophobia, and the ability to tolerate immobilization or prolonged imaging.¹¹

If you have significant back pain or mobility issues, getting into position and staying there may take longer, and the therapist will likely need extra time to find a comfortable setup that’s also reproducible. If you’re anxious about enclosed spaces, the CT bore itself can be stressful. Unlike an MRI tube, a CT scanner is open at both ends and the scan is fast, so claustrophobia is usually manageable, but if you anticipate trouble, mention it to your team in advance so they can plan accordingly.

What Happens After You Leave the Room

Your simulation appointment is just the beginning of the planning process. After you leave, the images are transferred to a treatment planning system where your radiation oncologist draws contours around the tumor and surrounding organs, and a medical physicist or dosimetrist designs a plan that delivers the prescribed dose while sparing healthy tissue. This planning phase is what takes the most calendar time before your first treatment.

At one large cancer center, the typical gap from CT simulation to the start of treatment was about two weeks before the institution implemented a policy to compress that timeline.¹² After imposing a five-day target, the average time between simulation and approval of the physician’s contours dropped to roughly four days, leaving about one day for the physics team to finalize the treatment plan.¹² So while your appointment in the simulation room takes under an hour, the behind-the-scenes planning work that depends on those images takes days. Most patients begin treatment somewhere between three days and two weeks after their simulation, depending on the complexity of the plan and the center’s workflow.

Re-Simulation and When You Might Need a Second One

Sometimes a single simulation isn’t enough. If your anatomy changes significantly during treatment, if weight loss alters the fit of your immobilization device, or if the tumor shrinks enough to justify re-planning, your team may schedule a re-simulation. This is essentially the same process all over again: positioning, scanning, new marks, new planning.

Re-simulations are more common in longer treatment courses (six or seven weeks of head-and-neck radiation, for instance) and when adaptive treatment approaches are being used. The logistical burden of re-simulation is one reason researchers are exploring alternatives. One group developed a system using computer-milled polystyrene molds to replace damaged or ill-fitting immobilization devices without requiring a full re-simulation visit. Their replacement molds were delivered in as little as four hours and at a fraction of the cost compared with the standard re-simulation process, which typically took a week or more and cost over $2,000.¹³

AI and Faster Simulations on the Horizon

One of the most time-consuming parts of the simulation process, positioning the patient correctly and centering the scan, may soon get faster thanks to artificial intelligence. AI-based automatic positioning and centering in CT imaging has been shown to reduce total positioning time by about 28% compared with manual positioning.¹⁴ For a step that currently requires a therapist to manually align your body to laser references and then verify the position with scout images, shaving off nearly a third of that time is meaningful, especially for patients with pain or mobility issues who struggle to hold still.

Surface-guided radiation therapy is another technology reshaping simulation. Instead of relying entirely on skin tattoos and lasers, surface-guidance systems use cameras to map the contour of your body in three dimensions and compare it to the reference shape from your simulation. Some centers are already moving toward “tattoo-free” setups for certain treatment sites, which eliminates the marking step from the simulation entirely and modestly shortens the appointment.

Radiation Dose from the Simulation Scan Itself

Patients occasionally wonder whether the CT simulation exposes them to meaningful radiation before treatment even starts. The short answer is that planning CT scans deliver a higher dose than a typical diagnostic CT, but the amount is small compared to the therapeutic doses that follow. A study comparing thoracic planning CTs with standard diagnostic chest CTs found that the planning scans delivered roughly four times the radiation dose on average.¹⁵ The ratio was even higher in patients with low body mass, where the planning CT delivered about six times the dose of the diagnostic scan.

The reason planning CTs run hotter is deliberate: higher image quality is needed for accurate tumor delineation and dose calculations. A separate study evaluating abdominopelvic scans confirmed that cancer incidence and mortality risks, while small in absolute terms, were greater for planning CTs than for diagnostic CTs.¹⁶ Still, the dose from even a high-quality planning CT is a tiny fraction of what the tumor will receive during treatment, so this is generally considered an acceptable trade-off. If you’re concerned, ask your team whether dose-reduction protocols are available for your specific scan, as some newer scanners offer iterative reconstruction techniques that maintain image quality at lower doses.