# CT Simulation and Immobilization Strategies

## Overview

CT simulation serves as the foundational step in radiation treatment planning by generating the anatomical dataset used to delineate target volumes and organs at risk. The primary goals of simulation are to establish a reproducible patient position, acquire high-quality volumetric imaging, and define reference coordinates essential for accurate treatment delivery. Immobilization devices play a crucial role in minimizing setup variability, which allows for smaller planning target volume (PTV) margins and enhances dose conformality. Additionally, site-specific considerations regarding positioning, immobilization, and motion management are vital to ensure high-quality treatment outcomes.

## CT Simulation Process

### Pre-Simulation Preparation

Before simulation, it is important to review the treatment intent, target site, planned technique, and any special considerations such as the presence of a pacemaker, claustrophobia, pain, or prior surgery. Coordination with nursing staff is necessary if contrast administration is required; intravenous (IV) contrast is typically used for vessel delineation, while oral contrast aids in gastrointestinal tract visualization. For pelvic sites, adherence to bladder and rectal filling protocols is essential, as consistent organ filling improves reproducibility. In head and neck cases, dental assessment and the fabrication of oral stents or bite blocks may be needed to optimize immobilization and protect normal tissues.

### Patient Positioning

The supine position is the most common during CT simulation because it is comfortable, reproducible, and compatible with most treatment techniques. The prone position is used selectively, for example, in rectal cancer to displace the small bowel, in breast cancer for patients with pendulous breasts, and in some spinal treatments. Arm positioning varies by site: arms up are preferred for thorax, breast, and upper abdomen treatments to allow beam access and reduce arm dose, while arms down are favored for head and neck or brain treatments to minimize shoulder obstruction for lateral beams, as well as for lower extremity and some pelvic setups. Maintaining consistent positioning between simulation and treatment is critical for accurate dose delivery.

### CT Acquisition

CT images are typically acquired with a slice thickness of 2-3 mm for most anatomical sites, while stereotactic treatments such as stereotactic radiosurgery (SRS) or stereotactic body radiation therapy (SBRT) require finer slices of 1-1.5 mm. The scan extent must cover sufficient anatomy above and below the treatment region to allow accurate beam entry and exit dose calculations. Large bore CT scanners with apertures ranging from 70 to 85 cm accommodate immobilization devices and larger patients. A flat tabletop insert is used to mimic the treatment couch, ensuring positional consistency. Laser alignment systems mark the isocenter position, and reference marks such as tattoos or surface markers are placed during simulation to guide treatment setup.

### Contrast Considerations

IV contrast enhances visualization of targets and vessels but can alter CT numbers, potentially affecting dose calculation accuracy. When IV contrast is administered, a non-contrast scan may also be obtained for dose calculation purposes. Oral contrast is useful for delineating gastrointestinal structures in abdominal and pelvic treatments, and rectal contrast is occasionally employed in prostate planning to better define rectal anatomy.

## Immobilization Devices by Anatomic Site

### Head and Neck / Brain

Thermoplastic masks are the most common immobilization devices for head and neck and brain treatments. These masks are custom-molded to the patient’s face, head, and shoulders. Standard open-face masks allow visualization of the patient’s face and reduce claustrophobia, whereas closed-face masks combined with bite blocks provide additional cranial immobilization, particularly for stereotactic radiosurgery (SRS). Masks may be designed as 3-point systems immobilizing the head only or 5-point systems that include the shoulders. Bite blocks or dental stents are used to open the mouth, displacing the hard palate and tongue away from treatment fields in oral cavity and oropharynx cases, and also contribute to head fixation. Frame-based systems, such as the Leksell frame, are invasive stereotactic frames pinned to the skull for single-fraction SRS but are declining in use due to frameless alternatives. Frameless SRS masks are reinforced thermoplastic devices that provide tighter immobilization with 2-3 mm accuracy and are combined with intrafraction image guidance.

### Thorax

For thoracic treatments, wing boards or breast boards are used to position patients with arms up, featuring adjustable arm supports to ensure consistent shoulder and arm positioning, which is important for breast and thoracic radiation. Vacuum cushions (Vac-Lok) create custom-molded body cradles using evacuated beads, offering excellent reproducibility for lung SBRT and other thoracic treatments. Abdominal compression plates limit diaphragmatic excursion, thereby reducing respiratory motion of lower lobe lung and hepatic targets during lung and liver SBRT. Stereotactic body frames are specialized SBRT immobilization devices that integrate abdominal compression and a fiducial coordinate system for precise targeting.

### Abdomen and Pelvis

Knee and ankle supports, such as the Combifix system, standardize leg positioning during pelvic treatments and reduce day-to-day variability in femoral head position and pelvic tilt. Belly boards are prone positioning devices with an aperture that allows abdominal contents to fall away from the treatment field, commonly used in rectal cancer to reduce small bowel dose. Vacuum cushions are also employed to create custom body molds for abdominal SBRT. Consistent bladder and rectal filling protocols serve as a form of internal immobilization; typically, a full bladder and empty rectum are preferred for prostate treatments, while protocols may vary for gynecologic sites.

### Extremities

Custom foam cradles or vacuum cushions are used to position and immobilize extremities. The alpha cradle, made from polyurethane foam, is custom-shaped to the patient’s anatomy for precise immobilization. It is important to ensure that the contralateral extremity is positioned away from the treatment beams to avoid unnecessary radiation exposure.

## 4D-CT and Motion Management

### Respiratory Motion

Respiratory motion, primarily diaphragmatic excursion, causes target displacement ranging from 5 to 25 mm in lower lung, liver, and upper abdominal targets. The greatest motion occurs in the superior-inferior direction, with intermediate motion anterior-posterior and the least motion laterally. Failure to account for respiratory motion can lead to target underdosage and geographic miss, compromising treatment effectiveness.

### 4D-CT Acquisition

Four-dimensional CT, also known as respiratory-correlated CT, is acquired using an external respiratory surrogate such as abdominal bellows, an infrared reflective block on the abdomen, or spirometry. The CT images are sorted into respiratory phase bins, typically ten phases representing 0 to 90% of the respiratory cycle. This process generates datasets for each respiratory phase, an average intensity projection (AvIP), and a maximum intensity projection (MIP). The MIP is used to define the internal target volume (ITV), which encompasses the full range of tumor motion, while the AvIP is used for dose calculation because it best represents the average tissue density along the beam path.

### Motion Management Strategies

Several strategies exist to manage respiratory motion during treatment. Free-breathing with an ITV approach is the simplest, where the ITV covers the entire range of motion, resulting in larger treatment volumes; this is appropriate when motion is less than 10-15 mm. Abdominal compression reduces diaphragmatic excursion from approximately 20 mm to 5-10 mm but may cause patient discomfort. Respiratory gating involves turning the treatment beam on only during a specific phase of respiration, typically end-expiration, which reduces effective motion but increases treatment time. Breath hold techniques, such as deep inspiration breath hold (DIBH), require the patient to hold their breath at a reproducible lung volume, eliminating respiratory motion during beam-on time; this requires patient cooperation and is commonly used for left breast treatments to spare the heart and for lung tumors. Tumor tracking involves real-time tracking of tumor position with beam adjustment, such as CyberKnife Synchrony or multileaf collimator (MLC) tracking; this is the most complex and has limited availability.

### Selection of Motion Management Approach

The choice of motion management depends on the magnitude of tumor motion. For motion less than 5 mm, free-breathing is usually acceptable. When motion ranges from 5 to 10 mm, either the ITV approach or abdominal compression should be considered. For motion between 10 and 15 mm, gating, breath hold, or compression is strongly recommended. When motion exceeds 15 mm, breath hold or tumor tracking is preferred, as free-breathing ITV results in unacceptably large treatment volumes.

| Tumor Motion Amplitude | Recommended Strategy | Notes |
|---|---|---|
| < 5 mm | Free-breathing | ITV approach acceptable |
| 5–10 mm | ITV or abdominal compression | Consider gating if resources available |
| 10–15 mm | Gating, breath hold, or compression | Strongly recommended over free-breathing |
| > 15 mm | Breath hold or tumor tracking | Free-breathing ITV yields unacceptably large volumes |

<image>A photograph-style illustration showing a CT simulation suite with labeled components: large-bore CT scanner with flat tabletop, wall-mounted laser alignment system (sagittal and lateral lasers), a patient lying supine on the table with a thermoplastic head and neck mask in place, and a respiratory monitoring device on the abdomen. Key equipment items are labeled with callout lines.</image>

<image>A four-panel figure showing different immobilization devices: (1) a 5-point thermoplastic head and neck mask on a head support, (2) a breast board with wing attachment and arm supports showing a patient positioned with arms above head, (3) a vacuum cushion (Vac-Lok) body mold shaped around a patient's torso for lung SBRT, and (4) a belly board prone positioning device with the aperture for small bowel displacement highlighted.</image>

<image>A 4D-CT respiratory cycle diagram showing 10 phase bins from 0% (peak inhalation) to 90%, with corresponding axial CT images of a lower lobe lung tumor at each phase. The tumor position shifts inferiorly during inhalation and superiorly during exhalation. Below, the MIP reconstruction shows the tumor envelope (ITV) encompassing all positions, and the AvIP reconstruction shows the averaged tumor appearance used for dose calculation.</image>

## Key Clinical Pearls

Simulation is an active clinical process rather than a passive imaging step; the radiation oncologist should be present or closely involved to verify patient positioning, assess the adequacy of immobilization, and identify any potential issues before the patient leaves the scanner. Precise placement of tattoos or surface markers is critical because even a 3 mm error at simulation propagates through the entire treatment course and cannot be fully corrected by daily image guidance. For head and neck patients, the thermoplastic mask should be made with the shoulders pulled down (retracted) to allow non-coplanar beam access and reduce shoulder interference with lateral fields. Four-dimensional CT is mandatory for any thoracic or upper abdominal target, as failing to account for respiratory motion constitutes a systematic error that no amount of image guidance can correct. Consistent bladder and rectal filling for pelvic treatments is among the most impactful and cost-effective quality measures in radiation therapy; clear patient instructions and verification of compliance at simulation and each treatment fraction are essential. When there is uncertainty about immobilization adequacy, it is better to err on the side of more robust immobilization, since the cost of devices like vacuum cushions is minimal compared to the consequences of geographic miss due to poor setup reproducibility.

## References

- Mutic S et al. "Quality assurance for computed-tomography simulators and the computed-tomography-simulation process: report of the AAPM Radiation Therapy Committee Task Group No. 66." *Med Phys*. 2003;30(10):2762-2792.  
- Keall PJ et al. "The management of respiratory motion in radiation oncology: report of AAPM Task Group 76." *Med Phys*. 2006;33(10):3874-3900.  
- Olsen JR et al. "Practical considerations for image-guided radiation therapy." *Semin Radiat Oncol*. 2018.  
- Brock KK. "Adaptive radiotherapy: moving into the future." *Semin Radiat Oncol*. 2019.