Simulation and Immobilization - CT Planning and Prep

Key Points

• Simulation images the patient in the actual treatment position and is fundamentally different from a routine diagnostic CT.
• Immobilization must be reproducible and tolerable for the expected treatment duration.
• Contrast should be used when it improves planning information and after appropriate allergy and renal-risk review.
• Site-specific bladder, bowel, fasting, or breathing preparation can reduce day-to-day anatomic variation.
• Respiratory motion should be explicitly assessed for targets that move with breathing.
• Document setup details precisely enough that another therapist can reproduce the position.
• Pregnancy and implanted-device screening are part of safe simulation when relevant.
• Image guidance during treatment should trigger replanning when anatomy no longer matches the simulation reference.

Algorithm

1. Verify treatment site, intent, consent, pathology/imaging, and special safety issues before simulation.
2. Screen for pregnancy, contrast reaction history, renal risk, implants, oxygen needs, pain, and positioning limitations.
3. Select site-specific immobilization and confirm that the patient can tolerate the full treatment position.
4. Apply bladder, bowel, fasting, or respiratory preparation according to the planned technique.
5. Acquire planning CT with adequate range, slice thickness, contrast, and motion data.
6. Document all device indices, body positions, preparation instructions, and respiratory parameters.
7. Register diagnostic MRI/PET or prior radiation imaging with quality assurance for contouring.
8. Use daily or periodic image guidance to confirm reproducibility and re-simulate when anatomy materially changes.

Clinical Synopsis & Reasoning

Simulation is a treatment procedure, not simply a diagnostic CT scan. The imaging must represent the anatomy that will exist during radiation delivery and must include enough longitudinal range and resolution to contour the target and every relevant organ at risk. The patient is positioned for treatment before scanning, using indexed devices such as masks, vacuum cushions, wing boards, knee supports, breast boards, or stereotactic frames depending on the site.

Reproducibility should be balanced against comfort and function. An immobilization setup that produces excellent geometry for five minutes but cannot be tolerated for a 30-minute treatment is not reliable. Pain, dyspnea, kyphosis, limited shoulder motion, obesity, neuropathy, claustrophobia, ostomies, wounds, and implanted devices should be addressed before finalizing position. Markings, tattoos, surface guidance, or mask reference points are then used according to the treatment system.

Contrast is used only when it improves target or organ definition enough to justify risk. Review prior iodinated contrast reactions, renal function when indicated, metformin/other medication considerations according to local policy, and venous access. Oral or rectal contrast is used selectively. For pelvic treatment, bladder filling and rectal emptying protocols can make target and organ position more reproducible. For upper-abdominal and thoracic targets, fasting or meal timing may matter if stomach or bowel filling changes geometry.

Respiratory motion must be characterized for lung, liver, pancreas, adrenal, and selected upper-abdominal targets. Four-dimensional CT, breath hold, gating, abdominal compression, or real-time tracking can be used. The chosen strategy must be practical for the patient and reproducible at the treatment machine. A motion-management technique should not be selected solely because it is technologically available.

Treatment Strategy & Disposition

Before simulation, confirm treatment site, laterality, intent, prescription concept, relevant surgery/pathology, imaging, and signed radiation consent. Screen pregnancy risk in patients for whom pelvic fetal exposure is possible. Review contrast allergy and renal risk, implanted cardiac devices, insulin pumps or continuous glucose monitors when relevant, oxygen needs, pain control, and the patient's ability to lie in the proposed position.

At simulation, construct and index immobilization, confirm patient comfort, mark reference points, and perform the site-specific preparation. Acquire the CT with appropriate slice thickness and scan range. Use IV contrast when it meaningfully improves anatomy and can be administered safely. For respiratory targets, obtain the required motion dataset and verify that the patient can reproduce the breathing maneuver.

After scanning, document every element needed to reproduce the setup: device type and index, arm and leg position, headrest, mask or cushion, bladder/bowel instructions, breath-hold parameters, bolus, wires or markers, and any special assistance. Fuse diagnostic MRI or PET/CT when needed for contouring, but remember that those images were usually obtained in a different position and require registration review.

At the first treatment and throughout the course, image guidance tests whether simulation assumptions remain valid. Weight loss, tumor shrinkage, pleural effusion, bowel or bladder changes, edema, mask looseness, or inability to maintain breath hold can trigger adaptive replanning. Simulation quality therefore continues to matter after the CT date; it establishes the reference against which all later anatomy is judged.

Epidemiology / Risk Factors

• Every external-beam radiation course begins with a simulation or equivalent image-acquisition process.
• Use of image-guided and stereotactic techniques has increased the importance of precise immobilization and motion assessment.
• Four-dimensional CT is widely used for thoracic and upper-abdominal respiratory targets.
• Adaptive radiotherapy is increasingly used when anatomy changes enough that the original simulation no longer represents treatment conditions.

Investigations

Common Prep Elements

Pharmacology

Prognosis / Complications

• Accurate simulation reduces geographic miss and unnecessary normal-tissue irradiation.
• Most simulation-related adverse events are minor, but contrast reactions, falls, pain, and inability to tolerate immobilization can occur.
• Poor reproducibility can lead to larger margins, treatment delays, or replanning.
• Early identification of positioning barriers improves completion and patient experience.

Patient Education / Counseling

• Tell the patient how long simulation and daily treatment positioning will take and why the position may feel different from diagnostic imaging.
• Give written bladder, bowel, fasting, or medication instructions and specify which instructions apply only to simulation versus every treatment.
• Explain breath-hold or motion-management practice in advance when the patient will need active participation.
• Review contrast risks and what symptoms should be reported immediately during injection.
• Encourage patients to report new pain, weight change, swelling, or difficulty reproducing the setup during the treatment course.

Notes

Simulation quality is a human-factors problem as much as an imaging problem. The most sophisticated immobilization device fails if instructions are inconsistent, the patient cannot tolerate it, or therapists cannot reproduce the setup. Written site-specific preparation and indexing reduce avoidable variation.

The planning CT should be considered a reference anatomy, not a promise that anatomy will remain unchanged. Image guidance is the mechanism for detecting when that reference has become stale and when adaptive replanning is safer than continuing the original plan.

References

1. ASTRO and AAPM practice parameters — Link
2. ESTRO teaching course materials on planning — Link