Simulation cheat sheet

CT simulation basics

The planning CT that defines the patient's treatment geometry

Done on a large-bore, flat tabletop CT scanner with the patient in the exact treatment position. Reproducibility is the goal: same couch index, same devices, same immobilization every fraction.

Acquire a full scan volume that covers all beams plus setup margin; record couch coordinates, device names, and any non-standard setup in the simulation note.

Positioning & reproducibility

Everything that makes day 1 identical to day 25

Supine is the default; prone or lateral only when dosimetry demands it. Arms at sides or above head — chosen to clear beam paths. Knee and foot rests stop pelvic rotation; head rests are standardized per site.

Document headrest type, arm position, and any supports in the plan record — an undocumented device is a setup error waiting to happen.

Immobilization devices

Thermoplastic masks, wing boards, belly boards, vac-lok bags

Thermoplastic masks (3-point brain, 5-point head & neck) fix the skull to a baseplate; wing boards hold arms overhead reproducibly; belly boards drop the abdomen for prone pelvic work; Vac-Lok bags and Body Fix conform to lower-limb and whole-body setups.

Warm masks to working temperature, mold quickly, and let them fully set before marking; label every device with the patient identifier.

Respiratory motion management

Controlling breathing so the target stays where the plan says it is

4D-CT captures the breathing cycle for ITV contouring; deep-inspiration breath-hold (DIBH) increases heart–lung distance for left breast and thoracic cases; abdominal compression reduces diaphragmatic excursion for liver and lung SBRT; gating or breath-hold (ABC, RPM) locks beam-on to a respiratory phase.

Train the patient on the technique before scanning — a breath-hold learned at simulation is the one reproduced during treatment.

Contrast & scan parameters

IV/oral contrast and slice thickness chosen per target

IV contrast delineates vessels and tumor for thorax, abdomen, and head & neck sims; oral or rectal contrast outlines bowel for GI targets. Slice thickness 1–2 mm for SRS/stereotactic targets, 2–3 mm for head & neck, 3–5 mm for most other sites.

Scan through the full treatment volume plus margins — a truncated scan forces a resim or an expanded field check later.

Isocenter, markings & tattoos

The reference point lasers and skin marks recreate daily

Isocenter is typically the geometric center of the target or a pre-defined anatomical reference; room lasers and permanent skin marks (tattoos) or semi-permanent marks anchor it. Marks are placed on stable anatomy — bony landmarks, not mobile tissue.

Verify marks against the scan geometry before the patient leaves the department; a mis-marked isocenter found at planning costs a return visit.

Fiducials & IGRT alignment

Implanted markers that anchor daily image registration

Gold fiducials (prostate, liver) and surgical clips (breast) are visible on CBCT/kV for soft-tissue-independent matching. Fiducial-based matching is standard where organ motion makes bony alignment unreliable; spacing and orientation matter — plan implant geometry in advance.

Confirm fiducial visibility on the planning scan before the first fraction; document the matching structure used for IGRT in the plan.

MRI fusion & advanced imaging

Registering MRI, PET, and 4D-CT to the planning CT

The planning CT is the geometric reference; MRI (T1/T2/FLAIR) adds soft-tissue target definition for CNS, PET adds metabolic avidity, and 4D-CT adds the motion envelope (ITV). Fusion is done in the planning system with anatomy-based or fiducial-based registration.

Use flat-tabletop MRI protocols or dedicated positioning where possible — scanning in the treatment position makes fusion and transfer far more accurate.

Patient preparation protocols

Bladder, rectum, fasting, and breath-hold prep

Full-bladder / empty-rectum protocols for pelvis sims (prostate, gynecologic, rectal); fasting standardizes stomach and bowel for upper-abdomen sims; breath-hold coaching for DIBH cases. Prep instructions are given before the sim appointment, not at the scanner.

Give written prep instructions and reinforce them at check-in — a non-prepped sim scan can invalidate the plan geometry.

Documentation & data transfer

What leaves the sim room with the patient

The sim note records position, devices, scan parameters, isocenter coordinates, tattoos, and any special instructions; DICOM data (CT, fused series, structures placeholder) moves to the planning system. Photos of setup and devices reduce ambiguity at first treatment.

Photograph the setup from two angles and attach it to the treatment record — the first-day therapist will thank you.