The radiation-oncology workflow begins long before the patient ever enters a treatment vault. CT-simulation, treatment planning, and immobilization-device ("mold-room") fabrication are the upstream spaces where each patient's treatment is defined, geometrically locked, and dosimetrically modeled. These rooms carry their own distinct construction, shielding, mechanical, and workflow requirements that are easy to under-scope when a project team is focused on the headline cost of the linear-accelerator (LINAC) vault.

This article covers the planning suite that sits between the clinic and the vault: the CT-simulator ("CT-sim") room, the treatment-planning and dosimetry workspace, the mold/immobilization room, and the closely associated quality-assurance and physics support spaces. It deliberately stays out of the lane of the LINAC vault and maze (covered in its own Article), brachytherapy/sealed-source rooms, and the infusion suite — referencing them only where adjacency matters.

The CT-sim room is a dedicated, oncology-configured CT space — not a borrowed diagnostic scanner

A radiation-oncology CT-simulator is a CT scanner whose purpose is geometric: it captures the patient in the exact position they will be treated in, on a flat-top couch matched to the LINAC, so the planning system can compute dose against true anatomy. Although some programs share a diagnostic CT early in their life, a mature radiation-oncology service plans for a dedicated CT-sim for several reasons that drive design.

Key differentiators from a diagnostic CT room:

• Flat treatment couch top matching the LINAC couch, rather than the curved diagnostic table, so the patient's setup is reproducible between sim and treatment.
• A large-bore gantry (commonly ~80–85 cm aperture versus the typical ~70 cm diagnostic bore) to accommodate immobilization devices, bulky setups (e.g., breast boards, arms-up positions), and larger patients.
• An external/overhead laser positioning system — typically a fixed sagittal/coronal laser set plus movable lasers — used to mark and tattoo (or apply surface-guided fiducials to) the patient's isocenter. Lasers are usually wall- and ceiling-mounted and require coordinated backing, conduit, and alignment provisions.
• 4D-CT and respiratory-gating capability for thoracic/abdominal sites, which adds surface-monitoring cameras, gating hardware, and the associated cabling and ceiling-mount points.
• A larger scan room footprint than diagnostic CT, to give staff room to set up immobilization, position lasers, and work around the bore from all sides.

Because the CT-sim is the geometric reference for everything downstream, floor flatness, structural stability of the slab, and coordination of laser mounting points are unusually consequential. Design intent and equipment selection should be locked early through the vendor's site-planning drawings.

CT-sim shielding is modest but real and must be designed by a qualified physicist

A CT-simulator emits ionizing radiation and therefore requires structural shielding, but at a far lower order of magnitude than a LINAC vault. Where a vault may need multiple feet of concrete (or high-density alternatives) and a maze, a CT-sim is typically shielded with lead-lined gypsum partitions, leaded doors, and leaded view-window glass — comparable in article to a diagnostic CT/imaging room, sized for the higher workload of simulation.

Design principles:

• The shielding calculation is performed by a Qualified Medical Physicist / Qualified Expert and reflects the specific scanner, expected workload, occupancy of adjacent spaces, and applicable methodology (the radiation-protection community generally follows NCRP-type shielding-design methods for medical imaging and therapy installations).
• Shielding extends to all six surfaces as required — walls, floor, and ceiling — because occupied spaces may exist above and below; vertical paths are checked, not assumed.
• The control alcove / operator position is shielded and positioned with a clear sight line to the patient through a leaded window or via in-room cameras.
• Penetrations are detailed and baffled — conduit, ductwork, and cable pass-throughs cannot create unshielded straight-line paths; sleeves and offsets are coordinated with the physicist.
• A radiation-warning signage and door-interlock/illuminated-warning scheme is installed per the AHJ and the state radiation-control program's requirements.

Final shielding drawings should be reviewed and approved by the physicist and, in many jurisdictions, submitted to the state radiation-control program (the AHJ for radiation-producing machines) before installation.