The linear-accelerator (LINAC) vault is the most construction-intensive room in any healthcare facility: a heavily shielded, structurally exceptional concrete box entered through a radiation maze. This article covers the architectural and spatial design of the vault and its maze — room geometry, the labyrinth, the door, and the surrounding support layout. The physics of how much concrete is required, and the engineered penetrations through the shell, are treated as a separate Article; here the focus is on designing the room a patient and therapist actually use.
A megavoltage external-beam LINAC produces high-energy photon beams (commonly 6 MV, 10 MV, 15 MV, and up to 18–20 MV on multi-energy machines) and electron beams. The vault exists to contain three distinct radiation sources so that occupied spaces beyond the walls stay below regulatory dose limits:
Because the gantry rotates and the patient table (couch) moves, the isocenter — the fixed point in space the beam rotates around, typically ~100 cm from the source — is the geometric origin of the entire design. The vault is laid out around the isocenter, not around the building grid. A consequence often missed in early planning: the slab, the ceiling, and the walls in the rotational plane are all primary barriers because the beam can be aimed straight down at the floor or straight up at the ceiling.
Shielding design is not an architectural exercise. A qualified medical physicist performs the shielding calculation using the methodology of NCRP Report No. 151 (Structural Shielding Design and Evaluation for Megavoltage X- and Gamma-Ray Radiotherapy Facilities), accounting for the specific machine, workload (weekly dose at 1 m), use factors, occupancy factors of adjacent spaces, and whether the machine is IMRT-heavy (which raises total monitor units and therefore leakage). Architecture follows that calculation — the room is sized and oriented to make the physicist's barriers buildable. Engaging the physicist before the floor plan is fixed is the single most important sequencing decision in vault design.
The defining architectural feature of a conventional vault is the maze (labyrinth) — the angled entry corridor that leads from the vault interior to the entrance door. Its purpose is purely physical: radiation that reaches the door has to scatter off the maze walls one or more times to get there, and each scatter event sharply reduces its energy and intensity. By the time radiation reaches the door, the maze has done most of the shielding work, so the door itself can be far lighter than it would otherwise need to be.
Maze design balances competing demands:
A typical conventional maze uses a single-bend or double-bend "L" or "Z" geometry. Maze walls in the direct scatter path are themselves secondary barriers and are detailed by the physicist. The maze inner wall facing the isocenter often serves double duty as a scatter barrier. A well-designed maze also keeps the line of sight from the door to the isocenter broken, so no occupant in the corridor can see the machine head directly during a beam-on condition.
On constrained sites — urban infill, basement retrofits, replacement vaults inside an existing footprint — the maze can be eliminated and replaced with a direct shielded door: a very heavy laminated door (steel, lead, and on high-energy machines borated polyethylene for neutrons) that does all the work the maze would have done. These doors can weigh several tons and are nearly always automatic and motorized. The tradeoff is well understood: a mazeless vault saves a large amount of floor area but shifts cost and risk to a massive, mechanically complex, failure-prone door that must be interlocked and have a manual-egress fallback. Mazeless designs are also increasingly viable for vendor-supplied modular/pre-engineered vaults. The maze-versus-direct-door decision is made jointly by the physicist, architect, and owner early, because it reshapes the entire suite layout.
Above roughly 10 MV, photonuclear reactions in the machine head, collimators, and high-Z shielding generate neutrons. Neutrons behave differently from photons: they bounce down the maze efficiently and, as they slow (thermalize), produce capture gamma rays. The practical design implications: