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Linac 6-DoF Treatment Couch

The six-degree-of-freedom (6-DoF) patient couch that has become standard on premium current-generation linacs — adds 3 rotational axes (pitch, roll, yaw) on top of the conventional 3 translational axes (longitudinal, lateral, vertical) for sub-millimeter, sub-degree patient-positioning correction during image-guided treatment delivery. The 6-DoF couch is the patient-side complement to the linac kV imaging / CBCT system in the IGRT workflow: kV/CBCT acquires the patient's actual setup position, the planning system computes the registration shift to the planned position, and the 6-DoF couch executes that correction including small rotational adjustments that conventional 3-DoF couches cannot deliver.

For SBRT / SRS programs where 1-mm geometric accuracy translates directly into clinical outcomes, the 6-DoF couch is essentially mandatory. Conventional 3-DoF couches force the planning team to either accept the rotational error or re-image / re-position the patient until the rotational components are minimized — workflow inefficient and not always achievable.

The two principal commercial products are the HexaPOD (formerly Medical Intelligence, now Elekta) and Varian PerfectPitch — both retrofittable to existing linac couches and both shipped as standard on current-generation premium systems.

Fits

6-DoF couches are typically retrofit to or factory-shipped on:

Distinctive technology

  • Pitch / roll / yaw rotation — added to longitudinal / lateral / vertical translation. All six axes motorized and closed-loop position-controlled.
  • Sub-millimeter / sub-degree precision — encoder + drive-system precision required for IGRT accuracy.
  • Carbon-fiber treatment-top — radiolucent for kV imaging during treatment without table-top attenuation artifacts.
  • Integration with the imaging-and-planning system — the planning system computes the 6-DoF correction; the couch executes it as a coordinated motion.
  • Patient-immobilization compatibility — frame mounts, mask-clamping accessories for cranial SRS, body-fix systems for SBRT.

Failure modes

  • Drive-motor wear on individual axes — manifests as slow or imprecise motion on the affected axis. Most-frequently the rotational axes (less-loaded by patient weight than the vertical translation but cumulatively moved many times per fraction).
  • Encoder drift — see patient-table positioning encoder drift — particularly clinically-impactful on linac couches given the SBRT / SRS tolerance.
  • End-of-life mechanical wear — bearings, gear-trains, and carriage components on the rotational axes after thousands of cumulative-fraction cycles.
  • Position-fault interlocks — the system flags position-feedback discrepancies during patient setup and refuses to proceed.
  • Couch-vertical end-stop wear — the vertical-translation system carries the patient's full weight; mechanical-stop wear affects positioning accuracy at the upper / lower extremes.
  • Carbon-fiber top damage — impact damage during patient transfer or accessory handling.

Diagnosis

  • Daily Winston-Lutz + IGRT QA — TG-142 protocol exercises 6-DoF accuracy.
  • End-to-end testing — phantom-based test that exercises the full image-guidance + couch-correction workflow.
  • Service-log motion-event review.
  • Per-axis encoder calibration trending.

Replacement path

  • Per-axis motor / encoder swap for component-level failures.
  • Drive-train service for cumulative-mechanical-wear cases.
  • Carbon-fiber top swap for surface-damage events.
  • Calibration suite post-swap: per-axis calibration, isocentricity, end-to-end IGRT testing.

Field notes

  • 6-DoF couch maintenance is a routine line item on premium-IGRT linac programs — annual or semi-annual comprehensive QA + service.
  • HexaPOD / PerfectPitch refurb / retrofit is a meaningful market — older 3-DoF linacs can be retrofitted with 6-DoF couch upgrades to extend their useful clinical life.
  • Refurb-linac due-diligence — couch-axis encoder calibration history + most-recent-IGRT-QA + cumulative-fraction count.
  • MR-linac couches (Elekta Unity / ViewRay MRIdian) are MR-compatible 6-DoF designs — different mechanical implementation but same architectural concept.

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