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Varian Millennium MLC

Varian's multi-leaf collimator product line — the leaf-shaping device mounted in the linac treatment head that conforms the radiation field to the planning target volume. The Millennium line spans 120-leaf and 80-leaf configurations (60 leaf pairs and 40 leaf pairs respectively) and was deployed across the long-running Clinac / Trilogy / TrueBeam platforms before the HD120 high-resolution MLC arrived as the premium option on TrueBeam.

The Millennium's leaf design (interlocking double-focused tungsten leaves with rounded ends) and leaf-pair count (120 leaves = 5 mm leaf width at isocenter for the central 20 cm, 10 mm leaf width for the outer leaves) became the de-facto standard for IMRT / VMAT delivery on Varian platforms throughout the 2000s and 2010s. The Millennium remains in service across a substantial installed base — most clinics that haven't upgraded to TrueBeam HD120 still run Millennium MLCs.

Fits

Family / configurations

  • Millennium 120 — 60 leaf pairs (120 leaves total). 5 mm central + 10 mm outer leaf width at isocenter.
  • Millennium 80 — 40 leaf pairs (80 leaves total). 10 mm leaf width.
  • HD120 (separate part) — TrueBeam-era successor with 2.5 mm central leaves for SRS / SBRT.

Failure modes

  • Leaf-positioning drift — individual leaves drift from commanded position over service intervals. Calibration suite restores baseline; cumulative drift past calibration tolerance is the principal end-of-service indicator. See MLC leaf-positioning drift.
  • Motor failure — each leaf has its own drive motor. Motor failure manifests as a stuck leaf, with the system flagging the affected leaf-pair on QA.
  • Encoder / readout failures — leaf position is closed-loop-controlled; encoder failure produces position-readout faults.
  • Leaf-bank carriage mechanical wear over very long service.
  • Calibration loss — typically triggered by service events, sometimes by drift accumulation.

Diagnosis

  • Picket-fence test and other QA patterns — the canonical periodic MLC integrity check.
  • Leaf-position log analysis — each delivered fraction logs leaf positions; drift trends are detectable in retrospective log analysis.
  • MLC QA software in the Varian service stack reports per-leaf health.
  • TG-142 / TG-50 quality-assurance protocols — clinical-physics standard for MLC QA frequency and tolerances.

Replacement path

  • Per-leaf motor swap for individual motor failures.
  • Full-bank service for severe drift or carriage-wear cases.
  • MLC replacement as a major service event — the Millennium leaf bank is integrated into the treatment head, not a quick-swap component.

Field notes

  • Long service life is the rule — the Millennium has been in production for over two decades with mature service depth.
  • Leaf transmission + interleaf leakage characteristics are Millennium-specific dosimetric properties that flow into the treatment-planning system; planning-system leaf-model accuracy is a clinical-physics QA item.
  • Refurb linac due-diligence — MLC condition + most-recent-comprehensive-QA date.

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