MLC Leaf-Positioning Drift
Gradual deviation of multi-leaf-collimator leaf positions from commanded values — the principal wear-out failure mode on linac MLCs across Varian Millennium, Varian HD120, and Elekta Agility platforms (and equivalent on Accuray / TomoTherapy MLCs). MLC leaves are closed-loop position-controlled, so single-event drifts are corrected automatically; the failure mode is cumulative drift past calibration tolerance, where the leaf-pair's position-readout no longer agrees with reality across the calibration suite.
The clinical consequence depends on the magnitude. Sub-millimeter drift is invisible to most planning models and undetectable on routine QA but progressively shifts dose distributions on highly-modulated VMAT / SRS plans. Multi-millimeter drift is detectable on picket-fence QA and produces visible artifacts on delivered fields. Beyond clinical-physics tolerance bands ([TG-142, TG-50]), the leaf is out-of-service and the MLC bank requires intervention.
Symptoms
- Picket-fence test failure — the canonical periodic QA test that surfaces drift before clinical impact.
- Per-leaf log analysis — modern Varian / Elekta linacs log per-fraction leaf positions, and retrospective analysis of cumulative delivered-vs-planned trajectories surfaces drift trends weeks-to-months before QA failure.
- Conformity-index drift in delivered VMAT / IMRT plans on patient-QA.
- Edge-of-field dose discrepancies detectable on film / array dosimetry.
- Leaf-positioning fault interlocks at the hard end — the system flags the leaf and refuses to deliver until calibration is restored.
Causes
- Motor wear — accumulated motion cycles wear the per-leaf drive motors.
- Encoder calibration drift — the position-feedback encoder's reference shifts.
- Mechanical wear in the leaf-bank carriage / guide-rail interfaces.
- Thermal effects — leaf-bank thermal stability issues during sustained high-duty-cycle delivery (Agility includes active leaf-bank cooling specifically to address this).
- Contamination — debris, lubricant degradation, or particulate ingress into the leaf-bank mechanism.
Diagnosis
- TG-142 / TG-50 quality-assurance protocols — clinical-physics QA standard.
- Picket-fence + dynalog / trajectory-log analysis — Varian and Elekta both expose per-fraction leaf-position logs that enable retrospective drift-detection analysis.
- MLC-vendor service tools — OEM-specific QA software reports per-leaf health.
- Patient-QA results trending — degraded patient-QA on highly-modulated plans precedes formal QA failure.
Affected parts
Operational implications
- Predictable, gradual end-of-service — log-analysis trending typically gives months of warning. Emergency MLC events are rare on well-instrumented modern linacs.
- Refurb-linac due diligence — most-recent-comprehensive-MLC-QA report + cumulative leaf-cycle counts (where exposed) are the principal inspection items.
- Plan-level mitigation — clinical physics teams sometimes adjust planning constraints on aging MLCs to reduce per-leaf cycle-count and slow drift accumulation.
- TG-142 tolerance bands are explicit on what drift magnitude is acceptable for a given delivery technique (IMRT vs SRS vs conventional 3DCRT).
Replacement path
- Per-leaf motor / encoder swap for individual leaf failures.
- Full leaf-bank service for systemic drift.
- MLC-head replacement as the major capital event at end-of-service.