Saxsons Group — India's trusted nuclear medicine, radiotherapy, oncosurgery, dosimetry and cyclotron supplier since 1986

Physicist's Notes · Moving Laser Positioning

Six-step verification, four drift patterns,annual recalibration cycle.

Motorised position-reproducibility verification protocol — six steps that the physicist runs annually to confirm the moving laser system still arrives at the same coordinates within ≤ 1 mm. This post walks the protocol, the four drift patterns to watch for, and the Saxsons annual recalibration service cycle.

Six-step verification protocol

Annual ≤ 1 mm reproducibility check

  1. 1

    Pick test positions

    Choose at least 5 test positions across the rail travel — extremes + centre + 2 mid-positions.

    Cover the full operational range; drift typically shows at the rail extremes first.

  2. 2

    Drive to position

    Operator enters the coordinates; motorised drive moves the laser carriage to the position; record arrival.

    Time the arrival; slow arrival can indicate worn drive belts or bearing drag.

  3. 3

    Verify cross-hair

    Place a precision target at the expected cross-hair position; measure laser-vs-target offset with a micrometre or precision rule.

    Use a thermally stable target (Invar or aluminium); plastic targets warp under ambient temperature changes.

  4. 4

    Drive away and return

    Drive the laser away to a different position, then return to the original position. Measure the offset again.

    Round-trip reproducibility — a single drive-to test misses the failure mode where drift accumulates on return trips.

  5. 5

    Compare against tolerance

    Offset must be ≤ 1 mm at each test position, for both drive-to and round-trip. Document the worst-case offset.

    The worst-case position is the lifecycle-replacement trigger when it exceeds the tolerance twice in successive annual cycles.

  6. 6

    Document and file

    Record positions, measured offsets, pass/fail and annotate any failed position. Sign and file in the annual QA dossier.

    The linac-facility QA dossier is reviewed at AERB facility inspection; document the protocol so the inspector can verify the result is repeatable. (The laser device itself is a CDSCO-imported passive accessory — not AERB-licensed.)

Four drift patterns

Diagnosing the root cause

  • Single-axis drift on one rail: typically a worn drive belt or shifted encoder coupling on that rail. Saxsons service-visit replaces the belt; recalibrates the encoder.
  • Bilateral lateral drift (both left + right rails drift equally): typically the room reference frame has shifted — recheck the CT-simulator isocentre before assuming the lasers drifted.
  • Sagittal-only drift (ceiling laser drifts, laterals stay): typically the ceiling-rail mount has shifted, often from HVAC vibration. Re-anchor the ceiling rail; recalibrate.
  • Drift increases over the rail length (small at centre, large at extremes): typically encoder-count drift — the motorised drive thinks it has moved further than it actually has. Saxsons service recalibrates the encoder.