Physicist's Notes · Fixed Laser Positioning

Three QA bands, four drift modes, and the AERB inspection dossier.

AAPM TG-142 frames patient-positioning-laser QA in three bands: daily by the radiation therapist, monthly by the physicist, annual full radiation-vs-laser-isocentre verification. The Saxsons own-brand fixed system carries the commissioning baseline through; this post walks the three bands, the four drift modes to watch for, and the six-item AERB dossier.

Three-band QA cadence

Daily, monthly, annual — per AAPM TG-142

Band	Test	Tolerance	Fail action
Daily — radiation therapist	Laser cross-hair vs linac light-field cross-hair at isocentre — visual / spirit-level check	≤ 2 mm (routine); ≤ 1 mm (stereotactic-class rooms)	Re-align operator-side if available; else escalate to physicist
Monthly — physicist	Optical-isocentre rotation test — set couch / gantry to all 4 cardinal positions, verify laser convergence remains within tolerance	≤ 1 mm at isocentre under all gantry / couch rotations	Investigate which axis (left / right wall, ceiling) drifted; re-align that mount
Annual — physicist + RSO	Full laser-vs-radiation-isocentre verification using star-shot or Winston-Lutz; complete documentation review	≤ 1 mm radiation-vs-laser-isocentre coincidence	Schedule Saxsons annual-verification service visit; re-align all three lasers against the radiation isocentre

Source: AAPM TG-142 — Quality Assurance of Medical Accelerators; AAPM TG-179 — CT-simulator + treatment-vault positioning; AERB Safety Code for Radiotherapy Facilities.

Four drift modes

What can go wrong between scheduled checks

Single-axis drift: one laser line moves off isocentre while the other two stay aligned. Cause is typically that one mount has shifted (wall vibration, thermal expansion, accidental impact). Re-align that mount only.
Multi-axis drift: all three lasers shift in the same direction. Cause is typically that the linac isocentre has drifted, not the lasers. Verify the linac before re-aligning the lasers.
Bulk laser-power drop: brightness / visibility of the laser line fades. Diode end-of-life or driver fault. Schedule a Saxsons service visit before the laser becomes unusable in clinical lighting.
Beam-divergence increase: the laser line becomes fan-shaped or fuzzy. Optical-component contamination or mechanical damage to the projection lens. Service-visit required; do not attempt cleaning unsupervised.

AERB inspection dossier

Six items the laser-QA programme contributes

Saxsons commissioning acceptance-test record from the original installation
Daily laser-vs-light-field-cross-hair check log (signed daily by the radiation therapist)
Monthly optical-isocentre rotation test result log (signed monthly by the physicist)
Annual Winston-Lutz / star-shot radiation-vs-laser-isocentre coincidence test record
Saxsons annual-verification service-visit record (re-alignment if performed)
Drift-incident log: any out-of-tolerance reading, the response taken, and the post-response verification result