Physicist's Notes · Fixed Laser Positioning
Three QA bands, four drift modes,and the linac-facility QA 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 facility-QA dossier reviewed at the annual AERB linac-facility inspection.
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 (linac-facility scope; the laser device is a CDSCO-imported passive accessory).
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.
Linac-facility QA dossier (reviewed at AERB facility inspection)
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