Varian HD120 Multileaf Collimator
Varian's high-definition multileaf collimator for SRS / SBRT / small-field radiation therapy. 2.5 mm projected leaf width at isocenter on the central 32 leaf pairs (covering a 10 × 10 cm central field); 5 mm outer leaf pairs. The HD120 is the MLC that distinguishes TrueBeam STx and Varian Edge from the standard TrueBeam (which uses the Millennium 120 MLC with 5 mm central leaves). The 2.5 mm leaves enable fine-aperture conformality that the Millennium 120 cannot deliver, specifically for single-fraction radiosurgery targets.
Fits
Specs
- 120 leaf pairs (60 per bank)
- 2.5 mm projected leaf width at isocenter — central 32 leaf pairs
- 5 mm projected leaf width at isocenter — outer leaf pairs
- Central field coverage: 10 × 10 cm at 2.5 mm resolution
- Interdigitating leaf design
Diagnosis
- Leaf binding — mechanical leaf motion failure; affects specific leaves and presents as blocked or slow-traveling leaves in QA testing.
- Position-sensor drift — leaf-position feedback drift; dosimetric impact on IMRT / VMAT plans. Routine MLC QC catches it.
- Leaf motor wear — common on high-utilization SRS / SBRT platforms with many small-aperture plans. Individual leaf-motor replacement is a field service.
- Radiation leakage — inter-leaf leakage measured annually per TG-142; HD120 leakage within tolerance despite fine leaf width.
Replacement
Individual leaf / motor service is a field procedure. Full MLC replacement is a larger service event. End-to-end dosimetric verification after any MLC service is mandatory, particularly for SRS / SBRT clinical programs.