Multi-leaf collimator

01

Choose leaf count, leaf width and mounting

The IAEA handbook Radiation Oncology Physics (2005) describes MLCs with 20 to 60 pairs of narrow, closely abutting tungsten leaves, each projecting a width of 10 mm or less at the linac isocentre; micro-MLCs project less than 5 mm and are used for irregular fields under 10 cm, such as head and neck, or under 3 cm, such as radiosurgery. An MLC may be integral to the linac head, replacing the upper or lower secondary jaws, or attached to the head and used with both sets of jaws. [1]

Each leaf is driven independently by a miniature DC motor under computer control, positioned to better than 1 mm, with a servomechanism using electronic or optical/video sensing to control and verify leaf positions. Beyond static field shaping, MLCs deliver intensity-modulated radiotherapy (IMRT) by multiple static shaped fields or by dynamic leaf motion. [1]

Editorial calculation: 40 leaf pairs at 10 mm projected width cover 40 cm of field length at isocentre, while 60 pairs at 5 mm cover 30 cm; finer leaves shape small targets more closely but cover a shorter field unless the leaf count rises, so match the leaf layout to the largest and smallest fields you treat. [1]

Give the supplier the linac make and model, mounting (integral or add-on), leaf count and projected width at isocentre, maximum field size, leaf travel and over-travel across the central axis, leaf speed, transmission and inter-leaf leakage limits, position-verification method, IMRT and VMAT capability, treatment planning and record-and-verify interfaces, acceptance test protocol and service terms.

02

Plan acceptance, commissioning and QA

The handbook says MLC acceptance testing must cover mechanical aspects (leaf motion and maximum travel, abutting leaves on and off the central axis, alignment with the secondary collimator axes, positional reproducibility, leaf interlocks and jaw tolerances), radiation aspects (leaf transmission, leakage between leaves and at abutting leaf ends, and penumbra along and across the leaves) and software (field shaping, the link from the treatment planning system and controller function). [1]

Commissioning should confirm that the MLC does not appreciably change open-beam flatness, symmetry, collimator, output and scatter factors and percentage depth dose, and ongoing QA should cover positional accuracy, leaf motion reliability, leakage, interlocks, networking and data transfer. In the handbook's sample linac QA programme based on AAPM TG-40, light and radiation field coincidence has a monthly action level of 2 mm or 1% on a side. [1]

Editorial note: agree the leaf-position test method, tolerances and frequency with your physicist before acceptance, and ask the supplier for the leaf-motor replacement procedure and expected service intervals.

Why positioning accuracy matters is quantified in a 2024 study in Reports of Practical Oncology and Radiotherapy by Katayama and co-authors, who added systematic leaf-position errors of ±0.1 to ±1.0 mm per bank (gap changes of ±0.2 to ±2.0 mm) to single-isocentre radiosurgery plans for multiple brain metastases on a 5 mm-leaf Millennium 120 MLC. Target D98% changed by 13.3% per millimetre of MLC error with dynamic conformal arcs and 15.3% per millimetre with VMAT, and the normal-brain V12Gy changed by 47.1% and 53.2% per millimetre. [4] Editorial calculation: at those rates, a systematic 0.5 mm error shifts target coverage by about 7–8% and the normal-brain V12Gy by about 24–27%, so agree with your physicist whether the 1 mm positioning figure above is tight enough for radiosurgery. [4] [1]

03

Check delivery features and the device class of the linac

Elekta describes the high-definition dynamic radiosurgery (HDRS) sequencing on its Versa HD linac as optimising the jaws across the full 40 cm field, using 1,024 dynamic control points and a 1 mm virtual leaf width, with low leakage through the leaves and jaws while delivering in flattening-filter-free high dose-rate mode; it states that SBRT and SRS treatments can be delivered in a standard 15-minute slot. [2]

In the United States the MLC is a component of a medical charged-particle radiation therapy system, which 21 CFR 892.5050 classifies as Class II, covering component parts and accessories along with planning software and patient supports; an add-on MLC is therefore cleared with or for the linac it fits. [3]

Editorial calculation: a 1 mm virtual leaf width across a 40 cm field gives 400 ÷ 1 = 400 steps across the field, against 400 ÷ 5 = 80 for physical 5 mm leaves, so ask how the virtual width is produced and verified in QA. [2] [1]

04

Two linac makers with 120-leaf MLCs built in

United Imaging (uRT-linac 506c) and Shinva (XHA600E) each list a linac with an integrated 120-leaf MLC; neither sells the MLC separately, so an add-on MLC for another linac is not offered here.

05

Leaves, motors, position sensing, carriage, controller and interlocks

All six parts are named in the IAEA handbook; the third column lists what each maker must state for its own collimator.

06

Leaf pairs, width, accuracy, leakage and QA values for the quotation

Field sizes and techniques come from your clinic. The middle column holds IAEA handbook figures and the Katayama error study, not values from a product data sheet.

07

Send the linac, field and QA data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Linac: [make; model; energies]
MLC: [integral or add-on; leaf pairs; width]
Fields: [largest; smallest; IMRT; radiosurgery]
Performance: [accuracy; leakage; speed]
Integration: [TPS; record-and-verify]
Service: [acceptance tests; QA; spares]

See manufacturer records Send an enquiry through CNFX