The m3 enables the delivery of superior homogeneous dose distributions with a steep dose gradient – an essential requirement for SRS treatments.

The leaves of the m3 are automatically placed around the target for all beam angles with the desired margin. Risk objects may be automatically protected by the leaves, with manual adjustments able to be made at any time.

Reduced irradiation of normal tissue and the homogeneous dose distribution within the PTV contributes to superior and more efficient patient care.

m3 setup

The detachable 'plug and treat' design of the m3 high-resolution multileaf collimator allows fast and easy slide-on attachment to almost any existing linac accessory mount, aided by the storage trolley and with the gantry at the 180° position.

Once m3 is physically mounted to the linear accelerator, the electrical and date connection is immediate.


Integrated safety

The m3 is the only solution fully integrated with Varian accelerators. It communicates with Varian C-Series Clinacs as if it were an internal part of the accelerator, providing superior Clinac safety interlocks, reliability and capabilities such as Dynamic Conformal Arcs and Dynamic IMRT.

Steep dose fall-off - a fundamental requirement for radiosurgery

Multileaf collimators with a leaf thickness of 10 mm are optimized for standard radiotherapy. For stereotactic treatments, a steep dose fall-off is required to protect normal tissue and, in particular, critical structures from high doses.

To accommodate a higher resolution dose delivery, new multileaf collimator designs with 5 mm thick leaves allow the delivery of fractionated stereotactic radiotherapy, but are not generally acceptable for single fraction radiosurgery where special requirements have been defined by the AAPM (1).

	Fig. 1: Dose Volume Histogram for single isocenter m3 plan

The steepness of the dose gradient in radiosurgery is a function of beam geometry and beam penumbra.

Care must be taken to minimize penumbra of the collimator system because the larger the beam penumbra, the shallower the dose fall-off (Figure 1).

For radiosurgery, the recommended limit for dose gradient in the beam penumbra (from 80% to 20%) is greater than or equal to 60%/3 mm (Figure 1).

	Fig. 2*

Most commercially available multileaf collimators have a penumbra specification from 4 to 8 mm. The m3 with its 3 mm-thin leaves has an effective penumbra of less than 3 mm for all SRS field sizes and meets all SRS requirements (Figure 2).

Thus, the dose can be tailored according to the shape of the lesion, while sparing large amounts of normal healthy tissue.

*An irregular field shape formed from diagonal, straight field edges. Measured 80%, 50% and 20% isodose lines are indicated. A sinusoidal variation in the isodose lines occurs due to the stepping effect between leaf ends and is smallest where the field is formed by the 3 mm thin leaves. Tangents between the 80% and 20% isodose lines show an increasing effective penumbra with increasing leaf thickness.

1. AAPM Report No. 54, Stereotactic Radiosurgery, Report of Task Group 42, American Institute of Physics, June 1995
2. Pike, Podgorsak, et. al., 'Dose distributions in radiosurgery' Medical Physics, Mar/Apr 1990
3. Cosgrove, et. al., 'Commissioning of a Micro Multi-leaf Collimator and Planning System for SRS.', Radiotherapy & Oncology, (50) 1999