A beam-shaping multileaf collimator from Varian Medical Systems driven by more than 100 motors from Maxon Motor directs radiation on tumours precisely while protecting the surrounding healthy tissue.
Until a few years ago, head and neck tumours could not be safely bombarded with radiation without the possibility of damaging organs. Doctors were forced to keep radiation doses low in order to prevent collateral damage to the patient, a situation that also prevented effective treatment.
Hospitals needed a machine that combined superior technology with an efficient, reliable process for targeting tumours while sparing normal healthy tissues.
The Healing Capabilities
Cancer treatment technology from Varian Medical Systems is focused on conforming a radiation dose very tightly to a small or irregularly shaped target. These systems have the ability to minimise hot spots, improve target dose homogeneity, and shape dose delivery around critical structures with precision.
These systems have been very effective in treating tumours very close to sensitive parts such as the optic nerves, spinal columns, glands and organs.
Varian offers the highly advanced high-definition multileaf collimator (MLC), an ultra-fine beam shaping device designed for radiosurgery therapy and consisting of a computer-controlled array of up to 120 parallel, individually adjustable tungsten slats or leaves that can block the path of an X-ray beam.
The HD120 MLC is attached to the head of the medical linear accelerator, which generates the beam. The leaves of the MLC are situated in two parallel rows, and can be moved in and out to create an adjustable aperture through which radiation beams are directed at a patient’s tumour.
The shape of the aperture is dynamically changed as treatment progresses to match the shape of the tumour. By using the MLC to precisely shape the beams from multiple angles, it is possible to deliver a radiation dose that closely matches the three-dimensional volume of the tumour.
The MLC also facilitates intensity-modulated radiation therapy (IMRT). Using the adjustable leaves of an MLC to shape the beam and by controlling exposure times, clinicians can deliver a different dose to different segments of the tumour, modulating the dose intensity across the entire treatment field.
Consequently, the dose can be higher in the most aggressive areas of the tumour and lower in areas where the beam is near or passing through sensitive healthy tissues.
Each side of the Varian collimator is configured with 60 leaves distributed in an 8cm wide central region with 32 x 2.5mm leaves, flanked by two 7cm wide outer regions with 14 x 5.0mm leaves for a total width of 22cm.
Packaging of the leaves in the collimator needed to be engineered carefully as they could jam if the leaves were too tightly together or cause radiation leaks if the leaves were loosely packed.
The leaves move in and out of the collimator through the use of 120 tightly packaged motors by Maxon.
The unique construction of Maxon motors manufactured on specialised machines enables long-term reliability. Their compact size also offered Varian engineers excellent packaging options. Packing 120 motors into a 40cm x 40cm space meant that motor size was almost as critical as motor specifications.
Through the use of Maxon’s RE 8, RE 10 and RE 13 motors, Varian was able to package 120 tungsten leaves into the limited space available.
While the motors are small, the truly impressive miniaturisation occurred with the required encoder feedback.
Each motor has an encoder to close the servo loop that must deliver suitably high resolution while remaining as small as possible in order to meet the packing requirement. Additionally, these encoders must offer radiation-resistance as the high field dose would destroy conventional encoder technology.
Varian’s radiation system is designed to deliver distinct radiation doses to thousands of different segments per tumour. To do this, the multileaf collimator reshapes the opening for radiation quickly and automatically according to preprogrammed data.
Consequently, each motor must handle high torque and high speed to create the programmed shape changes as quickly and accurately as possible. Motor torque needed to be at a level where the plates could operate in a tightly synched formation, even against the high friction that could occur.
Key features of Maxon’s motors used in the collimator
- Powerful rare earth magnets maximise the torque that is available in such a small package
- Patented rhombic moving coil design enables long life, low electrical noise, fast acceleration and high efficiency
- Ironless rotor allows for zero cogging and simple accurate control
- Motors are rated 0.5 watts - 1.5 watts, measure Ø8 x 17mm to Ø13 x 24.6mm and offer a maximum continuous torque up to 1.61 mNm
- Maximum motor efficiency is up to 76% depending on the winding
- Ambient temperature range is from –20ºC to 65ºC
- Different windings are available to match desired speed with available voltage
- Matching gearheads are also available with ratios ranging from 4:1 up to 1024:1 capable of delivering up to 200 mNm of intermittent torque
- Encoders are available down to 8mm in diameter with resolutions up to 100 lines per turn
Their high performance and small size make Maxon’s motors suitable for a variety of critical medical applications such as miniature pumps, surgical devices, air samplers, micro-stages and laser measuring devices.
Maxon Motor Australia is the Australian operation of the worldwide supplier of high-precision drive systems.