Stewart Goulding, managing director of high torque DC motor supplier Electro Mechanical Systems (EMS) explains how multileaf collimators (MLC) deliver accuracy in radiotherapy.
Provided to more than a quarter of cancer patients, today’s targeted radiotherapy has come a long way from its experimental, and Nobel Prize winning, origins. When administering the treatment, shaping of the beam is an important way of minimising the absorbed radiation in healthy tissue and delivering an accurate dose.
The X-ray was first discovered in 1895, when German physicist Wilhelm Röntgen took an image of his wife’s wedding ring on her hand. He had discovered a form of electromagnetic radiation, called X-rays, that allowed him to take images of body structures. The discovery led him to win the inaugural Nobel Prize in Physics in 1901.
The X-ray also laid the groundwork for the development of radiotherapy, which uses radiation to reduce the symptoms of cancer or, in some cases, cure it completely.
A targeted approach
Until a few years ago, delivering precision during radiotherapy was challenging. Particularly when targeting areas such as the head and neck, doctors had to proceed with caution and deliver lower doses of radiation to protect areas like the spinal cord. Delivering too much could impact healthy areas surrounding the tumour, too little could render the treatment ineffective.
Thankfully, several technologies have improved the precision of radiotherapy treatment. A widely used form of treatment today is intensity-modulated radiation therapy (IMRT). It uses multiple small photon or proton beams of varying intensities to irradiate a tumour, delivering precise control of individual rays within beams of radiation to enable better targeting, especially for tumours with complex shapes.
With IMRT, or any other form of radiotherapy, shaping of the beam is of extreme importance. It must target the treatment area without damaging any healthy tissue. Conventional collimator jaws are often used for shaping a rectangular treatment field but, as usually the treatment volume is not rectangular, additional shaping is required. One option is to use a Multi Leaf Collimator (MLC).
An MLC is a device that’s affixed onto the collimator of the linear accelerator system, which delivers radiation to the treatment area. It contains several sets of metallic “leaves” that, when opened and closed, can shape the beam of radiation as it exits the linear accelerator. Using an MLC to precisely shape the beams from multiple angles makes it possible to deliver a radiation dose that closely matches the volume of the tumour. The MLC also facilitates IMRT by using its adjustable leaves to shape the beam and by controlling exposure times, clinicians can deliver a different dose to different parts of the tumour.
Consequently, technicians can administer a higher dose to most aggressive areas of the tumour, and a lower dose where the beam risks passing through healthy tissue areas.
Delivering accuracy
To deliver the accuracy and control an MLC requires, DC motors move the leaves in and out of place. Motors need to be designed for durability, so they can withstand continual use in a busy clinic. An MLC can need over 100 motors packed into a small space, and so motors must be lightweight and capable of delivering high torque in a small package.
Handling high torque and speed is essential if the motors are to manage the quick, accurate shape changes an MLC delivers. Motor torque must be at a level where the leaves can operate in a tight formation and must account for the friction that occurs when the leaves brush against each other. Powerful rare earth magnets inside the motors can help achieve this high torque. To maintain motor control and adjust motor speed, when necessary, it may also be wise to attach an encoder to each motor.
EMS supplies motors that are capable to delivering such accuracy. For instance, FAULHABER drive systems are low noise, low weight and provide superior precision and reliability, making them ideal for applications such as in MLCs.
The FAULHABER SR series’ compact size means a large number of the motors can fit into the MLC. With no cogging torque and precise speed control, the SR series supports steady yet high-powered MLC operations and torque can be further increased by implementing a planetary gearhead.
Since the discovery of the X-ray, radiotherapy technology has come a long way. Today, thanks to the precision and accuracy an MLC can deliver, patients can benefit from a better controlled, more effective form of treatment.
