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Achieving sub-micron precision for radiotherapy treatment

04 December 2013

Varian Medical Systems is using a total of eleven Heidenhain rotary and linear encoders for the ultra-precise positioning of cancer patients being treated by its state-of-the-art TrueBeam radiotherapy system in use at leading hospitals and private clinics the world over.

Varian leads the world in the development and supply of radiotherapy and radiosurgery equipment and software for the treatment of cancer – indeed, more than 100,000 patients are treated every day on Varian systems. 

The company’s use of Heidenhain encoders instead of potentiometers, which are traditionally used for system positioning, was instigated after the team at Crawley in West Sussex, which is Varian’s UK manufacturing base for around 600 couches a year, began the highly-involved and groundbreaking project to develop the construction of TrueBeam and create the ultimate radiotherapy system.

The encoders – six supplied by Heidenhain (UK) and fitted by Varian engineers at its Crawley site – help ensure that sub-micron accuracies of patient positioning are achieved in a system that integrates imaging, beam delivery and motion management to produce unmatched synchronisation in the precise delivery of beam technology. 

Critically for Varian, the Heidenhain encoders’ EnDat serial interface technology provides secure data transmission of absolute positioning values so, whenever TrueBeam is turned off/on, the system knows its exact true position – which is vital when, for example, patients are being treated. Varian Project Manager Jeff Dowrick, takes up the story:

“To guarantee we had the best patient positioning system also meant we had to have the best absolute rotary encoders available. Concentrating on the system’s hardware, we took a ‘standard’ Varian couch design then fitted the Heidenhain encoders and brushless dc motors [which themselves incorporate Heidenhain MultiTurn absolute encoders], and integrated these with the system’s new Maestro control developed by our colleagues in the States. 

“Heidenhain encoders are used extensively throughout the design. Two Heidenhain encoders are used in the vertical lift motor package that is supplied to us as an assembled unit direct from the motor manufacturer in the USA. These combine with two additional Heidenhain encoders used on the ‘wire strings’ that operate in conjunction with the couch’s vertical/downwards travel.”

Mr Dowrick also explains that the couch has three independent axes of movement – vertical, longitudinal and lateral – and each of these axes uses Heidenhain EQN 1125 encoders. These can run at up to 12,000rpm and provide 8,192 (13 bits) of position value per revolution.

Each axis of movement also has secondary, ‘fail-safe’ encoders which during system operation constantly ‘check’ against the primary ‘master’ encoders for successful non-stop system operation. 

In addition, the new PerfectPitch couch system also aids positioning (and therefore precise beam targeting/delivery) by providing two additional rotational motion axes of pitch and roll. Four Heidenhain encoders are used here. The complete TrueBeam unit is supplied on a substantial four-axis turntable.

The incorporation of the encoders was not as straightforward as conventional system building, adds Mr Dowrick. 

“As well as very clever software integration that marries couch travel and positioning with beam delivery, another major aspect of system design was the need for built-in lead shields for every encoder, to protect them from the effects of radiation.”

The result is a system that features best-in-class engineering principles and intuitive operation. Indeed, TrueBeam is a system of such high-level complexity and sophistication that it illustrates perfectly Varian’s standing as a global leader in the design, development and manufacturing of such medical devices.

But the wheels of Varian’s progress never stop turning because, as project manager, Mr Dowrick’s task now continues not only in the ongoing support of existing products - which demand a ten-year end-of-life technical support and spares infrastructure – but, importantly, also ongoing investigations into cost reductions across the company’s global supply base. 

This, he emphasises, involves investigating savings on existing components and manufacturing practices as much as it does the potential use of substitute materials such as carbon fibres – to further enhance product performance in much the same way as the strategic replacement of potentiometers with Heidenhain encoders has successfully achieved.

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