A new linear accelerator for calibrating the energy doses delivered during radiotherapy has been unveiled at the National Physical Laboratory (NPL) in the UK.

Costing £1.5m, the new accelerator (or linac) will be used to calibrate radiation dosimetry equipment used in hospitals in the UK and Ireland. The linac replaces an accelerator installed in 1974, and will allow much quicker and more accurate equipment calibration than possible before.

Quality assurance plays a fundamental role in radiation treatment of cancer: while modern techniques offer the ability to deliver precise doses of radiation to tumour tissue, this advantage is lost if the equipment is not stable and accurate. Regular and precise calibration of radiotherapy apparatus is thus an essential procedure for hospitals.

The new clinical linac — officially launched at NPL last week — is scheduled to provide its first calibration services early next year. The custom-designed system will enable NPL to calibrate the full range of energies currently in therapeutic use in the UK, as well as characterize newer techniques such as intensity-modulated radiotherapy (IMRT) and image-guided treatments.

"I am absolutely convinced that this facility will play an important part in ensuring radiotherapy in this country is of the highest quality and contributes to the overall fight against cancer," Mike Richards, national clinical director for cancer at the UK's Department of Health, told the assembled crowd at the formal opening ceremony.

Absolute standard

As the UK's National Measurement Institute, NPL is tasked with developing, maintaining and disseminating the UK's primary standards of absorbed dose (as well as locking these into international standards). Radiotherapy facilities throughout the UK and Ireland (and potentially further afield) can then send their secondary standards — such as ionization chambers or other newer dosimetry devices — for calibration against the primary standards.

Such a service is not a new venture for NPL, which has been offering radiation dosimetry calibration since 1969. However, the existing linac was installed in 1974 (and was second-hand then) and is reaching the end of its operational life. In addition, while the old linac offered calibration using beam qualities "similar to those used for photon radiotherapy", the new system will provide calibration using identical set-ups to those employed clinically to irradiate cancer patients.

The machine in question is an Elekta Synergy digital linac, manufactured not too far from NPL at Elekta's Crawley facility. The Swedish medical technology vendor designed a one-off system for NPL that can deliver seven X-ray energies (as opposed to the two or three usually used in hospital linacs), plus up to ten electron-beam energies.

"Instead of spending two to three hours coaxing beams out of a 40-year-old linac, we can now just press a button and a stable beam comes out," said Martyn Sené, interim managing director of NPL. "This streamlined quality assurance maximizes the availability of radiotherapy facilities without compromising treatment quality."

Crucially, the Elekta Synergy enables dosimetry of the small fields and composite fields employed in advanced modalities such as stereotactic radiosurgery and IMRT. The machine also comes with a range of image-guidance facilities, including iViewGT portal imaging and Synergy XVI 3D X-ray volumetric imaging, and NPL plans to support research into new procedures for using such imaging capabilities more accurately.

Work in progress

NPL researchers are now pulling out the stops to test and commission the new equipment. Current work includes performing Monte Carlo modelling of beams from the old and new linacs to compare the standards operating in the two facilities. To date, three X-ray energies have been characterized (6, 10 and 15 MV photon beams), plus nine electron beams, enabling NPL to deliver first batch of calibration services early next year.

The first stage, scheduled for spring 2009, will be the transfer of existing reference dosimetry services (using the abovementioned X-ray beams) into the new facility. Further down the line, NPL will commission the remaining beams (4, 8, 18 and 25 MV) around June of next year and begin offering calibration of small and composite fields. The new facility will also be employed for training in dosimetry techniques, with courses scheduled to start next summer.

As well as offering calibration services, NPL is putting a strong emphasis on the R&D capabilities offered by the advanced linac. "We plan to exploit the new features on this facility to support radiotherapy developments," said Sené. "We want to ensure that all radiotherapy facilities can take advantage of the latest technologies such as IMRT or IGRT that enable delivery of dose exactly where it's needed."

Having been the first to come up with ideas for tomotherapy standards, NPL is focusing much research effort on developing new techniques and recommendations for reference dosimetry of non-standard fields. "We are building a calorimeter to perform absorbed-dose calorimetry on small-field treatments like IMRT, tomotherapy and the Gamma Knife," said Mark Bailey, senior research scientist at NPL.