NPL targets treatment

New equipment installed at the National Physical Laboratory could improve UK cancer survival rates by more accurately calibrating the dosage delivered by radiotherapy equipment in hospitals.


New equipment installed at the National Physical Laboratory (NPL) could improve UK cancer survival rates by more accurately calibrating the dosage delivered by radiotherapy equipment in hospitals.

Radiotherapy is one of the most effective treatments for cancer, and the targeted delivery of the precise dose of radiation is essential to ensure effective treatment without damaging the surrounding tissue.

This is especially true for head and neck cancers, where the dose must be within a few per cent of that required. Accurate dosimetry — the measurement of delivered dosage — can improve the outcome of treatment.

Radiotherapy is delivered using a linac, or linear accelerator. The UK primary standards for absorbed dose are defined using a calorimeter to measure the heating effect of the radiation in material.

The NPL has opened a dosimetry facility in Teddington, south-west London, with a Synergy model linac supplied by Swedish company Elekta, which manufactured the equipment at its Crawley facility. It features highly stable beams, accurate doses and the ability to be configured like different hospital linacs, which will speed up the process of calibrating radio- therapy instruments used in UK hospitals.

In NPL’s calibration trials, scientists test the sample on a patient couch to better reflect the many different angles and bursts of X-ray treatments used in hospitals. The Elekta Synergy machine has on-board imaging facilities that indicate where the patient or sample is lying on the table.

Penny Alissy-Roberts, head of the ionising radiation department at the Bureau International des Poids et Mesures, the international bureau of weights and measures, said: ‘Six million patients are treated every year, making a total of 100 million patient treatments. Of these, up to three million patients are estimated to have received the wrong dosage.

There are 200 linacs in the UK which, by 2011, the NPL will be able to calibrate to SI international measurement standard.’

Dr Martyn Sené, NPL interim managing director, said: ‘NPL has been carrying out calibrations on a linac that is 40 years old. The new model responds to the growing need for standards and will allow us to secure the existing support we give in standards, calibration, training and audit. We aim to simulate exactly what happens in radiotherapy facilities throughout the UK — except with no patients.’

The £1.5m equipment was funded through the DIUS National Measurement Service, the Department of Health, and NPL through its parent group Serco. There is additional support from Elekta and from Philips, which supplied its Pinnacle treatment planning system for the facility.

The NPL’s National Measurement System work for this project was defined in a report from 20 radiotherapy departments and via close links with the Royal Surrey County Hospital. As well as updated equipment, the facility will provide training, dosimetry R&D, imaging R&D and radiotherapy.

The Elekta linac was delivered in July, maximum energy was achieved in August followed by a full radiation survey in October, and three photon beams and one electron beam were characterised by the end of the month.

A carriage containing equipment mounted on a clinical couch is now being installed, and Monte Carlo modelling of the beams is under way. This is a realistic simulation of the physical scattering and absorption processes undergone by ionising radiation. From January until March the primary standards of reference dosimetry will be set, then secondary standards will begin.

Prof Mike Richards, national clinical director for cancer at the Department of Health, said: ‘One in three people get cancer; one in four will die of it in the UK. This represents a poor survival rate in the UK compared with the rest of Europe, but this partly reflects the fact we have better cancer data than most other European countries.

Richards said a major 1995 report on cancer care suggested the system was fragmented. The calibration services would help to address this.

The NPL’s facility will calibrate the instruments the hospitals use to measure the dose, while replicating the same conditions in which the patient is treated.

Berenice Baker