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Completion of the world’s largest accelerator facility for dosimetry in radiation therapy


For the success of a radiation therapy it is also decisive that the radiation dose is applied with sufficient accuracy. This dose is measured as absorbed dose to water in the unit gray (1 Gy = 1J/kg). The absorbed dose to water is the energy imparted to water per unit mass.

The realization and dissemination of the gray is one of PTB’s main tasks. The primary standard measuring device of Germany is a water calorimeter which measures the temperature rise of the water under well-defined irradiation conditions (reference conditions).

To realize the gray for irradiation conditions which are as practice-related as possible, PTB now has two clinical electron accelerators of the type "Elekta Precise Treatment SystemTM" at its disposal. With these accelerators, six reference fields for photon radiation and ten reference fields for electron radiation are available. The energy range extends from 4 MeV to 25 MeV. It completely covers the range occurring in practice.

In its fight against cancer, medicine is constantly developing new irradiation techniques. They generate irregularly shaped irradiation volumes which adapt the irradiated volume with an ever increasing exactness to the form of the tumour in order to treat sensitive organs and healthy tissue with maximum care. Examples are tomotherapy or "intensity-modulated radiation therapy" (abbreviated: IMRT). With its new accelerators, PTB can also generate such irradiation volumes and develop measurement procedures for exact dose determination in these volumes.

An electron linear acclerator - 11 m in length - especially developed for PTB by the company RI Research Instruments GmbH, Bergisch-Gladbach, serves to perform dosimetric basic research. The energy of the accelerated electrons can be varied continuously from 0.5 MeV to 50 MeV. Such a large energy range is nowhere else available for dosimetric research. It has been realized by dividing the accelerator into two sections. Behind the first section, electrons with energies below 10 MeV are extracted from the accelerator.

Accordingly, the research accelerator serves two irradiation set-ups: one for energies from 0.5 MeV to 10 MeV and one for energies from 6 MeV to 50 MeV. At the two irradiation set-ups, the electron beams can both be focussed and widened and, in addition, be moved by means of a fixed point scanning system. If a suitable target is installed at the fixed point, the system can even generate scanned high-energy photon beams.

By means of deflection magnets and diaphragms, quasi monoenergetic electron beams can be generated, with spectral widths down to 4 keV (1 s.d.). For energies above 6 MeV, the electron beam power is so large (up to 1 kW) that the photon radiation fields generated with it reach therapeutically relevant dose rates (4 Gy/min).

The electron current is measured contact-free with toroidal beam current transformers which have been developed at CERN. At present, they are regarded as the most exact measuring instruments for this application. For energy determination, the electron beam is deflected by 180° through a homogeneous magnetic field in spectrometers which have been especially designed for this purpose. The electron energy is obtained from the path diameter and the value of the magnetic field strength.

Now, the intended scientific operation has been started on all three accelerators. Although work is focussed on clinical dosimetry, other subjects are not excluded. Hamburg University, for example, studies at the research accelerator radiation damages in silicon diodes which are used at CERN in high-energy experiments.

Figure : Electron linear accelerator for dosimetry. In the foreground, the electrons reach the final energy of 50 MeV. Blue: Magnets for beam focussing and beam deflection.