Exploration of new treatment modalities offered by high energy (up to 50 MeV) electrons and photons

Abstract

Background and purpose: A number of deep seated tumours are difficult to treat conformally with photon beams mainly due to the almost exponential dose decrease with depth. Materials and methods: In order to improve the conformity of these treatments a number of useful characteristics of high energy (above 20 MeV) electron beams of the MM50 Racetrack Microtron have been systematically investigated and clinically applied. Results: A typical characteristic of electron beams with energies up to 20 MeV is the sharp dose fall-off with depth. At higher energies this effect is less pronounced but may be improved by adding a small fraction of photons with a matching dose gradient (wedge). With this technique, high energy electrons can be used close to sensitive organs down to 17 cm depth. Another physical characteristic of high energy electrons is the sharp penumbra at depths down to 4–5 cm and the possibility to use opposed electron beams in order to enhance the dose centrally or near the centre of a body. Skin sparing by delivering a part of the absorbed dose with photons through the same beam portal as the electrons has also been systematically studied. These characteristics of the high-energy electron beams have been utilised in the optimisation of some clinical treatments. Conclusions: Electron beams in this high energy region give increased possibilities to achieve dose conformity. Enhanced conformity can be obtained especially if electrons and photons are combined to augment some specific characteristics of the electron beams.