Phys. Med. Biol. 45 (2000) 1781–1805. Printed in the UK PII: S0031-9155(00)09776-1 150–250 MeV electron beams in radiation therapy C DesRosiers†, V Moskvin†, A F Bielajew‡ and L Papie˙ z† † Department of Radiation Oncology, Indiana University School of Medicine, 535 Barnhill Drive, Indianapolis, IN 46202, USA ‡ Nuclear Engineering and Radiological Sciences, University of Michigan, 2355 Bonisteel Blvd. Ann Arbor, MI 48109, USA Received 22 November 1999, in final form 11 February 2000 Abstract. High-energy electron beams in the range 150–250 MeV are studied to evaluate the feasibility for radiotherapy. Monte Carlo simulation results from the PENELOPE code are presented and used to determine lateral spread and penetration of these beams. It is shown that the penumbra is comparable to photon beams at depths less than 10 cm and the practical range (R p ) of these beams is greater than 40 cm. The depth dose distribution of electron beams compares favourably with photon beams. Effects caused by nuclear reactions are evaluated, including increased dose due to neutron production and induced radioactivity resulting in an increased relative biological effectiveness (RBE) factor of <1.03. 1. Introduction Megavoltage photon beams are the most widely used radiotherapy modality. Efforts to improve photon beam therapy, i.e. maximize tumour/normal dose ratio (TNR), include three- dimensional treatment planning (3DTP) and dynamic multileaf collimation (DMLC). 3DTP allows visualization of dose distribution from non-coplanar beams. DMLC ideally allows the operator to treat a range of beam orientations with radiation continuously ‘on’, minimizing the time for a given treatment. DMLC, combined with 3DTP, is perhaps the most promising development for photons to achieve truly three-dimensional intensity modulated radiation therapy (IMRT). Current modes of dose delivery incorporating IMRT are designed to maximize TNR by effectively maximizing the number of beam portals to be used in as short a treatment time as possible, increasing the total normal tissue volume irradiated and decreasing the maximum dose in that volume. Although the normal tissue complication probability (NTCP) is multifactorially dependent on the volume of tissue irradiated, the maximum dose in that volume and the irradiated organ (Jackson and Kutcher 1993, Kutcher and Burman 1989, Zaider and Amols 1998, Neal et al 1995, Lennernas et al 1995, Lee et al 1994), this paper assumes the principles upon which IMRT is based and proposes a new treatment design based on the same. The capability of scanning eliminates the need for blocks, multileaf collimators and DMLC, thereby reducing the functional dependence on several moving mechanical parts and increasing the reliability of accurate dose delivery. Additionally, intensity modulation could be achieved more efficiently with a scanned beam by allowing non-coplanar beam arrangements as relative beam angulation is possible with less reliance on mechanical motion (couch, gantry) and allowing the beam to ‘dwell’ longer in areas where additional dose is required. Therefore, a 0031-9155/00/071781+25$30.00 © 2000 IOP Publishing Ltd 1781