## 1 Introduction
Since photon beams of maximal energies in the range of 150 MV to 250 MV have characteristics of depth dose deposition similar to reversed very high-energy (VHE) electron beams, VHE photon beams may be used separately or in combination with VHE electron beams to regulate the depth of dose maximum so that it can coincide with the depth of target volume [8]. Clinical effects of VHE electron beams on cancer therapy cannot be directly verified by measurement, since this beam type with 100 MeV-level energies is not available in small-scale facilities. Therefore, most of the investigations for verifying beam characteristics and dose distributions in specific cancer treatment have been studied by Monte Carlo simulations [8; 9; 10; 11; 12]. Referring to the results of Monte Carlo simulations carried out by [PERSON] _et al._[8], depth-dose distributions from 200 MeV electron beams are illustrated for single-field, parallel opposed two-field, and orthogonal two-field arrangements as well as a 160 MeV proton beam with a Bragg peak at a depth of 15 cm, as shown in Figure 1. In the Monte Carlo simulation study [9] comparing intensity-modulated radiation therapy for prostate treatment with photon, proton, and VHE electron beams, the best conformity to the volume is achieved with proton beams, while target coverage for electron beams is comparable to or sometimes better than that provided by photon beams. Moreover, VHE electron beams provide a significant improvement over photon beams in the dose sparing of the sensitive structures and normal tissues. Monte Carlo simulations [10] of VHE electron beams with various energies have been performed to estimate dose distributions for a simulated lung tumor in comparison to state-of-the-art 6 MV photon beam therapy and a conventional 20 MeV electron beam therapy. This shows that electron energies of 75-100 MeV can produce superior quality compared to the photon beam and lower electron energies. It is also demonstrated that an 8 cc lung tumor could be treated with 100 MeV electrons to a 10 Gy in 1.3 s.  
It is important for clinical application of VHE electron beams to consider dose contribution to the patient from neutrons and from induced radioactivity, and to evaluate neutron and radioactivity production from the standpoint of radiation protection. The mechanism of neutron production in the region of electron beam energies under consideration is determined by photodivision of nuclei via photo-nuclear reaction channels such as (\(\gamma\),\(n\)), (\(\gamma\),\(p\)), (\(\gamma\),\(2n\)), and (\(\gamma\),\(pn\)) by Bremsstrahlung photons due to energy loss of VHE electrons and the small contribution of electrodivision by electrons. The analyses [8] on secondary particle production via the above-mentioned processes concluded that neutron and proton production in VHE electron beam therapy should not significantly affect equivalent doses in comparison with photon treatments, and that the required shielding should not be significantly greater than for 50 MeV electron and photon beams. Monte Carlo simulations [10] on contribution of secondary particles produced by Bremsstrahlung and electronuclear interactions to the dose from VHEelectron beams show that the estimated neutron dose is 1-2 orders of magnitude lower than that for scanning beam proton therapy and 15-18 MV photon IMRT. A proposed VHE electron beam therapy might utilize high-energy electron beams generated from a conventional RF-based linear accelerator comprising an electron injector, a main accelerating structure called LINAC [2], and a beam transport system, lastly followed by a gantry. The electron injector, typically composed of a photocathode RF gun or thermionic high-voltage DC gun and several buncher cavities for producing electron bunches. The LINAC is composed of a series of room-temperature or superconducting RF cavities with an accelerating gradient of the order of 10 MV/m. The beam transport system includes beam focusing and defocusing electro-quadrupole magnets.