Review of Energy Variation Approaches in Medical Accelerators

Most of cancer treatment Radiation Therapy (RT) machines rely on a linac as the source of the treatment beam which can be an electron beam or an X-ray beam. In either case, an approach to vary the energy of the linac’s output beam may be needed to target cancer tumors at different depths. Additionally, some of the Image Guide Radiation Therapy (IGRT) approaches use the linac as the source of both the treatment and imaging modes. This requires the linac to produce photon beams with different energies. Over the last two decades, multiple approaches for medical linac’s energy variation were proposed and some of them have been developed and implemented clinically. In this paper, we review some of these approaches and discuss the advantages and disadvantage of each technique. NEED FOR ENERGY VARIATION A fundamental strategy in cancer Radiation Therapy (RT) is to deliver a sufficiently high dose to the tumor while minimizing the dose to the neighboring healthy tissue especially those in the critical organs. RT treatment plans take advantage of dependence of the dose depth distribution on energy. Additionally, many of RT treatment plans utilize multi-energy mixed photon beam modalities, where the patient is irradiated with photon beams of two or more different energies. For example, in a dual-energy treatment, [1, 2], one beam can be at 6 MV and the other at 18 MV, in the same treatment. By a selection of weighing of the two photon beams, a depthdose characteristic for any energy between 6 and 18 MV can be obtained. An effective way of switching between the two energies of the generated X-ray beams is a requirement for such treatments. ENERGY VARIATION APPROACHES