Electron linear accelerators for radiation therapy: history, principles and contemporary developments

for Radiation Therapy 323 accelerating electrons because electrons quickly attain relativistic velocities and therefore require inordinately long drift tubes with consequent excessive power loss or, alternatively, microwave frequencies and more efficient structure designs.For example, a megavoltage electron will traverse a 10 cm drift tube in one third of a nanosecond, which corresponds to a frequency of 3000 MHz.During and shortly after World War 11, high-power, high-frequency microwave power sources were developed for radar applications.These pulsed diode sources, the magnetron and klystron, are capable of establishing intense electromagnetic fields in suitably designed microwave cavities.The field intensities and frequencies were high enough for linear acceleration of electrons to multimegavolt energies through appropriate microwave structures.During the mid-l940s, one group in England under Fry (1947, 1948, 1952) and another at Stmanford under Ginzton, Hansen andChodorow (Ginzton, Hansen and Kennedy 1948, Chodorow, Ginzton, Hansen, Kyhl, Neal and Panofsky 1955) succeeded in building a linear array of coupled cavities capa'ble of both transporting the microwave power and accelerating injected electrons.Xot surprisingly, these programs leaned heavily on the wartime hardware and technology in adopting 3000 MHz, a 10 cm free-space wavelength, as the design frequency.Fortunately, this proved to be an optimum frequency for highenergy electron linacs (Chodorow et al. 19%).Such pulsed linear accelerators can provide high average beam currents and large energy gains in structures which rema'in at DC ground potential.These were travelling wave linear accelerators in which the electrons are accelerated by an axial electric field advancing at a suitable velocity down the microwave accelerating structure, More recently, standing wave microwave electron linacs have been developed for radiation thereapy which obtain a given energy at a given microwave power in an accelerator of shorter length, A general review of linac theory is provided by Slater (1948) and a concise treatment may be found in Persico, Ferrari and Segri: (1968) and in Fry and Walkinshaw (1949) : see also Allen and Symonds (1947), Mnllett and Loach (1948), Walkinshaw (1948) andCockcroft (1949).