linac 2 ch2.pdf

Linear Accelerators (LINACS) ❑ Medical linacs are cyclic accelerators that accelerate electrons to kinetic energies from 4 to 25 MeV using microwave radiofrequency fields. ❑ In a linac the electrons are accelerated following straight trajectories in special evacuated structures ca...

Linear Accelerators (LINACS) ❑ Medical linacs are cyclic accelerators that accelerate electrons to kinetic energies from 4 to 25 MeV using microwave radiofrequency fields. ❑ In a linac the electrons are accelerated following straight trajectories in special evacuated structures called accelerating waveguides. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ Linac generations : During the past 40 years medical linacs have gone through five distinct generations, each one increasingly more sophisticated: (1) Low energy x rays (4 – 6 MV) (2) Medium energy x rays (10 – 15 MV) and electrons (3) High energy x rays (18 – 25 MV) and electrons (4) Computer controlled dual energy linac with electrons (5) Computer controlled dual energy linac with electrons combined with intensity modulation M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) Safety of linac installations ❑ Safety of operation for the patient, operator, and the general public is of great concern because of the complexity of modern linacs. ❑ Three areas of safety are of interest ❑ Mechanical ❑ Electrical ❑ Radiation ❑ Many national and international bodies are involved with issues related to linac safety. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) Components of modern linacs ❑ Linacs are usually mounted isocentrically and the operational systems are distributed over five major and distinct sections of the machine: ✓ Gantry ✓ Gantry stand and support ✓ Modulator cabinet ✓ Patient support assembly ✓ Control console M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ The main beam forming components of a modern medical linac are usually grouped into six classes: (1) Injection system (2) Radiofrequency power generation system (3) Accelerating waveguide (4) Auxiliary system (5) Beam transport system (6) Beam collimation and monitoring system M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) In the simplest and most practical configuration: Electron source and the x-ray target form part of the accelerating waveguide and are aligned directly with the linac isocentre obviating the need for a beam transport system. Since the target is embedded into the waveguide, this linac type cannot produce electron beams. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) Configuration of modern linacs ❑Accelerating waveguides for intermediate (8 – 15 MV) and high (15 – 30 MV) energy linacs are located: ❑ Either in the gantry parallel to the gantry axis of rotation ❑ Or in the gantry stand. ❑ In both cases, a beam transport system is used to transport the electron beam from the accelerating waveguide to the x-ray target. ❑ The radiofrequency power source in both configurations is mounted in the gantry stand M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) Configurations for intermediate and high energy linacs Waveguide in the gantry Waveguide in the gantry stand RF power source in gantry stand RF power source in gantry stand M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑Typical modern dual energy linac, incorporating imaging system and electronic portal imaging device (EPID), ❑Elekta, Stockholm M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑Typical modern dual energy linac, with on board imaging system and an electronic portal imaging device (EPID), Varian, Palo Alto, CA M.Sc. Medical Physics Dr. Khalid I Hussein