Linac

• What is the primary type of beam used in therapeutic radiography?

  Megavoltage X-ray (photon) beams
• Why are megavoltage photon beams preferred over kilovoltage beams for deep-seated tumors?


  Megavoltage beams have higher energy and can penetrate deeper tissues
• What is the energy range of megavoltage photon beams used in therapeutic radiography?

  4-25 MV
• What is the primary equipment used to deliver megavoltage photon beams in radiotherapy?

  Isocentric gantry-mounted linear accelerators (LinAcs)
• What is the shape of the LinAc referred to in some textbooks?

  C-arm
• Why are proton beams increasingly used in radiotherapy?

  They offer better precision in targeting deep-seated tumors
• What was the focus of early radiotherapy (teletherapy)?

  Kilovoltage (kV) beams
• What was the main goal of developing LinAcs in the 1950s?

  To use higher energy beams (MV) for deep-seated tumors
• What are the three major core sections of a LinAc?

  Electron producing core sections, Gantry assembly, Head
• What is the function of the electron gun in a LinAc?

  To produce electrons by heating tungsten
• What material is commonly used in the electron gun filament assembly?

  Tungsten
• Why is lanthanum hexaboride (LaB6) used in some electron gun units?

  It can replace tungsten for electron production
• What are the two main types of electron guns used in LinAcs?

  Diode and triode
• Why is the triode structure preferred in most LinAcs?

  It allows independent regulation of gun current and beam energy
• What is the role of the anode in a linear accelerator?

  To focus electrons through a small hole
• What happens to electrons after they pass through the anode in a LinAc?

  They enter the accelerating structure
• What are the two sources of RF electromagnetic waves in a LinAc?

  Klystron and magnetron
• How does a klystron generate high-power RF waves?

  By amplifying a low-power RF signal with a high-power electron stream
• What is the primary function of the accelerating waveguide in a LinAc?

  To accelerate electrons using RF electromagnetic waves
• Why do electrons need to gain energy in the accelerating waveguide?

  To achieve the required energy for therapeutic use
• What are the two types of waveguides used in LinAcs?

  Travelling wave waveguide and standing waveguide
• Why is copper used in the travelling wave waveguide?

  It has high electrical conductivity, reducing power loss
• What happens to the RF wave in a standing waveguide?

  It is reflected back, forming a standing wave
• What is the purpose of SF6 gas in the waveguide?

  To maintain a stable environment for electron acceleration
• What is the primary function of the cooling system in a LinAc?

  To maintain a stable operating temperature
• Why is a vacuum maintained in the accelerating structure?

  To prevent electrons from losing energy through collisions
• What is the approximate vacuum pressure maintained in the accelerating structure?

  10⁻⁷ torr
• What is the role of focusing coils in a LinAc?

  To prevent electrons from diverging during acceleration
• What causes electrons to diverge during acceleration?

  RF wave's radial component and electrostatic repulsion
• What is the purpose of steering coils in a LinAc?

  To keep electrons in their optimal position
• Where are steering coils typically located in a LinAc?

  At either end of the accelerating waveguide
• Why are bending magnets used in C-arm LinAcs?

  To change the direction of electrons before hitting the target
• What happens to electrons in the flight (drift) tube?

  No further acceleration occurs
• What are the components of EPID and OBI verification imaging?


  Electronic Portal Imaging Device (EPID) and On-Board Imager (OBI)
• What is the role of the heat exchanger in the cooling system?

  To dissipate heat from the cooling water
• What toxic byproducts are formed when SF6 decomposes?

  Sulphur tetrafluoride and hydrogen fluoride
• What happens to the RF wave after it exits the travelling wave waveguide?

  It is either absorbed or fed back into the input end
• What is the purpose of reflective discs in the standing waveguide?

  To reflect the RF wave, forming a standing wave
• What affects the electrons in the accelerating waveguide besides the waveguide itself?

  The Earth’s magnetic fields or other magnetic fields
• How many steering coils are typically present at each end of the accelerating waveguide?

  Four (two on the x-axis and two on the z-axis)