shelding 2 ch3.pdf

SHIELDING CONSIDERATIONS ❑ External beam radiotherapy is carried out mainly with three types of equipment that produces either x rays or electrons for clinical use: ❑ X-ray machines (superficial or orthovoltage). ❑ Cobalt-60 teletherapy machines. ❑ Linacs. ❑ All...

SHIELDING CONSIDERATIONS ❑ External beam radiotherapy is carried out mainly with three types of equipment that produces either x rays or electrons for clinical use: ❑ X-ray machines (superficial or orthovoltage). ❑ Cobalt-60 teletherapy machines. ❑ Linacs. ❑ All radiotherapy equipment must be housed in specially shielded treatment rooms in order to protect personnel and general public in areas adjacent to the treatment rooms. M.Sc. Medical Physics Dr. Khalid I Hussein  SHIELDING CONSIDERATIONS ❑ During the planning stage for a radiotherapy machine installation, a qualified medical physicist: ❑ Determines the required thickness of primary and secondary barriers ❑ Provides the information to the architect and structural engineer for incorporation into the architectural drawing for the treatment room. M.Sc. Medical Physics Dr. Khalid I Hussein  SHIELDING CONSIDERATIONS ❑Superficial and orthovoltage x-ray therapy rooms are shielded: ❑ Either with ordinary concrete (density: 2.35 g/cm3) ❑ Or lead (density: 11.36 g/cm3, atomic number: 82) ❑In this energy range the photoelectric effect is the predominant mode of photon interaction with matter, making the use of lead very efficient for shielding purposes. M.Sc. Medical Physics Dr. Khalid I Hussein  SHIELDING CONSIDERATIONS ❑ Megavoltage treatment rooms are often referred to as bunkers because of the large primary and secondary barrier thicknesses required for shielding. ❑ Megavoltage bunkers are most commonly shielded with ordinary concrete so as to minimize construction cost. ❑ The Compton effect is the predominant mode of photon interaction with shielding material in the megavoltage energy region. The barrier thickness is thus scaled inversely with density of the shielding material. M.Sc. Medical Physics Dr. Khalid I Hussein  SHIELDING CONSIDERATIONS ❑Typical teletherapy installation: isocentric machine ❑ Primary barriers shield against the primary radiation beam. ❑ Secondary barriers shield against leakage radiation and radiation scattered from the patient. M.Sc. Medical Physics Dr. Khalid I Hussein