Week 1 Flashcards
what are the room design requirements for superficial machine?
Adequate room Console area Interlocked door Warning lights / signs Shielding Machine mountings Water supply Storage space
What are the shielding materials for <500 kV?
lead lined walls – similar or slighty more than
diagnostic imaging conditions
Shielded doors – heavy duty hinges
What are the shielding materials for >500 kV?
Concrete, high density concrete walls
Maze – no door required if well designed and less
than 15MV
What is a kilovolt?
What is a Megavolt ?
What is kilovoltage?
What is an electron volt?
Kilovolt: A unit of force equal to 1000 volts
Megavolt: A unit of force equal to 1,000 000 volts. Beams with the
energy range of 4-25 MV are used. Linacs.
Kilovoltage: The amount of electrical energy applied so that the electrons
accelerate from the cathode/filament towards the anode.
Electron Volt: a unit of energy equal to the energy acquired by an electron
accelerated through a potential difference of 1 volt.
Control panel Info required
Machine warmed up Interlocks set Patient ID checked (name/d.o.b/db number) Filter correct Applicator correct Exposure time set
Filters for beam hardening
Filters preferentially remove the softer photon beam
components, which would simply irradiate the most
superficial layers and produce an undesirable rapid
attenuation through those layers. Hardens the beam
Prevents excessive photons with unwanted energies
contributing to the dose. Smooths the beam.
Makes the beam more homogeneous therefore able to penetrate more.
Filters used in Superficial RT
Quality is a measurement of beam’s penetrating
power (hardness).
• Knowing the kV is insufficient .There is no direct
relationship between the stated generating
voltage (kV) and beam penetration as there is
with MV.
• We express beam quality (penetration) in terms
of kV and Half-Value Layer (HVL).
What is half value layer?
HVL = The thickness of the material at which the
intensity of radiation entering it is reduced by one
half.
What does increasing filtration do?
Increasing filtration decreases the number of x-ray photons, intensity. • Most of this decrease is in the photons with low energies that do not penetrate but contribute to dose.
