Scope

Outline the calculation model for IQM in terms of the physical characteristics and behavior of linear accelerators.

Expected inputs for signal prediction

  • Chamber characterization
  • Treatment unit (Linac) characterization
    • Collimation attenuation
    • Fluence profiles
    • Patient treatment description
  • Both static field-in-field and dynamic delivery modes

IQM chamber properties

  • Sloped electrode chamber
    • Spatial gradient = delivery position encoding
  • Characterized by
    • Reference field normalization
    • Gradient (sensitivity) map
    • CSM, (𝑆𝐼𝑄𝑀)

Linac characterization

  • Behaviour to capture
    • Output change with field size
    • Radial Profile
    • Transmission through collimating elements
  • Source Assumption
    • Primary point source
    • Extended secondary source

Fundamentals of signal calculation

  • 𝑈 = MU setting for segment
  • 𝐴𝑂𝐹 = output change with field size (residual…)
  • 𝑁𝐼𝑄𝑀 / 𝑛×𝑚= normalization (electrometer reading)
  • 𝐼p, 𝐼s = primary and secondary source intensity matrix
  • 𝑓𝑠 = fractional contribution from secondary source
  • 𝑆𝐼𝑄𝑀 = chamber positional sensitivity matrix

Primary source intensity

  • Starts with open source profile
    • Assume radially symmetric intensity profile
    • Apply effect of collimation attenuation
    • Works on an area weighted average rather than an intensity to a point

Primary Source Modulation

  • Area-Weighted Transmission through collimating elements subdivided in regions of transmission and time for each pixel

Secondary source

  • Extended source geometry
    • Positioned at bottom of flattening filter

 

Secondary source modulation

  • More complex geometry
    • Non-divergence matched
    • Multiple off-axis sources
    • Complex element shape shading
  • Simplify calculation
    • Static “snapshot” calculation
    • Sampling point geometry
    • Layered collimating element

Area output factor characterization

  • Captures changes in output due to field size effects
  • Derived from a series of rectangular field measurements
  • Behaves as a “residual”
    • Some effects accounted for byextended source
    • Rederived for tweaks in source description & transmission
  • Look-up according to average field width, length