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Related Experiment Videos

Is the diaphragm motion probability density function normally distributed?

R George1, P J Keall, V R Kini

  • 1Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298, USA.

Medical Physics
|March 26, 2005
PubMed
Summary
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Internal motion from respiration is not normally distributed. However, combining patient motion data allows for normal distribution approximation in radiotherapy planning, simplifying margin calculations.

Area of Science:

  • Medical Physics
  • Radiation Oncology
  • Radiotherapy Treatment Planning

Background:

  • Radiotherapy planning margins account for internal motion and set-up errors.
  • Existing margin formulas often assume normal distributions for both errors.
  • Internal motion, unlike set-up error, may not follow a normal distribution.

Purpose of the Study:

  • To assess if respiratory-induced internal motion probability density functions (PDFs) are normally distributed.
  • To determine if the combined geometric uncertainty (internal motion + set-up error) can be approximated by a normal distribution.

Main Methods:

  • Collected 60 fluoroscopy diaphragm motion datasets across free breathing, audio, and visual feedback.
  • Used diaphragm motion as a proxy for liver and lung tumor motion.

Related Experiment Videos

  • Performed normality tests on single, per-patient combined, and all-patient combined motion data; convolved PDFs with set-up error PDFs.

    • Main Results:

    • Individual and per-patient combined diaphragm motion PDFs were not consistently normal.
    • Total geometric error PDFs approached normality at specific set-up error standard deviations (e.g., ≥0.27 cm for free breathing).
    • When all patient motion data were pooled, diaphragm motion PDFs approximated a normal distribution across all feedback types.

    Conclusions:

    • Diaphragm motion PDFs can be approximated as normal for radiotherapy planning when individual patient data is unavailable.
    • Caution is advised when calculating margins from individual patient measurements due to potential non-normal total geometric error.
    • This finding aids in refining margin strategies for internal motion in radiation oncology.