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Related Experiment Videos

Abutment region dosimetry for serial tomotherapy.

D A Low1, S Mutic, J F Dempsey

  • 1Division of Radiation Oncology, Mallinckrodt Institute of Radiology, Washington University Medical Center, St. Louis, MO 63110, USA. low@castor.wustl.edu

International Journal of Radiation Oncology, Biology, Physics
|September 7, 1999
PubMed
Summary

Precise patient positioning is crucial in serial tomotherapy to prevent dose variations. Even with accurate setup, beam divergence can cause significant hot and cold spots, impacting treatment efficacy.

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Area of Science:

  • Medical Physics
  • Radiation Oncology

Background:

  • Serial tomotherapy utilizes dynamic multileaf collimators (DMLC) for optimized dose delivery.
  • Accurate patient indexing is critical for serial tomotherapy, especially for longer targets requiring patient movement between treatment fractions.

Purpose of the Study:

  • To investigate the dosimetric consequences of incorrect patient indexing in serial tomotherapy.
  • To evaluate dose heterogeneity as a function of gantry rotation angle, leaf width, and position relative to the isocenter.
  • To assess the impact of random daily patient positioning errors.

Main Methods:

  • Treatment plans were generated for cylindrical targets using 1 cm and 2 cm leaf modes with varying gantry rotations (300, 240, 180 degrees).
  • Dosimetric measurements were performed using film dosimetry in a homogeneous phantom.

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  • The effects of couch indexing errors were specifically investigated.

    • Main Results:

    • Dose error due to couch indexing was measured at 25% mm(-1).
    • Smaller gantry angles and larger leaf projections resulted in more heterogeneous dose distributions.
    • Significant hot (7-22%) and cold (15-27%) spots were observed at 7 cm from the isocenter, increasing with reduced gantry rotation.
    • The 2 cm leaf mode exhibited 70% greater heterogeneity than the 1 cm mode.

    Conclusions:

    • Serial tomotherapy requires strict patient immobilization to mitigate indexing errors and resultant dose heterogeneities.
    • Beam divergence inherently causes dose variations, particularly in abutment regions, even with precise setup.
    • Optimizing treatment by using larger gantry angles, smaller leaf widths, and strategic rotation axis placement can minimize these dosimetric challenges.