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Gapping fields on sloping surfaces.

R Keys1, P W Grigsby

  • 1Radiation Oncology Center, St. Louis, MO 63110.

International Journal of Radiation Oncology, Biology, Physics
|May 1, 1990
PubMed
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Calculating radiation therapy gaps for Hodgkin's patients is challenging due to sloping surfaces. A modified formula accurately accounts for patient thickness variations, preventing dangerous radiation dose overlaps between adjacent treatment fields.

Area of Science:

  • Medical Physics
  • Radiation Oncology
  • Radiotherapy Treatment Planning

Background:

  • Accurate positioning of adjacent radiation therapy fields is critical to avoid under-dosing or over-dosing.
  • The standard gapping formula assumes a flat patient surface, which is often inaccurate, particularly in Hodgkin's lymphoma patients.

Purpose of the Study:

  • To address the inaccuracies of standard radiation therapy field gapping formulas on sloping surfaces.
  • To develop and validate a modified formula for calculating gaps between adjacent treatment fields, considering patient surface topography.

Main Methods:

  • Investigated the impact of sloping surfaces and patient thickness variations on radiation field gapping in Hodgkin's patients.
  • Formulated a modified gapping formula applicable to both source-skin-distance and isocentric treatment setups.

Related Experiment Videos

  • Analyzed positioning challenges related to mantle fields in prone and supine patient positions.

    • Main Results:

    • The standard gapping formula can lead to significant overlap (7-9 mm) and dose escalation (up to 140%) on sloping surfaces.
    • The modified formula accurately accounts for variations in patient thickness (up to 5-6 cm) across treatment fields.
    • Identified specific positioning issues for mantle fields in different patient orientations.

    Conclusions:

    • A modified gapping formula is essential for accurate radiation therapy field placement in patients with non-flat surfaces.
    • The new formula mitigates the risk of geographic misses and hot spots, improving treatment delivery accuracy.
    • Addressing patient positioning and surface geometry is crucial for optimizing radiotherapy for Hodgkin's lymphoma.