Gapping fields on sloping surfaces

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.
• 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.