Dosimetric comparison of centered and off-centered posterior neck electron fields

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Electron beams at extended source-to-surface distance (SSD) are used for spinal cord boosts in head and neck cancer treatment after tolerance is reached.
• Photon fields abut the electron fields anterior to the spine, typically with different central axes.
• Optimizing field matching is crucial for accurate dose delivery and minimizing irradiated tissue.

Purpose of the Study:

• To investigate the dosimetric impact of off-centered electron cutouts compared to centered cutouts.
• To determine if a common central axis can be used for abutting photon and electron fields in radiotherapy.
• To assess the feasibility of simplifying treatment planning for head and neck cancer boosts.

Main Methods:

• Measured percent depth dose (PDD), output factors, and beam profiles for centered and off-centered rectangular and square cutouts.
• Utilized 6, 9, and 12 MeV electron beams from Varian linear accelerators.
• Employed 10x10 cm and 15x15 cm cones at SSDs of 100 cm and 110 cm.

Main Results:

• Differences in PDD and profiles between centered and off-centered cutouts were less than 1%.
• Output factor variations between centered and off-centered cutouts were less than 5%.
• These dosimetric differences are clinically insignificant, validating the use of off-centered cutouts.

Conclusions:

• The central axis of electron fields can be aligned with the central axis of abutting photon fields without significant dosimetric consequences.
• Off-centered electron cutouts simplify treatment planning by enabling the use of a single central axis for matched fields.
• This approach eliminates the need for additional dosimetric data for off-center electron cutout placements, streamlining radiotherapy procedures.