Effects on the photon beam from an electromagnetic array used for patient localization and tumor tracking

Area of Science:

• Medical Physics
• Radiation Oncology
• Medical Imaging

Background:

• The Calypso 4D localization and tracking system uses an electromagnetic array for real-time target monitoring during radiation therapy.
• Understanding the array's impact on radiation beams is crucial for treatment accuracy.

Purpose of the Study:

• To investigate the beam attenuation and beam spoiling properties of the Calypso electromagnetic array.
• To assess the clinical significance of dose differences introduced by the array.

Main Methods:

• Measurements were conducted using 6 MV and 15 MV photon beams on a Varian Clinac iX linear accelerator.
• Narrow and broad beam attenuation were measured using a photon diode and Farmer chamber, respectively.
• Beam spoiling was assessed by measuring depth-dose changes at various distances with an embedded Markus parallel plate chamber.

Main Results:

• Beam attenuation was less than 2%-3% for angles <40° and increased at more oblique angles.
• Spoiling effects were significant in the buildup region but negligible beyond the maximum dose depth (dmax).
• Dose differences in IMRT and VMAT plans were less than 2.5%.

Conclusions:

• The Calypso electromagnetic array exhibits minimal beam attenuation and dose spoiling effects.
• The observed dose differences are clinically insignificant, supporting its use in radiation therapy monitoring.