In-vivo EPID dosimetry for IMRT and VMAT based on through-air predicted portal dose algorithm.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) are advanced radiation techniques.
• Accurate in-vivo dosimetry is crucial for ensuring treatment efficacy and patient safety.
• Existing portal dose prediction methods may require adjustments for fixed source-to-imager distances.

Purpose of the Study:

• To adapt and validate a portal dose prediction methodology for IMRT and VMAT at a fixed source-to-imager distance (SID).
• To introduce a correction factor for air gap variations to maintain accuracy.
• To assess the accuracy and sensitivity of the adapted method for in-vivo dosimetry.

Main Methods:

• Adapted Berry et al. (2012) methodology for IMRT/VMAT with a fixed 150 cm SID.
• Implemented a correction factor for air gap variations.
• Commissioned using Varian aS1200 EPID data with varying field sizes, phantom thicknesses, and air gaps.
• Verified with six IMRT and seven VMAT plans on multiple phantoms.
• Assessed accuracy using a global 3%/3 mm gamma index.

Main Results:

• The effect of air gap variation on EPID signal was significant (up to 30%).
• Over 95% of IMRT plans passed the 3%/3 mm gamma criteria across three phantoms.
• 23 out of 24 VMAT arcs passed the 3%/3 mm gamma criteria by over 95%.
• The method demonstrated sensitivity to potential errors.

Conclusions:

• The adapted method provides fast and accurate in-vivo EPID dosimetry for IMRT and VMAT.
• This approach can potentially replace numerous pre-treatment verification procedures.
• The correction factor effectively accounts for air gap variability, enhancing accuracy at a fixed SID.