Dose calibration of EPIDs for segmented IMRT dosimetry

Area of Science:

• Medical Physics
• Radiation Oncology
• Imaging Technology

Background:

• Electronic Portal Imaging Devices (EPIDs) are crucial for verifying radiation therapy delivery.
• Understanding EPID dose response is essential for accurate dosimetry, especially with Intensity-Modulated Radiation Therapy (IMRT).
• Gain ghosting is a known artifact affecting EPID image quality and dose accuracy.

Purpose of the Study:

• To investigate the dose response of amorphous silicon (a-Si) EPIDs under various acquisition settings.
• To compare dose linearity for open fields versus segmented IMRT fields across different EPID models.
• To evaluate pixel calibration methods for clinical IMRT dosimetry.

Main Methods:

• Four EPIDs (2 Siemens, 1 Elekta, 1 research) were tested with different acquisition settings.
• Dose response linearity was measured for static open fields and simple segmented IMRT fields.
• Simultaneous measurements with an ionization chamber array (ICA) and EPID were performed.
• Three pixel calibration methods were demonstrated and compared for IMRT fields.

Main Results:

• Nonlinear dose response was observed across EPIDs, primarily attributed to gain ghosting.
• Elekta EPID showed good agreement (within 2.5%) between IMRT and static fields for monitor units (MU) ≥ 2.
• Siemens EPIDs exhibited poor reproducibility for segmented IMRT at low MU (≤ 5).
• Pixel calibration at 20 MU yielded results equivalent to ghosting correction models.

Conclusions:

• The nonlinear dose response of a-Si EPIDs is largely insensitive to acquisition settings and mainly caused by gain ghosting.
• A specific pixel-to-dose calibration method at small MU obviates the need for additional ghosting correction factors.
• Clinical implementation of EPID dosimetry requires careful consideration of device-specific performance, particularly for segmented IMRT.