Generalized EPID calibration for in vivo transit dosimetry

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• In vivo dosimetry is crucial for accurate radiotherapy.
• Electronic Portal Imaging Devices (EPIDs) offer potential for simple and efficient in vivo dosimetry.
• Time-consuming calibration measurements with solid water phantoms can hinder EPID implementation.

Purpose of the Study:

• To propose and validate a calibration procedure for amorphous silicon (aSi) EPIDs for in vivo transit dosimetry.
• To investigate the dosimetric equivalence of aSi EPIDs regarding signal stability, linearity, and field dependence.
• To develop generalized correlation functions for accurate isocenter dose reconstruction.

Main Methods:

• Investigated signal reproducibility, long-term stability, linearity with monitor units (MU) and dose per pulse, and field size dependence for three aSi Varian EPIDs.
• Determined correction factors for MU linearity (k(lin)) and verified their independence from beam quality and dose per pulse.
• Developed generalized correlation functions F(TPR,w,L) and empirical factors f(TPR,d,L) based on Tissue Phantom Ratio (TPR), phantom thickness, field size, and distance for dose reconstruction.

Main Results:

• Signal reproducibility within ±0.5% (2SD) and long-term stability within ±2%.
• Signal linearity with MU within ±2% (IAS 2) and ±0.5% (IAS 3).
• Developed generalized functions enabling isocenter dose reconstruction with tolerance levels of ±5% to ±6%.

Conclusions:

• The proposed calibration procedure effectively calibrates aSi EPIDs for in vivo transit dosimetry.
• The method reduces implementation effort by utilizing generalized correlation functions.
• This approach enhances the clinical applicability of EPID-based in vivo dosimetry for photon beams.