MO‐EE‐A3‐02: Three‐Dimensional Dose Reconstruction Using Non‐Transmission Portal Dosimetry and Monte Carlo Calculations

Abstract

Purpose: To evaluate a convolution and Monte Carlo based non‐transmission portal dosimetry method used to reconstruct a 3D dose distribution. Method and Materials: Raw images acquired by an amorphous silicon electronic portal imaging device (EPID) were deconvolved with a scatter kernel to determine a 2D primary energy fluence distribution. The scatter kernel was characterized by two components: a Monte Carlo simulated dose deposition kernel scored in the scintillation screen, and an iteratively optimized optical photon‐spreading (or “glare”) kernel initialized by the tail of a measured in‐air fluence profile. The derived primary energy fluence was converted into a phase space file and used as a source input for the Electron Gamma Shower (EGS) user code DOSXYZnrc for Monte Carlo calculation. The reconstructed 3D dose distribution was verified by comparison with a measured beam profile and PDD data. Results: The glare kernel was determined to be the dominating component of the scatter kernel, while the dose kernel correction exhibited longer‐range radial extent from the central axis of the beamlet. The derived primary energy fluence demonstrated a similar trend as the in‐air fluence measurement, and was in good agreement with the penumbra tails. The reconstructed beam profile and PDD for a 10×10 cm2 open‐field were in excellent agreement with photodiode and ionization chamber measurements. Conclusion: This study demonstrates the potential for pretreatment IMRT verification using the primary fluence extracted from an EPID to reconstruct a Monte Carlo calculated 3D dose distribution in a patient's planning CT‐scan. Future work will include dose normalization and a more comprehensive validation measurement scheme before moving to a comparison of a 3D dose volume between treatment planning system and EPID reconstruction.