Monte Carlo-based adaptive EPID dose kernel accounting for different field size responses of imagers

Abstract

The aim of this study is to present an efficient method to generate imager-specific Monte Carlo ͑MC͒-based dose kernels for amorphous silicon-based electronic portal image device dose prediction and determine the effective backscattering thicknesses for such imagers. EPID field sizedependent responses were measured for five matched Varian accelerators from three institutions with 6 MV beams at the source to detector distance ͑SDD͒ of 105 cm. For two imagers, measurements were made with and without the imager mounted on the robotic supporting arm. Monoenergetic energy deposition kernels with 0-2.5 cm of water backscattering thicknesses were simultaneously computed by MC to a high precision. For each imager, the backscattering thickness required to match measured field size responses was determined. The monoenergetic kernel method was validated by comparing measured and predicted field size responses at 150 cm SDD, 10ϫ 10 cm 2 multileaf collimator ͑MLC͒ sliding window fields created with 5, 10, 20, and 50 mm gaps, and a head-and-neck ͑H&N͒ intensity modulated radiation therapy ͑IMRT͒ patient field. Field size responses for the five different imagers deviated by up to 1.3%. When imagers were removed from the robotic arms, response deviations were reduced to 0.2%. All imager field size responses were captured by using between 1.0 and 1.6 cm backscatter. The predicted field size responses by the imager-specific kernels matched measurements for all involved imagers with the maximal deviation of 0.34%. The maximal deviation between the predicted and measured field size responses at 150 cm SDD is 0.39%. The maximal deviation between the predicted and measured MLC sliding window fields is 0.39%. For the patient field, gamma analysis yielded that 99.0% of the pixels have ␥ Ͻ 1 by the 2%, 2 mm criteria with a 3% dose threshold. Tunable imager-specific kernels can be generated rapidly and accurately in a single MC simulation. The resultant kernels are imager position independent and are able to predict fields with varied incident energy spectra and a H&N IMRT patient field. The proposed adaptive EPID dose kernel method provides the necessary infrastructure to build reliable and accurate portal dosimetry systems.

Simple Proposal for Dosimetry with an Elekta iViewGTTM Electronic Portal Imaging Device (EPID) Using Commercial Software Modules

“…The most accurate way of modeling scattering processes is to utilize Monte Carlo computation [1,4,23]. However, this is time consuming, demands knowledge, and the appropriate software.…”

A portal dosimetry dose prediction method based on collapsed cone algorithm using the clinical beam model

“…According to Antonuk et al, the proximity of the photodiodes to the scintillator make this effect negligible in a‐Si EPID. Wang et al . also pointed to a negligible glare effect.…”

Calibration of a detector array through beam profile reconstruction with error‐locking

“…The method was not validated on the EPID as the robotic arm of the imager introduces backscattered radiation, (10) which changes fluence of a wide beam irradiation at the unshifted and shifted positions of the proposed procedure. This significant variation of fluence is not acceptable for the iterative method due to error propagation.…”