Quantifying uncertainty in calibration and clinical application of EBT-4 Gafchromic film dosimetry

Abstract

Background: GAFchromic films (GAFs) are essential tools in radiotherapy dosimetry, but uncertainties in calibration methods limit their accuracy and hinder consistency across clinical practice.

Purpose: This study aimed to perform a comparative analysis of different calibration functions for GAFchromic EBT4 films in single- and double-channel dosimetry to quantify the uncertainty associated with their calibration and clinical use.

Methods: GAFchromic EBT4 films were irradiated to doses between 0.2 and 10 Gy using a 6 MV photon beam. A custom irradiation setup in a water-equivalent phantom ensured reference conditions, with dose verified by an ionization chamber. Films were scanned on an Epson Expression 13000XL flatbed scanner at 72 dpi, with pre- and postirradiation scans taken at optimized time points (48 h postirradiation to ensure polymer growth stabilization). A custom-developed Python script was used for comprehensive film data processing, including lateral response artifact (LRA) correction. Eight calibration functions, including invertible and non-invertible forms from existing literature, were evaluated and compared. A comparison of the performance of the proposed approach with an established commercial software was performed.

Results: Among the tested functions, two of them consistently emerged as the most accurate for EBT4 film calibration, yielding also the lowest relative errors. Specifically, the 2.5th order polynomial function showed an average percentage residual error of 2.7% (ranging from 1.6-4.4%), while the same function on corrected net optical density (netOD_c) demonstrated a comparable 2.9% (ranging from 1.9-4.4%). In general, calibration functions relying on netOD_c showed larger uncertainty compared to their analog using the red channel only. The best performing function showed residual values comparable to those obtained with the commercial benchmark. The LRA was observed to cause deviations of up to 5% at scanner extreme lateral positions, reinforcing the necessity of correction for large fields.

Conclusions: We identified an optimal calibration function that incorporates a linear term and a 2.5th-order component to accurately model the relationship between radiation dose and the netOD_c in the red channel. The significant variability in performance among calibration functions underscores the critical need for independent verification and transparent dosimetry tools. To address this, our study makes the full analysis code and data publicly available, facilitating independent validation and enhancing the accuracy, consistency, and reproducibility of film dosimetry in clinical practice.

Keywords: Gafchromic EBT‐4; film calibration; film dosimetry; lateral response artifacts; uncertainty analysis.

FIGURE 1

Picture of the mask used to ensure the good positioning of the films on the Epson Expression 13000XL scanner.

FIGURE 2

Residuals of the validation points for the tested calibration functions, employing both netOD and netOD_C.

FIGURE 3

Fit (upper plot), experimental (middle plot) and total (lower plot) error components for each of the calibration functions tested within the calibration range.

FIGURE 4

Comparison of residuals on the validation films between Function 5 and benchmark.

FIGURE 5

Parameters for the correction of the netOD at different lateral positions, for channels R, G, and B. An offset was added in the x‐axis to help the visualization of the different channels.