Gafchromic ebt3

Owing to their superior spatial resolution and established DR independence^{1 (link),22 (link)}, EBT3 Gafchromic® (Ashland Advanced Materials, Bridgewater, NJ) films were employed for the dose measurements used throughout the experimental campaign at TRIUMF. Prior to their first use for absolute dosimetry, the film batch was cross-calibrated against a reference 0.6cc ionization chamber (Farmer TN30010, PTW, Freiburg, Germany) and SuperMAX electrometer (Standard Imaging, Middleton, USA) whose calibration is traceable to the standard laboratories of the National Research Council of Canada (NRC). The film calibration was handled according to supplier recommendations and TG-235 guidelines^{30 (link)} while the reference dose measurement was conducted according to the procedure set out by AAPM’s TG-51 protocol^{41 (link)}. The films were located at a depth of 10 cm from the phantom surface at 100 cm SAD and the chamber reading was therefore scaled to the film depth by applying a previously measured tissue-maximum ratio conversion that was also verified at the time of irradiation.
In all, 18 calibration films were irradiated to equally-separated doses between 0 and 35 Gy using a 10 MV clinical photon beam delivered on a Varian TrueBeam STx (Varian, Palo Alto, USA). The feasibility of using EBT3 film beyond 20 Gy has been previously demonstrated^{42 (link),43 (link)} though future work would benefit from the use of EBT-XD with enhanced dose sensitivity and accuracy out to $\approx 30-40$  Gy^{42 (link),44 (link)}. The energy independence of EBT3 films in the MV energy range was exploited to enable their use in our experimental beam despite the differences in beam quality as compared to the clinical 10 MV calibration beam^{29 (link),45 (link)}.
Irradiated films were scanned approximately 24 h after exposure on an EPSON^® 10000XL flatbed scanner (Epson America, Long Beach, CA) using the central-axis to minimize lateral response artifacts and a resolution of 200 dpi. A Matlab^® (Mathworks, Nattick, MA) script was written to process the scanned films using both red and green color channels, from which the absorbed dose was interpolated for each channel independently. Red channel responses were used for $\le$ 15Gy dose to exploit the improved dynamic range and reduced variance while for > 15Gy irradiations the green channel was instead used^{29 (link),30 (link),46 (link),47 (link)}. Our film uncertainty budget follows from the prescription of TG-235 and was ascribed to be $\pm 5\%$ , consistent with the literature comparing the red and green channels^{47 (link)}.

In a calibration process of the dose calculation engine, we adjusted the focal spot width $\sigma$ such that calculated and measured dose profiles matched. The peak doses, valley doses, and dose profiles were assessed in a 100 × 54 × 54 mm³ PMMA phantom at different depths using GafChromic™ EBT3 (Ashland, Wilmington, DE, USA) films. The entire 20 × 20 mm² microbeam field was used for the irradiation. The same phantom and field were defined in the hybrid algorithm, and the resulting doses were compared against the experimental results.
For quantitative dosimetry, the films were calibrated with uniform doses in a RS225 X-ray irradiator (Xstrahl, Suwanee, GA, USA). Its dose rate was measured using a Farmer TM30010 ionization chamber (PTW Freiburg GmbH, Freiburg, Germany), and six different films were irradiated with a dose in the range of (0–10) Gy. After irradiation, the films were scanned with a Reflecta ProScan 10T (Reflecta GmbH, Eutingen, Germany) film scanner, and their gray values were correlated to the irradiated dose by fitting the data points with: $$D\left(gv\right)=a+\frac{b}{gv+c},$$
where D is the dose, $gv$ is the grey value and a, b, and c are the fit parameters. More details about film handling are provided in [26 (link)].