Gafchromic film

We carried out absorbed dose measurements using GAFCHROMIC™ films (Ashland Specialty Ingredients G.P., Bridgewater, NJ, USA) type EBT3.³⁶ EBT3 film are suitable for dose map measurements in radiation fields with high dose gradients, as in our measurements, because of the high spatial resolution and energy independence. To convert the response of the film into dose, we measured a calibration curve. For this purpose, we exposed n = 8 pieces of cut films (10 cm × 13 cm) to known different doses (0, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.9 Gy), obtained by changing the exposure time. To ensure the stabilization of the polymers, 24 hours after irradiation, the films were scanned with an Epson Expression 11000XL scanner of San Raffaele Hospital (Milan, Italy) in transmission mode at a resolution of 72 dpi (0.35 mm pixel size). To obtain the calibration curve, we utilized the FilmQA™ Pro Software (Ashland Inc.), a quantitative analysis tool designed for film scanning, pixel value extraction, and dose conversion. This software works with a method called “triple channel evaluation,” which uses pixel values from all color channels together to construct the dose map, providing “corrected” red, green, and blue dose maps. It is also possible to separate the dose‐dependent part of the signal from the dose‐independent part, where the latter derives from nonhomogeneity related to film manufacturing, scanner artifacts, or film manipulation (dust, fingerprints).³⁷ Practically, the method varies the dose values until the corresponding pixel values are best matched for all three‐color channels.
In the case of 6 MV irradiation, to plan the fields and beam arrangements for cell lines irradiations, the Medical Physics team of San Raffaele Hospital acquired a CT image of the experimental setup, then they imported it into the treatment planning system (TPS) Eclipse (Varian Medical Systems, Inc., USA) and simulated the irradiation session. To deliver a dose of 1 Gy to the cells, with an uncertainty of 5%, the TPS indicated the necessity to deliver 92 Monitor Unit (MU); for 2 Gy the TPS indicated a value of 184 MU, where MU measures the output of the clinical accelerator, measured by ionization chambers.³⁸ The correct delivery of 1 and 2 Gy, respectively, was also checked placing a piece of EBT3 above the cell arrangement.