A study on the use of Gafchromic™ EBT3 film for output factor measurements in kilovoltage X-ray beams

Abstract

Relative output factors are used in radiation therapy for treatment planning purposes including treatments using kilovoltage X-ray beams. The output factor is the relative dose output for a particular applicator relative to a reference applicator. Due to the differences in the scatter contribution from the inside of an applicator, it is more accurate if output factors are measured for all combinations of X-ray beam energy and applicator. Previously published papers and various kilovoltage X-ray beam dosimetry protocols have discussed the validity of using various cylindrical and parallel plate ionization chambers for relative output factor measurements. In this work, we evaluated the suitability of Gafchromic™ EBT3 film for the determination of output factors for kilovoltage X-ray beams. Output factors were measured with Gafchromic™ EBT3 film for beam qualities of 50, 75, 100 and 125 kVp and applicator sizes of 2, 3, 4 cm diameter, 8 × 8 and 12 × 12 cm(2) square applicators. The film read out was performed with a flatbed EPSON Expression 10000XL scanner. The measured data was compared with benchmark data from measurements using an Advanced Markus ionisation chamber as well as comparing with ratios of published backscatter factor values. The agreement between output factors measured with EBT3 film and the ionisation chamber was generally better than 2 %, with the largest difference of 3.3 % occurring for the 2 cm diameter field with the 50 kVp X-ray beam. These differences were consistent with the estimated total uncertainty in the measurements as calculated by the ISO GUM. The agreement between the output factors measured with film and the published BSFs was generally better than 5 % but differences of up to 12 % occurred for the smallest field size. The results demonstrate that Gafchromic™ EBT3 film is a suitable dosimeter for output factor measurements of kilovoltage X-ray beams.

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“…Our dose output measurements using films are only 1–2% off from the calibrated ionization chamber measurements. The agreement is similar to the level reported in literature on the use of EBT3 films for kV energies 29 …”

Monte Carlo dosimetry modeling of focused kV x‐ray radiotherapy of eye diseases with potential nanoparticle dose enhancement

“…The differences in the dose-response of EBT3 films between kV and MV had been reported to be 3%-10%. 21,22,[56][57][58] Larger difference for low kV was reported by Villarreal-Barajas et al 59 However, the authors also noted that their results are larger than most other studies and suspected the continuous energy spectrum of the source and different film composition of the film batches they used. The energy spectrum of the source we used (Fig.…”

“…As a result, a calibration curve between the absolute dose (to water) and film intensity was established. The rationale is that EBT3 films have shown minimal energy dependence over a wide energy range from 6 MV to 50 kVp and 21 keV (Pd‐103) …”

Off‐axis dose distribution with stand‐in and stand‐off configurations for superficial radiotherapy treatments

“…The intention of these measurements was to independently verify that the GAFChromic EBT‐3 film is suitable for use with kilovoltage photons. The GAFChromic EBT‐3 film is already backed up in the literature as suitable for use with kilovoltage photons. The PTW LA‐48 array measurements were taken under 5 mm of liquid water in a PTW MP3 Water Tank (PTW GmbH, Freiburg, Germany), allowing a 1‐mm spatial resolution despite the 8 mm spacing between each chamber of the array.…”