In vivo Dosimetry Using In-House Fabricated 16-Mm EBT4 Radiochromic Film Discs and Imagej Software
Abstract
Purpose
In-vivo dosimetry requires compact dosimeters suitable for patient placement, with multiple detectors often deployed simultaneously for spatial sampling. The 2023 nanoDot OSLD recall left many clinics without accessible, low-cost alternatives. Our purpose is to validate the use of EBT4 radiochromic 16-mm diameter film circles for in-vivo dosimetry and brachytherapy end-to-end testing.
Methods
We punched 16-mm EBT4 film circles using a leather punch and an arbor press. Small circles are more conformal to patient contours and do not present painful, sharp corners. After cutting, film circles maintain an effective measurement area circle >10mm in diameter. We created two calibration curves by irradiating with a 6MV Linac and Ir-192 source, scanning with an EPSON 12000XL scanner, and analyzing with ImageJ software. We validated our calibration curves with 1). 6 MV PDD measurements (d=1,1.5,2,5,10 cm) 2). 6 MV off-axis and out-of-field measurements (r=3,7,12 cm) and 3). Ir-192 end-to-end test (Friedberg Flap to solid water).
Results
6 MV and Ir-192 calibration curves agree (≤5% for 0-6 Gy and ≤10% for 0-10 Gy). PDD measurements, off-axis measurements (up to 2 cm out-of-field), and Brachytherapy end-to-end test results are within tolerance (≤5.5%). Consistent with TG-203, the 7 cm out-of-field measurement is not accurately predicted by the TPS (+13.4%).
Conclusion
EBT4 is relatively energy independent in spectra dominated by lower-energy photons, as extra-focal surface radiation has been shown to approach the ~500 keV range as distance from the field edge increases. We demonstrate that 6 MV calibration of 16-mm EBT4 film circles is acceptable for Ir-192 (380 keV), and, therefore, it is also acceptable for out-of-field 6 MV linac exposures (~500 keV). Uncut and cut films behave similarly. Ongoing work includes validation for 15 MV photon, electron, and low-dose-rate brachytherapy.