Comparison of Film Scanner and Single-Point Transmission Densitometer EBT4 Radiochromic Film Dose Calibration
Abstract
Purpose
Film is typically read using a flatbed scanner, but these scanners are prone to Type A errors (orientation, placement, etc.). We compare dose calibration of EBT4 radiochromic film from 6MV exposures using a flatbed film scanner and a single-point transmission densitometer for the purpose of measuring point doses for in-vivo dosimetry.
Methods
16 mm discs were cut from sheets of EBT4 film for potential use as in-vivo dosimeters because they may be more conformal to skin contours and present no sharp corners to the patient. The film disc formats were calibrated in solid water with incremental doses up to 10 Gy from 6MV exposure. Dose calibration curves were obtained by a) digitization to pixel values with an EPSON 12000XL using a one scan technique to minimize Type A errors in the uncertainty budget analysis and b) point measurements on an X-rite Model 331 transmission densitometer with a 3d printed centering jig for reproducibility of aperture placement. Least square fits of readings versus dose data were obtained and compared. The uncertainty budgets and general procedures of each type of measurement were also compared.
Results
The uncertainty budget and procedure involved with use of the densitometer measurements are simpler. Acceptable 3rd order polynomial least square fits were created for both plots. The densitometer calibration curve (R-sq.=0.9998) and the scanner calibration curve (R-sq.=0.9995) are on different scales, but both curve fits were well fit.
Conclusion
For simple IVD-related point dose measurements, single-point transmission densitometers exhibit acceptable dose measurement accuracy and may be effectively used to streamline clinical workflows. The added complexity from type A uncertainties and physical workspace inefficiency of flatbed film scanner systems with their concordant PC requirement may be avoided by the use of simple densitometers.