Calibration of GafChromic XR-RV3 radiochromic film for skin dose measurement using standardized x-ray spectra and a commercial flatbed scanner

Abstract

Purpose: In this study, newly formulated XR-RV3 GafChromic film was calibrated with National Institute of Standards and Technology (NIST) traceability for measurement of patient skin dose during fluoroscopically guided interventional procedures.

Methods: The film was calibrated free-in-air to air kerma levels between 15 and 1100 cGy using four moderately filtered x-ray beam qualities (60, 80, 100, and 120 kVp). The calibration films were scanned with a commercial flatbed document scanner. Film reflective density-to-air kerma calibration curves were constructed for each beam quality, with both the orange and white sides facing the x-ray source. A method to correct for nonuniformity in scanner response (up to 25% depending on position) was developed to enable dose measurement with large films. The response of XR-RV3 film under patient backscattering conditions was examined using on-phantom film exposures and Monte Carlo simulations.

Results: The response of XR-RV3 film to a given air kerma depended on kVp and film orientation. For a 200 cGy air kerma exposure with the orange side of the film facing the source, the film response increased by 20% from 60 to 120 kVp. At 500 cGy, the increase was 12%. When 500 cGy exposures were performed with the white side facing the x-ray source, the film response increased by 4.0% (60 kVp) to 9.9% (120 kVp) compared to the orange-facing orientation. On-phantom film measurements and Monte Carlo simulations show that using a NIST-traceable free-in-air calibration curve to determine air kerma in the presence of backscatter results in an error from 2% up to 8% depending on beam quality. The combined uncertainty in the air kerma measurement from the calibration curves and scanner nonuniformity correction was +/- 7.1% (95% C.I.). The film showed notable stability. Calibrations of film and scanner separated by 1 yr differed by 1.0%.

Conclusions: XR-RV3 radiochromic film response to a given air kerma shows dependence on beam quality and film orientation. The presence of backscatter slightly modifies the x-ray energy spectrum; however, the increase in film response can be attributed primarily to the increase in total photon fluence at the sensitive layer. Film calibration curves created under free-in-air conditions may be used to measure dose from fluoroscopic quality x-ray beams, including patient backscatter with an error less than the uncertainty of the calibration in most cases.

Figures

Figure 1

Free-in-air exposed XR-RV3 film calibration setup.

Figure 2

GafChromic XR-RV3 film layers (Ref. 31). Note: The “yellow” polyester layer side of the film actually looks orange.

Figure 3

Reflective density versus air kerma for UW80-M beam quality. Red, green, and blue channels isolated and analyzed separately.

Figure 4

Air kerma versus reflective density of the calibration films exposed free-in-air to the four beam qualities. (a) Orange side of the film facing the x-ray source. (b) White side of the film facing the x-ray source.

Figure 5

Energy dependence of XR-RV3 film versus air kerma. Normalized to UW120-M beam quality film response.

Figure 6

Comparison of the primary GSF 80 (120 kVp) spectrum to the primary plus scatter spectrum incident upon the simulated air at 20 °C and the active layer of the simulated film (the orange side facing the x-ray source) for the on-phantom geometry.

Figure 7

Air kerma versus reflective density of the calibration films exposed free-in-air, plotted together with uncorrected free-in-air air kerma versus on-phantom RD and corrected on-phantom air kerma versus on-phantom RD. (The orange side facing the x-ray source; UW120-M beam.)

Figure 8

Energy deposited in the film layers under free-in-air conditions, sampled in 1 μm thick segments (GSF 80 primary spectra). Primary x rays enter the film from left side of the plot (1 μm) and exit on the right side of the plot (225 μm).

Figure 9

Contour plot of the pixel values of the uncorrected stitched image of the 6 cm×6 cm calibration film exposed free-in-air to 570 cGy air kerma, positioned and scanned separately at 42 different scanner bed locations on the Epson 10000XL scanner.

Figure 10

Measured scanner profiles for each of the ten film darkness levels. The polynomial best-fit curves, excluding the outliers, are plotted as the dashed lines through the data points.

Figure 11

Contour plot of the pixel values of the corrected stitched image of the 6 cm×6 cm calibration film exposed free-in-air to 570 cGy air kerma, positioned and scanned separately at 42 different scanner bed locations on the Epson 10000XL scanner.