SU-E-T-276: Radiochromic Film Dosimetry: Solving the Lateral Response Puzzle

Authors


Abstract

Purpose:

A known factor affecting the accuracy of radiochromic film dosimetry is the lateral response artifact induced by off-center placement of film on flatbed scanners. This work reports a practical solution to eliminate such artifacts for IMRT dose QA.

Methods:

EBT3 film calibration strips (3.5×32cm2) were exposed at a depth of 5cm on a Varian Trilogy to a 40×40cm2 open field at six doses between 120 and 900cGy using 6MV photons. A sheet of film was similarly exposed with a large IMRT field. Together with a strip of unexposed film from the same lot the films were digitized at different lateral positions on an Epson 10000XL scanner. The calibration strips covered nearly the full width of the scanner window. Calibration and response data were used to generate the IMRT dose map using multi-channel dosimetry. The scanner response was determined as a function of lateral position of the scanned film. The response at any lateral position L is related to the response at the center, C, of the scanner by Response(C,D) = aL+bL*Response(L,D) where D is dose and the coefficients aL and bL are constant for each lateral location. The coefficients aL and bL were determined by measuring films exposed to different doses at the center of the scanner and various lateral positions.

Results:

Applying this relationship to raw calibration responses, the values at any lateral position can be corrected to an equivalent response as if that film was located centrally. Our study found that calibration curves at the different lateral positions are correlated by a simple two-point re-scaling and thereby the correction could be made.

Conclusion:

The work reported elaborates on the process and demonstrates improvements in radiochromic film dosimetry using this simple procedure to eliminate the lateral response artifact. Acknowledgements: Supported in part by a grant from Ashland Inc.

D Lewis is an employee of Ashland Inc., the manufacturer of radiochromic film; M Chan is supported in part by a grant from Ashland Inc.

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