TITLE:
Is It Possible to Publish a Calibration Function for Radiochromic Film?
AUTHORS:
Maria F. Chan, David Lewis, Xiang Yu
KEYWORDS:
Dosimetry; Quality Assurance; IMRT; Radiochromic Film; Multichannel
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.3 No.1,
January
29,
2014
ABSTRACT: Purpose: To assess the possibility of using a public
calibration function for radiochromic film dosimetry in dose QA of highly
conformal treatment plans. Methods: EBT3
film calibration strips (3.5 × 20 cm2 from lots A101212 and A011713) were exposed on a Varian Trilogy at a facility
to a 10 × 10 cm2 open field at doses of 80, 160, 320 cGy using 6MV photons. Together with a
strip of unexposed film from the same lot, the exposed films were digitized in a single scan using different
Epson 10,000 XL scanners at two different facilities. The
dose-response data for each color-channel from each facility were generated
using the same calibration function X(D) = a + b/(D - c), where X(D) is the response at dose D and a, b and c are the
coefficients. Different batches of EBT3 film were exposed to a VMAT beam. These
films, plus two reference strips exposed to doses of zero and 160 cGy, were digitized
on the scanners at the two facilities. Using the multi-channel dosimetry method
and One-scan protocol (Med Phys, 39: 6339-6349, 2012) the recorded doses on the VMAT films were calculated and
the results were compared with the VMAT plan using a Gamma index of 3%/3 mm. Results:
The passing rates obtained for dose maps calculated for all combinations
of VMAT images and calibration functions were nearly unchanged, using the
One-scan protocol. Also, in all cases a passing rate of >99% was obtained
for Gamma index of 3%/3 mm. On
the other hand, if the One-scan protocol was not employed, the dose maps for
VMAT images and calibration functions from different scanners showed poor
correlation with the treatment plan. This is probably due to the scan-to-scan
variability. Conclusions: The
authors have found that it is feasible to use a public calibration function for
a given radiochromic film lot using the same methodology, One-scan protocol,
for patient-specific QA.