Author: Ruiming Chen Edmondson, Darwin Abraham Garcia, James A. Kavanaugh, Joseph John Lucido 👨🔬
Affiliation: Mayo Clinic 🌍
Purpose: Characterize the dose response and reproducibility of a low-dose film for 6 MV photon beam and assess the film’s energy dependence between kV and MV energies.
Methods: A single 8’x10’ LD-V1 Gafchromic film was cut into 20 2’x2’ strips. The film strips were organized into 5 groups of 4 to minimize dose uncertainties due to each strip’s positioning within the original film sheet. Each group of films were irradiated with 6 MV to 0, 5, 10, 20, and 40 cGy. Scans were repeated 5 times on a flatbed scanner 24h after irradiation. The red channel of an ROI taken at film center was used for analysis. Additional measurements were made for select known doses with 10MV FFF and four orthovoltage energies. Two independently irradiated test films of 5 cGy and 10 cGy were used to assess dose determination accuracy of the 6 MV calibration curve.
Results: For the 6 MV films, the pixel-wise coefficient of variation (CoV) within center ROI was less than 0.01 for all dose levels. Maximum scan-to-scan variation for any film was 0.05%. Inter-film CoV between 4 films of same dose levels was 0.02 or less, with the largest difference of 4.5% at 0 cGy. For the test films, measured dose was 5.8 cGy at 5 cGy and 10.0 cGy at 10 cGy. There is substantial signal response difference between kV and MV beam energies; for 10 cGy irradiation, red channel net OD is 26.8% higher for 100 kVp compared to 6 MV.
Conclusion: LD-V1 exhibits low pixel-wise, scan-to-scan, and inter-film variation. The difference in dose response with MV versus kV energies demonstrates the need of independent characterization of MV energy film response. Due to the lower dose range, characterization of relevant dependencies such as scanning direction, scanning uniformity, and post-irradiation signal change, may be necessary.