Purpose: To evaluate the accuracy of Ir-192 film dosimetry using the calibration curve from 6 MV photon beams, and design a novel method for direct film calibration using the Ir-192 source.
Methods: EBT3 films were calibrated with the 6 MV photon beams on one Truebeam and the Ir-192 source on the GammaMed unit of our institution. We irradiated 18 film pieces (0 to 10 Gy) for each radiation source. All films were scanned 1-day post-irradiation on an EPSON EXPRESSION 10000XL scanner with “Portrait” placement using 48 bits, 150 dpi, and no color correction. Pixel values and calibration curves were acquired from RIT V6.8 using all 3 channels. The irradiated films using the Ir-192 source were reloaded in RIT and a different ROI was chosen to test the accuracy of both calibrations. Moreover, we irradiated an independent test case with the Varian surface applicator (1.5 cm cone) and performed composite analysis between dose maps generated by these two calibrations.
Results: It was found the red channel has the best performance in point dose measurement (on average, 6 MV: 3.3% from the expected value, Ir-192: 0.3%), comparing to green (6 MV: 3.91%, Ir-192: 0.65%) and blue (6 MV: 4.43%, Ir-192: 2.15%) channels. The discrepancy was less significant (~1.82%) in the high dose region (300 cGy~1000 cGy) for red channels as well. With red channel, the composite analysis showed 83.15% passing rate for 1mm&1%, 89.23% for 1mm&2%, and 98.71% for 1mm&3%. The number of points failed only the percent dose difference (%Diff) criteria is found to be 0 when %Diff > 4%.
Conclusion: This preliminary study showed a higher error for Ir-192 film dosimetry if films are calibrated using 6 MV photon beams. We suggest the films should be calibrated directly using the cylinder applicator and Ir-192 source for more accurate film dosimetry.