Developing a Simple Commissioning Protocol for Leipzig Brachytherapy Surface Applicators Using Only Radiochromic Film
Abstract
Purpose
To develop a simple protocol for dose verification of Leipzig surface HDR applicators using Gafchromic EBT3 film.
Methods
Although radiochromic film is considered to display an energy independent response to high energy photons, calibration measurements were taken using both a 6MV linear accelerator beam and an Iridium-192 HDR source. The 6MV irradiations were performed with a 10x10cm field size at 100SSD and under 10cm solid water with backscatter. Iridium-192 irradiations were performed with four dwell positions in a Freiburg flap applicator above a solid water phantom. The assembly was scanned in a CT simulator and expected dose at a reference point centered between the dwell positions was calculated with the TG-43 algorithm. Film was scanned after 24 hours and red channel pixel value to dose calibrations were generated by fitting a 6th-order polynomial and a rational function of the form y=a+b/(x-c) to each dataset. Leipzig measurements were also performed using solid water, with the film under a 3mm sheet, to verify 30, 35, 40, and 45mm applicators. Four dose points were measured: 1, 3, 5, and 7Gy.
Results
When fit to the same calibration dataset, the rational function and polynomial fits represented essentially equivalent curves. However, the Iridium-192 and 6X calibrations diverged strongly with increasing dose. At a pixel value of 15000, the iridium curve reached 885cGy, while the 6X curve computed 710cGy. Applicator film dose was calculated pixel-wise, averaged over a 1cm diameter ROI, and compared to average treatment planning dose over a similar contour. Using the 6X rational function calibration, the average film difference from TG-43 dose was -4.96%+/-4.44% (standard deviation). The 7Gy dose point and 40mm cone showed distinctly better agreement than the others.
Conclusion
Further investigation is necessary to develop a film dosimetry protocol which responds accurately and linearly to surface applicator dose.