Purpose: Radiochromic film provides excellent spatial resolution, which is advantageous for brachytherapy sources, however energy dependence poses a challenge in low energy film dosimetry. Rather than measuring a correction factor from an external megavoltage source, a novel technique was developedto calibrate film directly with low-energy sources including I-125 and Cs-131 (Eavg= 28.5 and 30.4 keV, respectively). This method circumvents the process of measuring a correction factor and removes the uncertainty of energy dependence. In this work a custom phantom was designed to create a precise and reproducible calibration curve using low energy sources.
Methods: A small slot was machined into the surface of a Solid Water® HE piece (Gammex, 30cm x 30cm x 1cm) to fit a brachytherapy seed (length: 4.5mm, radius: 0.4mm). An IsoAid Advantage I-125 or an IsoRay Cs-131 source could be placed in the slot, level with the Solid Water surface. Film was placed at 5 mm from the seed plane. 8-10 dose exposures over the range of 50-800 cGy were made to the film. Doses to a point were calculated using TG-43 formalism. Film was read with a flatbed scanner (Epson Expression 11000XL) and analyzed with ImageJ. A small region of interest (5x5 pixels) was selected around the highest dose to represent the point dose where dose was calculated to. A calibration curve was established that converts net optical density (NOD) to dose.
Results: A fourth-degree polynomial was used to fit each calibration curve. Both I-125 and Cs-131 calibration curves fit the dose-NOD relationship well, with R²=0.999 and 0.987, respectively. When accounting for statistical and systematic errors, the estimated uncertainty is less than 6%.
Conclusion: A custom-made phantom can reduce distance errors and provide an accurate calibration curve for low energy brachytherapy sources and Radiochromic film without the need to use an external beam source.