C Oare, A Alshreef*, C Ferreira, University of Minnesota, Minneapolis, MN

• 
  A Alshreef

PO-GePV-T-40 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

ePoster Forums

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.

Keywords

Film, Low Energy X-rays, Calibration

Taxonomy

TH- Radiation Dose Measurement Devices: Radiochromic film

Contact Email