Near-Field Validation of an I-125 Brachytherapy Lab-on-a-Chip Using Gafchromic EBT-Xd Unlaminated Films In a Novel Seed-through-Film Geometry
Abstract
Purpose
A microfluidic lab-on-a-chip integrating an I-125 brachytherapy seed has recently been developed for radiobiological irradiation of spheroids. This study investigates a novel seed-through-film geometry suitable to validate the dosimetry of this device in the near-field region (i.e., from 1 to 10 mm).
Methods
Gafchromic EBT-XD unlaminated films were irradiated within the microfluidic device. To accommodate the seeds, 3.2 x 2.7 cm2 film pieces were pierced using a needle to create small pinholes, allowing the insertion of I-125 seeds upright inside each film’s plane. Films were then irradiated to ~8 Gy at a radial distance of 1.5 mm and scanned using a flatbed scanner in both transmission and reflection modes to generate 48-bit RGB .tif images. An 80 kV orthovoltage beam provided calibration curves relating dose with pixel values using a rational function fit of all three-color channels. Transverse dose profiles perpendicular to the seed’s longitudinal axis were averaged over all measured films by sampling doses from either all pixels in 2D (averaged over concentric radial bins) or pixels along a few selected 1D angular profiles.
Results
Statistical quantification of the agreement between measured and AAPM TG-43U1 calculated doses was done by evaluating R2 and RMSE values over the entire near-field region. A comparison of the different measurement protocols shows that scanning the unlaminated EBT-XD films in reflection mode and analyzing them with a multichannel method (as opposed to single channel (red)) with full 2D dose sampling yields the best agreement with R2 = 99.2% and RMSE = 0.21 Gy.
Conclusion
The proposed seed-through-film geometry enables the capture of the complete 360° radial dose distribution around the seed at a longitudinal position. This methodology is also suited for other applications where near-field doses are of interest like intravascular brachytherapy and eye plaque treatments.