Using EBT3 Films and Monte Carlo Simulations to Determine the Percent Depth Dose of a New Y-90 Disc Source for Episcleral Brachytherapy

Abstract

<h3>Purpose/Objective(s)</h3> A novel Y-90-disc beta source was recently developed for episcleral eye plaque brachytherapy. The purpose of this study was to determine the percent depth dose (PDD) of this new source using the EBT3 film measurements and Monte Carlo simulations. <h3>Materials/Methods</h3> Radiographic films of the same batch were calibrated using the 6MV beam of a linear accelerator, and used to measure the dose distribution of this Y-90-disc source in water. Monte Carlo simulations were performed to calculate the in-water dose distribution using the "GATE," a software based on a Monte Carlo simulation toolkit developed by the international OpenGATE collaboration. The simulation codes were first validated with the TG43, ICRU56, and ICRU72 results, and then used to simulate the radiation transport in the Y-90-disc source, its capsule and the surrounding water medium. The standard Y-90 beta spectrum from ICRU 72 was used, and the physics processes for beta and photon interactions with matters were all included. Three-dimensional dose distribution was simulated, from which the PDD curve along the central axis perpendicular to the disk surface was extracted and compared with that obtained from the film measurements. <h3>Results</h3> The measured PDD curve, normalized to the point at 1 mm above the source capsule, agreed well with the simulated PDD curve with an average difference of -0.27±1.5%. Since the eye plaque brachytherapy might be prescribed to a point with a low PDD (e.g., 5%-10%), we further analyzed the agreement in this area. The difference between the measured and simulated PDDs gradually increases for depths larger than 3.5 mm (measured 16.7% vs. simulated 16.6 %). At 5 millimeters (5.9% vs. 4.3%), the simulated PDD is 26.2% lower than the measured, which we believe is still a clinically acceptable agreement. For larger depths, the difference increased quickly to a level that we could not confidently conclude that the measured PDD is accurate. <h3>Conclusion</h3> PDD of this new Y-90 source was successfully measured with the EBT3 films and confirmed by Monte Carlo simulations for depth up to 5 mm from the surface of the plaque. Large discrepancies were observed for depths beyond 5 mm. More studies are needed to confirm the PDD if the treatment needs to be prescribed to a depth larger than 5 mm.