Room: Room 209
Purpose: The purpose of this work is to evaluate the simulated dose conformity and homogeneity delivered by a conformal superficial brachytherapy device for various source configurations. The simulated device is comprised of 37 hexagonally distributed Yttriumâ€‘90 (Y-90) disks, each capable of independent extension to improve contact and coverage of a 3-dimensional target.
Methods: MCNP simulations were performed to obtain Y-90 doses deposited to circular and irregularly shaped targets. Each source (1 mm thick, 1 mm radius) is held by a polyacrylic acid tip affixed to a translatable rod. The maximum diameter of the source array is 26 mm. Variations in dose distributions were investigated as functions of: source holder geometry, asymmetric source configurations, air gap between source and water phantom, and tip wall thickness. Subsequently, to quantify the performance of the device, pseudo planning target volumes (PTV) were defined as the volume within the 40% isodose line for each source configuration with a gap of 1 mm. Finally, the conformity index (CI) and homogeneous index (HI) were calculated, with the optimal homogeneous dose distribution occurring where HI equals zero.
Results: Neglecting effects of the tips, the maximum dose rate of the device with a 1 mm gap and 2 mm gap was 130 mGy/hr/MBq and 110 mGy/hr/Mbq, respectively; decreasing 10 mGy/hr/MBq for each 1 mm gap increase. For a circular PTV and a gap of 1 mm, the CI was 0.91 and the HI was 0.61. For an irregularly â€œCâ€?-shaped PTV and gaps of 1 mm, 3 mm and 5 mm; the CI(HI) values were: 0.86(0.63), 1.04(0.53) and 1.14(0.45), respectively.
Conclusion: Increasing the air gap improves target dose homogeneity; however, it results in increased dose outside the PTV. The CSBT device can deliver conformal doses to irregularly shaped targets at optimal gap distances of 1 mm to 3 mm.