Room: Exhibit Hall
Purpose: A nomogram was created to determine required seed activity for Cesium-131 (Proxcelan, Rev2, Isoray Medical Inc., Richland, WA) permanent seed implant (PSI) brachytherapy for recurrent head and neck cancer and meningiomas based on a single academic institutional experience. Single planar implants are prescribed to a point 5mm perpendicular to the implant plane, 5mm from its central seed. A nomogram table for seed activity determination significantly reduces planning time and resources required, compared to conventional planning systems.
Methods: TG-43 source data was used to create a point-source seed model in MATLAB (MathworksÂ®, Natick, MA), using linear interpolation between data points. The total dose delivered to the prescription point was calculated for an individual 1 U seed at each possible seed location in a 16x16cm grid, and for rectangular implant geometries up to 16x16cm. Dividing the prescription dose by the sum of dose contributions from an array of 1U seeds (Gy/U) yields the activity per seed (U) required to deliver the prescribed dose to the prescription point. Implant geometry was re-created in MIM Symphony (version 6.5, Cleveland, Ohio); individual seed and rectangular implant dose contributions were compared to those calculated in MATLAB.
Results: All individual seed contributions varied by <1.5% between MATLAB and MIM, except seeds 8,5 (1.7%) and 8,8 (3.1%) cm from the prescription point. Distant seeds contribute minimal dose to the prescription point, so even large percent difference are small absolute differences. For implants from 1x1 to 16x16 seeds, the largest percent difference was <0.4%, which occurred for 4x2 seed implants or smaller.
Conclusion: A table containing the total dose per U delivered to a prescription point 5mm from the implant plane is provided. It can be used to accurately determine the required seed activity for a planar Cesium-131 implant aiding the preplanning process and/or providing secondary dose verification.