Room: AAPM ePoster Library
Purpose: The Bonvoisin-Gerard esophageal applicators (Elekta Inc.) are manufactured with materials of high x-ray contrast so they can be easily visualized and reconstructed for image-based brachytherapy treatment planning. The Hounsfield Unit of these applicators can be over 3000. However, little study can be found in literature regarding their radiation attenuation characteristics. This study investigated the dosimetric characteristics of these applicators thereby verify the validity of water equivalent dose calculation scheme used in current treatment planning systems.
Methods: The Bonvoisin-Gerard applicators (6, 8, 10, 12mm diameter) were wrapped in air bubble sheets to maintain a straight and central configuration before been fully inserted into an IVB1000 well chamber. Air kerma rate and accumulated radiation dose of 30s exposure were measured for each applicator, with Ir192 source (Flexitron afterloader) dwelling at the same position relative to the well chamber. Same measurement was performed with a LumenCare Azure applicator (5F thin-walled blue plastic catheter). For each Bonvoisin-Gerard applicator, a treatment plan delivering 5Gy to the applicator surface was generated. Dose distributions were measured with EBT3 gafchromic films. Identical plans were also delivered by the LumenCare applicator with 3, 4, 5, and 6mm boluses, approximating the buildup thickness of the esophageal applicators.
Results: Air kerma rate through 6, 8, 10, and 12mm diameter esophageal applicators were 100.6, 98.9, 98.5 and 97.0% of the LumenCare applicator measurement. The 30s accumulated dose were 100.5, 98.9, 98.5, and 96.9% of the LumenCare applicator measurement. Dose distribution of the surface plans displayed good agreement between the two delivery configurations, with the mid-point dose agreeing with 100.3, 95.6, 101.5 and 104.1%.
Conclusion: Although the Bonvoisin-Gerard esophageal applicators present high contrast in x-ray and CT images, their dosimetric characteristics follow water equivalent behavior with <5% difference. The water equivalent dose calculation method is pertinent to these applicators for Ir192 source.