TU-F-12A-08: Verification of APBI Balloon Integrity Using Ultrasound Imaging

Abstract

Purpose: An overview of the ultrasound imaging artifacts encountered in Mammosite balloon APBI is presented. The goal was to determine whether clinically relevant changes in balloon size resulting from leakage can be detected by ultrasound imaging and also to compare the magnitude of size variations using both ultrasound and CT imaging. Methods: Ultrasound imaging of a Mammosite balloon phantom was performed to better understand measurement errors and accuracy. The dose to the prescription point as a function of balloon diameter was computed for different sized balloons. The results were compared to phantom measurements of balloon diameter versus filling volume to assess the dose change that would result from tissue moving inward with a shrinking balloon boundary. In APBI patients undergoing a course of 10 treatment fractions, an assessment was made of the variability and accuracy of balloon size measured with ultrasound imaging compared to CT. Results: Ultrasound artifacts combine to form a false image of the distal balloon boundary. Proper US probe orientation and choice of measurement point locations improves distance measurement accuracy. A 1 mm change in balloon diameter is measurable with ±0.1 mm error; this 1 mm change corresponding to <4% change in dose at a distance 1 cm from the balloon. In patients, the implanted balloon diameters ranged from 3.4 to 5.8 cm. Measurement errors relative to CT averaged less than 1.4 mm and variability (standard deviation) over the course of treatment averaged 1.9 mm. Conclusion: Complex and misleading image artifacts can be minimized by proper probe orientation, and accurate balloon diameter measurement requires consistent image analysis methods. Ultrasound imaging can detect dosimetrically relevant changes in the size of a leaking Mammosite balloon, therefore ultrasound imaging can be used for verification of balloon integrity over the course of APBI treatment.