APBI Brachytherapy Device Stability and the Importance of Pre-fraction Device QA

Purpose: Recently, several new single-entry, intracavitary breast brachytherapy applicators have emerged with multiple catheters that allow dose modulation to spare normal tissue, such as skin, chestwall, and lung. However, since these catheters are differentially loaded with the HDR source, care must be taken to ensure there is no rotation, change in the device, or change in cavity conformance that could potentially alter the dose modulation from beneficial to detrimental. In this study, we retrospectively assess the rotational stability, strut symmetry, and cavity conformance of the SAVI device. Methods and Materials: Fifty patients treated at UC San Diego with the SAVI device were retrospectively reviewed for the number of patients that needed re-planning as a function of rotation, strut asymmetry (splay), cavity conformance, or a combination of these factors. At our institution, the radiation oncologist surgically places the SAVI device followed immediately by a planning CT scan. A treatment plan is generated immediately after the surgical insertion. Results: Of the 50 patients treated, 4 were re-planned for device rotation, 8 were re-planned due a change in strut symmetry, and 2 for cavity conformance; all within a 24 hour window following surgical insertion of the device. An additional 7 of the 50 (14.0%) were re-planned outside of the 24 hour window, making them unexpected re-plans, primarily due to rotation. Conclusion: The interfraction stability of multicatheter, intracavitary breast devices is crucial for efficacy and patient safety. All cases were planned immediately after insertion since they were inserted by the treating radiation oncologist in the HDR suite. However, our recommendation is to allow 24 hours after device insertion for settling of the device, especially in the case of strut splay. Heeding this advice, 14 of 50 cases presented would not have needed replanning. Obvious strut splay or air pockets necessitate planning to be delayed for 24 hours to allow stable conformance. In practice, with a 24 hour time window between insertion and planning only 14.0% of cases reviewed needed re-planning due to changes in the position or symmetry of the device. All of these cases were re-planned regardless of the amount of change; however, further analysis will evaluate the dosimetric relevance of the re-plan. This study also demonstrates that daily evaluation of the device before each fraction is essential to evaluate rotation, strut symmetry, and cavity conformance.