Analysis Of Air Cavity Volume Change In Balloon-Based High-Dose-Rate Accelerated Partial Breast Irradiation

To track and model the decay of air cavity volume for balloon-based HDR accelerated partial breast irradiation (APBI) using fractional or daily CT images. The decay data and model may be useful for future guidance, as a large air volume around the balloon could compromise target coverage. CT images of 26 patients treated with balloon catheter high dose rate brachytherapy HDR APBI at our institution were retrospectively analyzed. The patients were planned on a simulation CT acquired 1-4 days before the first fraction, with initial air volumes <10% of the PTV_eval to ensure a V90>90%. At least one CT image was taken to verify the balloon volume and orientation for each treatment day. The treatments were administered over five consecutive days. Air cavities were contoured for each patient and the volume changes were analyzed to determine if air volume decay over time could be described with a mathematic model. The 26 studied patients had a median initial air volume (in percent of PTV_eval volume) of 2.3% (range 0-6.2%). A mathematic model could not be fitted due to the significant heterogeneity in air volume change across all patients. For 13 patients the air volume decreased monotonically over time. For the remaining 13 patients, the air volume increased during the treatment course for at least one fraction. The results also showed substantial air volume variation for some patients. For one patient, the largest single-day variation was observed at a 5.4% increase (of PTV_eval volume) between the two fractions. For another patient, the air volume continued to increase during the treatment, reaching 4.5% of PTV_eval volume at the last fraction from 0.1% at the initial plan. No clear correlation between the PTV_eval volume and the level of air volume change was found in this study. Table 1. Number of patients for different levels of air volume increase compared with corresponding initial treatment plans.Abstract 2109; Table 1.Largest Air Volume Increase (in Percent of PTV_eval Volume)<0%0-1%1-2%2-3%3-4%>4%Number of Patients1354112Percentage in Total Number of Patients50%19%15%4%4%8% Open table in a new tab No brachytherapy treatment course in this study was terminated or delayed due to an air cavity volume exceeding 10% of the PTV_eval (V90>90%) as most patients had a small initial air volume in our study. If large air volumes are found at simulation, air volume fluctuation during the treatment course may compromise PTV_eval coverage. In selected patients, air volume may need to be monitored with pre-treatment imaging.