Heart-Sparing Capability and Reproducibility of Continuous Positive Airway Pressure (CPAP) in Left-Sided Breast Radiation Therapy

Abstract

<h3>Purpose/Objective(s)</h3> CPAP is a novel method to avoid heart radiation exposure during radiation therapy, recently proposed as an alternative to the current standard-of-care deep inspiration breath-hold technique. In this study, the heart dose, inter- and intrafraction motions in CPAP-based treatment were investigated. <h3>Materials/Methods</h3> A retrospective study on heart dose level and CPAP compliance included 237 left-sided breast cancer patients treated with CPAP-based VMAT in hypofractionation (with (n = 116) and without (n = 105) internal mammary node irradiation (IMNI)) between June 2020 and January 2021. During simulation, CT was obtained with free breathing (FB) and CPAP. For each treatment fraction, CBCT was acquired to confirm geometric accuracy within 3 mm tolerance. Two different datasets were used for reproducibility evaluations. Dataset 1: interfraction reproducibility was evaluated by measuring heart-to-PTV distance (MinHD), defined as the minimum distance between the line posteriorly tangential to the PTV and the heart, on 300 CBCT and 20 planning CT data of 20 women. Using dataset 1, heart-sparing effect was further explored by comparing dosimetric metrics of the whole heart and its substructures (e.g., atrium, ventricles, and vessels) between CPAP- and FB-based VMAT plans. Dataset 2: intrafraction reproducibility was evaluated in 20 women who were treated for different malignancies and underwent 4D CT with CPAP. <h3>Results</h3> Among 237 patients recruited, the CPAP compliance was 93% with a mean heart dose of 1.6 ± 0.7 Gy (no IMNI, 1.1 Gy; IMNI 2.0 Gy). The use of CPAP during treatment did not require additional time resource. The analysis of dataset 1 (i.e., total of 300 CBCT and 20 planning CT images) showed small variations of MinHD (0.6 ± 0.2 cm), demonstrating a reasonable agreement of the heart-to-PTV distance between the planning CT and CBCTs (MinHD difference < 0.2 cm, on average, for all patients). Compared with FB, the use of CPAP significantly reduced the mean heart dose (2.1 to 1.7 Gy), the mean dose to the left ventricle, the maximum dose to the left anterior descending artery, and the mean lung dose by 21%, 23%, 36%, and 23%, respectively. In dataset 2, the breast surface motion amplitude was 0.5 ± 0.5, 2.5 ± 2.0, and 1.8 ± 1.4 mm across the patients in the mediolateral, craniocaudal, and anteroposterior directions, respectively, and exceeded 5 mm for only two patients. <h3>Conclusion</h3> To the best of our knowledge, this is the first and largest study to evaluate the heart-sparing capability and reproducibility of CPAP-based VMAT for left-sided breast cancer treatment. A large population study confirmed a reasonably low irradiation to the heart with CPAP. This heart-sparing effect can be consistent across treatment fractions as demonstrated in sub-centimeter reproducibility of the heart position. For most of the women, the intrafraction breast motion with CPAP was minimal.