Deep-Inspiration Breath-Hold Stereotactic Body Radiation Therapy by Combining Spirometer-Guided Breath-Hold and a Real-Time Tumor Tracking System: A Novel Approach

Abstract

There are several methods used against respiratory motion (RM). Expiratory breath-hold (BH) is considered more stable and reproducible than inspiratory BH; therefore, BH with spirometry is often used for expiration. The real-time tumor tracking radiotherapy (RTRT) system is a highly effective method for reducing the margin of RM. This system ambushes and irradiates tumors during the expiratory phase when tumors move slowly. Although these methods usually involve expiration, it is advantageous to expand the lungs with inspiration to reduce the risk of adverse events. Here, we developed a new approach of performing stereotactic body radiation therapy (SBRT) under deep-inspiration BH (DIBH) by combining these two methods. Lung tumors with respiratory motion ≥ 1 cm were included. Three or four fiducial markers were placed near the tumor via bronchoscopy. DIBH CT (CT-IN) was performed under the guidance of spirometer. The PTV was obtained by adding a 5-mm margin to the GTV delineated on CT-IN. The prescribed dose was 42 Gy in four fractions for the D95 of the PTV. An error of 2.0 mm around the planned position of the fiducial marker on CT-IN was permitted along each orthogonal axis as a gating box. In preparation for cases in which the reproducibility of DIBH is low and treatment cannot be performed, light expiration BH CT (CT-EX) was also performed, and a radiotherapy plan was prepared for the conventional RTRT system so that it could be switched at any time. Lung volumes and doses (mean dose, V20 Gy, V10 Gy, and V5 Gy) on CT-EX and CT-IN were compared. Five patients underwent SBRT with DIBH, and all completed the planned irradiation course. The median treatment time per fraction was 27.86 min (range, 25.5-40.6). Four tumors were located in the left lower lobe and one in the right lower lobe. The median volume of PTV was 12.4 (range, 5.2-26.2) mL. The lung volumes and doses on CT-EX and CT-IN are shown in the Table. The lung volume on CT-IN was 1.6 times larger than that on CT-EX. The PTV-to-lung ratio on CT-IN was significantly lower than that on CT-EX. V20 Gy and V10 Gy on CT-IN were significantly lower than those on CT-EX. SBRT with DIBH was achieved by combining the spirometer and RTRT system. This can help to eliminate concerns about reproducibility and high-speed tumor movement during inspiration, which are weaknesses of spirometer-guided breath-hold and the RTRT system, respectively, while ensuring the accuracy of the RTRT system. DIBH SBRT is a promising method that can reduce lung dose.