Dosimetric impact of respiratory motion on proximal bronchial tree during lung cancer stereotactic body radiation therapy: A patient-specific phantom study

Introduction: The respiratory motion of the proximal bronchial tree (PBT) adjacent to the tumor is not taken into consideration in the treatment plan, and there is a risk that the PBT may be irradiated with a higher dose. Therefore, this study aimed to measure the PBT dose directly using patient-specific phantoms of patients treated with SBRT for a centrally located lung tumor and to determine the dose variation with the respiratory motion of the PBT. Materials and methods: This study included 10 patients who received SBRT for central lung tumors. We utilized patient-specific tumor phantoms fabricated with a three-dimensional printer using the patient's computed tomography (CT) data to analyze the dose variation in PBT owing to respiratory motion. Two SBRT plans were generated: a full-phase plan and a 30-70 gating plan. Dose measurements were performed using glass dosimeters. We measured the dose three times for each plan, with the phantom either in motion or fixed at the 50% phase. Results: We analyzed the target volumes and measured doses for each SBRT plan. Compared with the gross tumor volume on the 50% phase CT image, the internal target volume was larger by approximately 13.1% and 45.9% during the 30-70% phase and full phases CT images, respectively. Similarly, the planning target volumes increased by 9.3% and 34.4%, respectively. The average variations in the measured dose of the glass dosimeter owing to patient-specific motion were 3.85% for the full-phase plans and 1.92% for the 30-70 gating plans. Conclusion: Respiratory motion can affect the absorbed dose of the PBT, and this effect is particularly pronounced when the PBT is near the tumor. The gating method reduces the degree of dose variation. This study highlights the importance of understanding the respiratory motion of the PBT in SBRT for centrally located lung tumors.