International Journal of Radiation Oncology*Biology*Physics
Physics ContributionImplementation of Feedback-Guided Voluntary Breath-Hold Gating for Cone Beam CT-Based Stereotactic Body Radiotherapy
Introduction
Stereotactic body radiation therapy (SBRT) 1, 2 has recently gained prominence as a preferred treatment for early-stage lung cancer. In SBRT, a high dose of radiation (50-60 Gy) is delivered in a small number of fractions 4, 5. Surrounding normal tissue is spared by use of tight margins and sharp dose gradients. Tight margins are achieved by the combination of good patient immobilization, daily soft tissue image guidance, and appropriate respiratory motion management. Several studies 1, 3, 4, 5 have reported implementation of SBRT with good patient immobilization and daily image guidance.
Respiratory-induced tumor motion is a limiting factor in the delivery of SBRT to the thoracic and abdominal regions. Various approaches have been developed to actively manage this motion during SBRT, including abdominal compression (6), motion accommodation (the internal tumor volume approach) (7), tumor tracking (8), gating during normal respiration, and breath hold (BH) 5, 9. Wurm et al. (5) reported the feasibility of image-guided free-breathing-gated SBRT by using implanted fiducials and a stereoscopic X-ray imaging system for liver and lung tumors. Nelson et al. (10) first presented the system for feedback-guided BH used in this study. Stock et al. (4) demonstrated that the use of a similar system could improve patient compliance with BH instructions. Duggan et al. (9) described a process of acquiring cone beam computed tomography (CBCT) under repeated voluntary deep inspiration BHs for SBRT treatments.
BH gating is a desirable option for a number of reasons including maximization of separation between the gross tumor volume (GTV) and nearby critical structures and minimization of delivery time compared to free-breathing gating, and the most compelling reason is the ability to perform volumetric verification imaging (9).
In this study, we investigated the combination of deep inspiration and visual feedback guidance 9, 10, 11, 12 to implement feedback-guided breath-hold (FGBH) gating for SBRT treatments. We examined tumor position reproducibility for FGBH-gated SBRT for lung tumors exhibiting motion greater than 1 cm in four-dimensional CT (4DCT). We investigated the benefits of FGBH during treatment planning and the differences in GTV centroids between simulation and treatment.
Section snippets
Visual feedback system
In this study, the respiratory management system (Figs. 1 and 2) was a Varian RPM system (Varian Medical System, Palo Alto, CA) combined with VR goggles (iWear model AV230; Vuzix Co., Rochester, NY) to provide visual feedback to the patient. The RPM system has been described extensively in the literature (13). We have modified the RPM system in our institution so that the monitoring camera is mounted at the end of the couch rather than on the wall (Fig. 2), as is the default arrangement. This
Results and Discussion
Patients who passed our initial screening for FGBH also tolerated the procedure well. The typical time for coaching/training patients to perform FGBH was less than 5 minutes, hence, no special appointment was required to prepare for this procedure.
Conclusions
We have presented a system for treating patients with FGBH-gated SBRT, including multiple FGBH CTs for assessing BH reproducibility and FGBH CBCTs to develop a relationship between the tumor and the external fiducial markers used for gating on each treatment day. The extent of tumor motion at normal respiration does not influence the reproducibility of the GTV centroids under BH conditions. FGBH-gated SBRT with CBCT can improve the reproducibility of GTV centroids, reduce required margins, and
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Conflict of interest: none.