Physics Contribution
Implementation of Feedback-Guided Voluntary Breath-Hold Gating for Cone Beam CT-Based Stereotactic Body Radiotherapy

https://doi.org/10.1016/j.ijrobp.2010.08.011Get rights and content

Purpose

To analyze tumor position reproducibility of feedback-guided voluntary deep inspiration breath-hold (FGBH) gating for cone beam computed tomography (CBCT)-based stereotactic body radiotherapy (SBRT).

Methods and materials

Thirteen early-stage lung cancer patients eligible for SBRT with tumor motion of >1cm were evaluated for FGBH-gated treatment. Multiple FGBH CTs were acquired at simulation, and single FGBH CBCTs were also acquired prior to each treatment. Simulation CTs and treatment CBCTs were analyzed to quantify reproducibility of tumor positions during FGBH. Benefits of FGBH gating compared to treatment during free breathing, as well treatment with gating at exhalation, were examined for lung sparing, motion margins, and reproducibility of gross tumor volume (GTV) position relative to nonmoving anatomy.

Results

FGBH increased total lung volumes by 1.5 times compared to free breathing, resulting in a proportional drop in total lung volume receiving 10 Gy or more. Intra- and inter-FGBH reproducibility of GTV centroid positions at simulation were 1.0 ± 0.5 mm, 1.3 ± 1.0 mm, and 0.6 ± 0.4 mm in the anterior-posterior (AP), superior-inferior (SI), and left-right lateral (LR) directions, respectively, compared to more than 1 cm of tumor motion at free breathing. During treatment, inter-FGBH reproducibility of the GTV centroid with respect to bony anatomy was 1.2 ± 0.7 mm, 1.5 ± 0.8 mm, and 1.0 ± 0.4 mm in the AP, SI, and LR directions. In addition, the quality of CBCTs was improved due to elimination of motion artifacts, making this technique attractive for poorly visualized tumors, even with small motion.

Conclusions

The extent of tumor motion at normal respiration does not influence the reproducibility of the tumor position under breath hold conditions. FGBH-gated SBRT with CBCT can improve the reproducibility of GTV centroids, reduce required margins, and minimize dose to normal tissues in the treatment of mobile tumors.

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.

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