Cone-Beam Computed Tomography for On-Line Image Guidance of Lung Stereotactic Radiotherapy: Localization, Verification, and Intrafraction Tumor Position

Purpose: Cone-beam computed tomography (CBCT) in-room imaging allows accurate inter- and intrafraction target localization in stereotactic body radiotherapy of lung tumors.

Methods and Materials: Image-guided stereotactic body radiotherapy was performed in 28 patients (89 fractions) with medically inoperable Stage T1-T2 non–small-cell lung carcinoma. The targets from the CBCT and planning data set (helical or four-dimensional CT) were matched on-line to determine the couch shift required for target localization. Matching based on the bony anatomy was also performed retrospectively. Verification of target localization was done using either megavoltage portal imaging or CBCT imaging; repeat CBCT imaging was used to assess the intrafraction tumor position.

Results: The mean three-dimensional tumor motion for patients with upper lesions (n = 21) and mid-lobe or lower lobe lesions (n = 7) was 4.2 and 6.7 mm, respectively. The mean difference between the target and bony anatomy matching using CBCT was 6.8 mm (SD, 4.9, maximum, 30.3); the difference exceeded 13.9 mm in 10% of the treatment fractions. The mean residual error after target localization using CBCT imaging was 1.9 mm (SD, 1.1, maximum, 4.4). The mean intrafraction tumor deviation was significantly greater (5.3 mm vs. 2.2 mm) when the interval between localization and repeat CBCT imaging (n = 8) exceeded 34 min.

Conclusion: In-room volumetric imaging, such as CBCT, is essential for target localization accuracy in lung stereotactic body radiotherapy. Imaging that relies on bony anatomy as a surrogate of the target may provide erroneous results in both localization and verification.

Introduction

The importance of target localization in stereotactic body radiotherapy (SBRT) cannot be overstated, because high doses are delivered in a limited number of treatment fractions. Targeting errors are often unrecoverable in this context. Therefore, the major focus in SBRT has been the development of stereotactic body immobilization devices and image-guidance systems for target localization. The stereotactic body frame (SBF) provides stereotactic localization when in-room volumetric imaging is not available (1, 2, 3, 4, 5, 6, 7). Novel image-guidance systems have also been used in lung SBRT. Uematsu et al. (8) have pioneered this approach by incorporating a linear accelerator and computed tomography (CT) scanner with a common couch in the treatment room. Onishi et al. (9) developed a CT on rails system that also allows gating using a patient self-breath hold device. Similar to the image-guidance system presented in the present study, these systems provide excellent target visualization with volumetric imaging and the ability to assess tumor motion. Imaging-guidance systems that rely on fluoroscopic imaging of implanted fiducial markers for target localization and tracking have also been used in lung SBRT (10, 11).

Although the clinical data for SBRT are still maturing, the investigators using these image-guidance systems have contributed much of the current clinical lung SBRT data. Their results have demonstrated excellent local control using various dose-fractionation schedules (10, 12, 13, 14, 15).

Conventional linear accelerators have also been used to acquire volumetric megavoltage (MV) images for target localization in the treatment room, although these have not been specifically applied to lung SBRT. MV cone-beam CT (CBCT) has been used for treatment verification by acquiring imaging portals over a partial gantry rotation (16). The acquired two-dimensional (2D) projections are reconstructed using standard CBCT algorithms to generate volumetric images, which provide improved visualization of low-contrast structures. Similarly, the MV treatment beam has been used to acquire CT images on the treatment unit for localization of thoracic lesions treated with single fraction SBRT (17). The slice thickness of the CT images was 2 cm, which correspond to the width of the multileaf collimator leaves.

In this report, we present an image-guided approach for lung SBRT using kilovoltage (kV) CBCT for target localization. We retrospectively matched all localization imaging data sets to the planning CT data sets using bony anatomy to determine the applicability of the vertebrae as a surrogate for tumor position. In addition, we assessed residual errors after target localization and the intrafraction reproducibility of target position using repeat CBCT imaging.