Cone-beam computed tomography imaging in stereotactic body radiotherapy allows for more than target localization

Abstract

Image-guidance plays a crucial role in positioning the target during stereotactic body radiotherapy (SBRT). Despite current limitations, on-line kilo-voltage cone-beam computed tomography (CBCT) also has the potential to identify changes in target volume geometry and the location of important organs at risk. These capabilities could be enhanced by further technical improvements in CBCT, including image quality.

1 INTRODUCTION

Stereotactic body radiotherapy (SBRT) frequently delivers high doses of radiation to tumors located within or near to organs at risk (OAR), some of which are critical, dose-limiting structures. Although image-guidance for SBRT typically focuses on precise target localization [1,2], the benefit of being able to visualize changes in the volume and geometry of the target and the position of critical organs can be inferred from descriptions of SBRT-related toxicity and symptomatic progression during treatment [3,4]. In this report we illustrate how, despite current limitations in image quality, volumetric kilo-voltage (kV) cone-beam computed tomography (CBCT) is able to do more than confirm the position of the target.

2 CASE VIGNETTE: VERTEBRAL COLLAPSE AND TUMOR PROGRESSION DURING SPINE SBRT

A patient was receiving spine SBRT for metastatic disease (Figure 1). This was delivered with volumetric-modulated arc therapy (RapidArc®, Varian Medical Systems Inc., Palo Alto, USA) and on-line CBCT image-guidance (TrueBeam™, Varian Medical Systems). The planning target volume (PTV, volume 142cm³) included a single-level upper thoracic vertebra, the proximal part of a corresponding rib and a dorsally located paraspinal mass. There was tumor extension into the spinal canal. Five fractions of 7Gy were prescribed to the 80% isodose with further dose escalation in the gross tumor volume (GTV). The mean total dose in the PTV and GTV was 42Gy and 48Gy respectively. At the 3^rd fraction partial collapse of the vertebra (Figure 2, thin arrow) and an increase in tumor bulk (Figure 2, thick arrow) were identified on CBCT. An urgent diagnostic MRI scan of the spine confirmed these findings and showed spinal cord compression. SBRT was discontinued and surgical decompression and stabilization were performed while the patient was still ambulant.

Figure 1.

Target lesion (arrow) prior to SBRT as seen on planning CT.

Figure 2.

On-line CBCT demonstrates vertebral collapse (thin arrow) and suggests increased tumor bulk (thick arrow).

3 DISCUSSION

We have illustrated that with a currently available commercial kV CBCT system it is possible to detect clinically significant changes in target geometry during spine SBRT. In other situations on-line CBCT may be able to verify the position of important critical structures, even in challenging locations (Figures 3 and 4). However this is by no means always the case. On-board kV CBCT imaging systems have generally evolved to deliver ever lower doses to the patient rather than with a dominant emphasis on image quality, with the result that this is typically rather less than that offered by diagnostic CT for example [5]. While imaging dose is important, SBRT is being carried out in increasingly complex scenarios, and necessitates different considerations from a conventional treatment delivered over several weeks. Incorrect OAR positioning by no more than 2-3 millimeters during SBRT may result in a substantial discrepancy between planned and delivered dose. Recognition of the importance of high-quality on-line imaging of normal structures as well as the target is one factor driving the development of MRI-guided linear accelerators for hypo-fractionated treatments [6]. Although in-room diagnostic quality CT scanners can be installed, this solution is not in common use whereas the use of gantry-mounted kV CBCT is widespread. Clinical teams should be aware of the possibilities for verifying OAR position and target volume geometry with current generation kV CBCT, however there is a need for future improvements in imaging quality and optimization of clinical imaging routines.

Figure 3.

On-line imaging of vascular structures adjacent to the brachial plexus (arrow) on CBCT during SBRT for an apical lung tumor.

Figure 4.

Confirmation with on-line CBCT that the ureter lies within the planning organ at risk volume (light contour, double arrow) adjacent to the planning target volume (darker contour, single arrow). The patient was receiving SBRT for a para-aortic lymph-node metastasis located adjacent to the hilar region of their remaining kidney.

3 CONFLICTS OF INTEREST

The Department of Radiation Oncology, VU University Medical Center has research collaborations with Varian Medical Systems Inc., USA and Brainlab AG, Germany.

WV has received travel support and honoraria from Varian Medical Systems Inc.

BS has received travel support and honoraria from Varian Medical Systems Inc., and Brainlab AG and has served as a consultant.

MD has received travel support from Varian Medical Systems Inc., and Brainlab AG and honoraria from Varian Medical Systems Inc.