Practical Assessment of Bronchoscopically Inserted Fiducial Markers for Image Guidance in Stereotactic Lung Radiotherapy

IntroductionStereotactic radiotherapy is a high-dose precision technique necessitating accurate target visualization through either cone beam computed tomography (CBCT) or planar imaging with implanted fiducial markers. We have investigated the properties for image guidance using fiducial markers implanted through minimally invasive bronchoscopy.MethodsTwo fiducial marker types were implanted endobronchially in 10 patients undergoing radical radiation treatment for non-small cell lung cancer (eight using Visicoil linear fiducial markers [IBA Dosimetry GmbH, Schwarzenbruck Germany] and two using superDimension and superLock two-band markers [Covidien Inc., Minneapolis, MN]). Patients underwent four-dimensional computed tomography imaging for treatment planning and after completion of treatment to investigate marker movement. As part of the image guidance assessment, megavolt electronic portal images (EPIs) were acquired in addition to kilovolt planar and CBCT (Varian Medical Systems, Palo Alto, CA) images.ResultsIn two of 10 patients (both receiving Visicoil markers), marker migration was observed before treatment. In patients with stable markers, both types were clearly visible in planar kilovolt imaging; however, in EPIs the markers could be detected only in selected beam directions in which bony interference was minimal. Diagnostic computed tomography scanning was able to demonstrate the markers with clarity, but significant starring artifacts were observed in CBCT. This was particularly problematic in patients with some lateral component of tumor motion during breathing.ConclusionsThe potential for fiducial migration must be considered and investigated if bronchoscopic implantation of fiducial markers is performed. The choice of marker is a compromise between trying to minimize CBCT artifacts while enabling visualization in EPI imaging, which is an ideal tool to verify gated radiotherapy delivery.