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Automated fiducial marker planning for thoracic stereotactic body radiation therapy

Stereotactic body-radiation therapy (SBRT) has gained acceptance in treating lung cancer. Localization of a thoracic lesion is challenging as tumors can move significantly with breathing. Some SBRT systems compensate for tumor motion with the intrafraction tracking of targets by two stereo fluoroscopy cameras. However, many lung tumors lack a fluoroscopic signature and cannot be directly tracked. Small radiopaque fiducial markers, acting as fluoroscopically visible surrogates, are instead implanted nearby. The spacing and configuration of the fiducial markers is important to the success of the therapy as SBRT systems impose constraints on the geometry of a fiducial-marker constellation. It is difficult even for experienced physicians mentally assess the validity of a constellation a priori. To address this challenge, we present the first automated planning system for bronchoscopic fiducial-marker placement. Fiducial-marker planning is posed as a constrained combinatoric optimization problem. Constraints include requiring access from a navigable airway, having sufficient separation in the fluoroscopic imaging planes to resolve each individual marker, and avoidance of major blood vessels. Automated fiducial-marker planning takes approximately fifteen seconds, fitting within the clinical workflow. The resulting locations are integrated into a virtual bronchoscopic planning system, which provides guidance to each location during the implantation procedure. To date, we have retrospectively planned over 50 targets for treatment, and have implanted markers according to the automated plan in one patient who then underwent SBRT treatment. To our knowledge, this approach is the first to address automated bronchoscopic fiducialmarker planning for SBRT.