Fiducial markers are gold seeds or stainless steel screws that are implanted in and/or around a soft tissue tumor, or within the bony spine, to act as a radiologic landmark, to define the target lesion's position with millimeter precision. They are typically placed using a CT or other image-guided percutaneous method. There may be other appropriate fiducial placement methods as well, including endoscopic or surgical approaches, if determined best by the participating physicians. To track lesions in 6 degrees (translational and rotational movements), fiducials may be recommended, depending upon the exact circumstance and lesion. Fiducials must be fixed relative to other fiducials and relative to the tumor to ensure targeting accuracy.
Approved Fiducials
Stainless Steel Screw (2.0 x 5.0mm) embedded within bone. Self-drilling and self-tapping screws provide optimal contrast against the bone and minimize migration. These are typically used for spine applications.
Gold Seeds ( 0.8 mm x 5 mm) are typically used for soft tissue lesions
Specific Clinical Situations Potentially Requiring Fiducial Placement
- Spine lesions – In cases where spine fiducials are required, we will normally recommend 3-6 Stainless Steel Screws, placed by a neurosurgeon or interventional radiologist. In some cases, only one screw may be required, and in still other cases, due to an advanced CyberKnife software feature known as X-Sight, there may be no fiducials required. The patient’s attending doctors will individualize the recommended fiducial approach for each CyberKnife spine case. If neurosurgical spine stabilization is required, any required fiducial screws may be integrated into that same surgical procedure, eliminating a separate fiducial placement step for the patient.
- Head and Neck soft tissue lesions – In these cases, 3-6 fiducial seeds may be placed under anesthesia by the participating head and neck surgeon, or percutaneously by a radiologist under CT or other radiologic guidance. Depending upon the initial clinical presentation, it may be feasible for the head and neck surgeon or radiologist to place the CyberKnife targeting fiducials at the time of the patient’s original biopsy, if clinical suspicion is high and potential CyberKnife use is anticipated, avoiding the need for a separate fiducial placement procedure.
- Intrathoracic lesions (e.g. NSCLC) – The majority of Intrathoracic lesions will be lung cancers but the specific fiducial requirement will vary according to the specific clinical presentation.
- Peripheral pulmonary nodules – Typically, these lesions will be targeted using 3-6 transthoracically placed CT-guided fiducials. In certain clinical situations where the nodule is small and the medical risk high, it may be acceptable to place a single fiducial for translational motion tracking only. Because there is a pneumothorax risk with the transthoracic CT-guided approach, this will ordinarily be done in a hospital setting, and the patient observed for a period of time. If no pneumothorax ensues the patient will be discharged the same day and treatment planning will ensue approximately one week later.
- Central or recurrent lesions – Central intrathoracic lesions such as endobronchial or mediastinal presentations may be effectively targeted for fiducial placement bronchoscopically or transesophageally, greatly reducing the pneumothorax risk. Depending upon the initial clinical presentation, it may also be feasible for the pulmonologist or thoracic surgeon to place the CyberKnife targeting fiducials at the time of the patient’s original biopsy, if clinical suspicion is high and potential CyberKnife use is anticipated, avoiding the need for a separate fiducial placement procedure. CT +/- PET based CyberKnife treatment planning will then ensue approximately 7 days after fiducial placement.
- Liver, Pancreatic, Retroperitoneal and Kidney Lesions – These will ordinarily be marked using CT-guided fiducial placement technique, delivering 3-6 seed fiducials in and around the target lesion. If surgical exploration is otherwise indicated then open or laparoscopic fiducial placement may also be accomplished at the time of surgical exploration in lieu of scheduling a separate invasive CT procedure. CT +/- PET based CyberKnife treatment planning will then ensue approximately 7 days later.
- Prostate Cancer – Typically, 3-6 seed fiducials will be placed in the prostate using ultrasound guidance, either transrectally or transperineally, by the urologist or radiation oncologist. CT +/- co-registered MRI CyberKnife treatment planning will then ensue approximately 7 days later.
- Miscellaneous – Invariably, there will arise some sort of tumor situation that is not specifically addressed by any of the fiducial placement protocols described above. In such a case the patient’s participating physicians will decide the most effective fiducial placement and treatment planning approach.
- Technology note – Over time, as the CyberKnife device continues to evolve, improved software and image-processing capability may eliminate the need for fiducials in some cases, though at present, fiducials are required for all non-cranial lesions, except for X-Sight targeted spine lesions.
Key Fiducial Placement Principles
- Implant 3 - 6 fiducials with a minimum of 2.0 cm spacing between fiducials to minimize uncertainty in measuring rotation. They should be placed no more than 5 -6 cm from lesion (20 cm FOV for live images)
- There must be at least a 15° angle between any grouping of 3 fiducials - not collinear - encompassing the tumor volume
- In soft tissue, use gold seeds and place 3 - 6 fiducials around the perimeter of the tumor, 2 -3 cm apart.
- If hardware or other radio-opaque materials are present, place the fiducial above, below or lateral to and not along the axis of the X-ray tracking system (i.e. - Do not place fiducial(s) at or near 45 degrees axial orientation from the potentially obstructing hardware).
Fiducial Migration
Any fiducial migration will degrade the accuracy of fiducial-based targeting. If CyberKnife targeting fiducials migrate more than 1.5 mm from time of CyberKnife CT planning to actual CyberKnife treatment, it may render the patient untreatable without repeating the entire CyberKnife treatment planning process.
As there may be some migration or “settling” of fiducials for up to 7 days following their placement into soft tissue, it is recommended that the CyberKnife planning CT study be obtained approximately 7 days after fiducial placement for soft tissue lesions, to allow them to settle into stable position. For spine lesions, where fiducials are anchored into bone, the planning CT may be accomplished as soon as the following day. Once the CyberKnife planning CT has been accomplished, the time to the actual CyberKnife treatment itself should be minimized, to reduce the probability of fiducial mis-registration between the planning and treatment stage due to additional potential fiducial migration.
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