Fiducials, short for fiducial markers, are tiny reference points placed inside or on the body to help doctors aim radiation therapy or navigate surgery with pinpoint accuracy. Most are small metal seeds, often made of gold, that show up brightly on imaging scans and act as fixed landmarks so treatment can be targeted to exactly the right spot. They’re used most commonly in cancer treatment, where even a few millimeters of misalignment can mean the difference between hitting a tumor and hitting healthy tissue.
How Fiducials Work
The core idea is simple: tumors and soft organs are difficult to see clearly on the imaging used during treatment. Fiducials solve this by giving the treatment system something unmistakable to lock onto. Because they’re made from dense, radio-opaque materials like gold or platinum, they appear as bright, sharp dots on CT scans, X-rays, and even MRI. Before each radiation session, the system takes a quick scan, finds the markers, and compares their position to the original treatment plan. If anything has shifted, the system adjusts in real time.
This matters because your body isn’t perfectly still. You breathe, your organs settle slightly differently each day, and your bladder or stomach may be more or less full than during planning. Without internal landmarks, clinicians would have to estimate tumor position based on external anatomy or organ edges, which introduces significantly more variability and error in daily setup. Studies comparing fiducial-based alignment to organ-edge alignment have found that markers reduce both the inconsistency between different clinicians and the overall positioning error.
What They Look Like
Most fiducial markers are gold seeds roughly the size of a grain of rice or smaller. Dimensions vary by application. Prostate markers, for instance, can be as small as 3 mm long and 0.6 mm wide. Others used in prostate localization measure about 1.2 by 2.0 mm. The gold is biologically inert, meaning it doesn’t react with your body’s tissues or cause an immune response, and it stays visible across multiple imaging types.
In neurosurgery, fiducials take different forms. Invasive markers, typically titanium screws, are anchored directly to the skull before a preoperative scan and stay in place through the procedure. Non-invasive options include adhesive skin markers, dental splints, or simply using anatomical landmarks like bone surfaces. The choice depends on how much precision the procedure demands.
Where Fiducials Are Used
Prostate Cancer
Prostate radiation therapy is one of the most common applications. Typically, three gold seeds are implanted into the prostate gland under ultrasound guidance. The markers form a triangular pattern that lets the treatment system identify both the position and rotation of the gland before each session. A five-year study covering 795 patients confirmed this as a reliable, well-established practice.
Lung Cancer
Lung tumors present a unique challenge because they move with every breath. Fiducials placed in or near the tumor allow stereotactic body radiation therapy (SBRT) systems to compensate for respiratory motion. Usually two or three markers are placed adjacent to or within the lesion. Advanced platforms like the CyberKnife system use a robotic arm that tracks the fiducials in real time during treatment, adjusting the radiation beam as the tumor moves with each breath cycle. Treatment planning often uses 4D imaging, where the fourth dimension is time, to map exactly how the markers move throughout the breathing cycle.
Liver Cancer
Liver SBRT also relies heavily on fiducials. The liver shifts position with breathing and varies in shape depending on factors like stomach contents. Implanted markers provide a consistent, reproducible reference that has been shown to deliver safer and more accurate treatment compared to aligning based on the liver’s outer edge alone.
Brain Surgery
Frameless stereotactic neurosurgery uses bone-anchored fiducial markers to register preoperative scans to the patient’s actual anatomy in the operating room. One commercialized system, the StarFix microTargeting Platform, uses fiducial screws placed into the outer layer of the skull to build a custom tripod guide manufactured specifically for that patient’s anatomy.
How They’re Implanted
For prostate placement, the procedure uses an ultrasound probe and a thin needle (typically 18-gauge, roughly the diameter of a standard blood draw needle). The doctor watches the ultrasound screen to identify the right locations within the gland, carefully positioning each marker with adequate spacing so the treatment system can distinguish them from one another. Key technical details include orienting the needle correctly and confirming it’s fully within the gland before releasing the marker.
For lung tumors, markers can be placed through a bronchoscope (a thin tube threaded through the airway) or via a needle guided by CT imaging through the chest wall. Liver fiducials are similarly placed using CT-guided needles inserted through the skin.
The procedures are generally outpatient. For prostate fiducial placement, Brigham and Women’s Hospital instructs patients to limit activity for 24 hours afterward and to drink extra fluids, at least one glass per hour until bedtime and about two quarts the following day. Some rectal pressure is normal and usually manageable with over-the-counter pain relief. Blood in the urine can occur and typically resolves with increased fluid intake.
Risks and Complications
The most notable risk is marker migration, where a seed shifts from its original position after placement. If a marker moves far enough, it could mislead the treatment system. Reported migration rates in published studies range from 0.7% to 2.7%, though individual institutions have reported rates up to 5%. Migration is uncommon but important enough that treatment teams verify marker positions on pre-treatment scans.
Other possible complications are minor and similar to any needle-based procedure: temporary bleeding, mild discomfort, or localized swelling. Serious complications like infection are rare.
MRI Compatibility
Gold fiducial markers are visible on MRI, CT, X-ray, and portal images, making them versatile across different stages of treatment planning and delivery. On MRI, how clearly a marker appears depends on its shape and its orientation relative to the scanner’s magnetic field, a phenomenon caused by susceptibility effects. Research using both computer simulations and physical testing found that the position distortion caused by these effects is less than 1 mm, small enough to be clinically acceptable. Cylindrical gold markers work well as internal fiducials for MRI-guided workflows.
Liquid Fiducial Markers
A newer alternative to solid gold seeds is liquid fiducial markers, injectable gels or solutions that solidify at the target site. These have several practical advantages: they’re less likely to migrate over time, they produce fewer imaging artifacts on scans, and they’re easier to inject precisely where needed. Because they don’t require endoscopic implantation, they can be a simpler option for patients and a more practical choice in settings with limited procedural resources. Liquid fiducials have been studied for use in pancreatic, bladder, and prostate cancers, and some newer formulations are being designed to combine tumor marking with additional therapeutic functions.