A fiducial is a fixed reference point used to ensure precise alignment or positioning. The term comes from the Latin word “fiducia,” meaning trust or confidence, and it shows up across surprisingly different fields: cancer treatment, brain surgery, robotics, and even circuit board manufacturing. In every case, the core idea is the same. You place a known marker in a known location so that a system (a radiation beam, a surgical instrument, a robot’s camera, or an assembly machine) can orient itself accurately.
Fiducials in Radiation Therapy
The most common context where people encounter fiducials is cancer treatment. In radiation therapy, a fiducial marker is a tiny object placed inside or near a tumor so the treatment beam can target it with millimeter-level precision. These markers are typically small seeds or coils, roughly 1 mm wide and 3 mm long, implanted directly into or around the tumor before treatment begins.
The reason they matter is that tumors move. Your liver shifts when you breathe, your prostate shifts as your bladder fills, and even small changes in body position between treatment sessions can throw off targeting. Fiducial markers give the radiation system a reliable landmark to lock onto, session after session. In liver radiation therapy, aligning the beam to fiducial markers produces an average positioning error of just 1.5 mm, compared to 5.3 mm when aligning to the liver’s outer edge. That difference matters when you’re delivering high doses of radiation and trying to spare healthy tissue nearby.
The placement procedure is usually quick. For prostate cancer, you lie on your side, a doctor applies local anesthetic, and uses an ultrasound probe to guide the markers into position. The whole process takes about five minutes. For tumors in the lung or liver, markers are typically placed using CT-guided needles, which is slightly more involved.
What Fiducial Markers Are Made Of
Gold is the most widely used material because it shows up brightly on X-ray imaging, making it easy for the treatment system to detect during each session. But gold has drawbacks. It creates streaky artifacts on CT scans that can interfere with treatment planning, and it’s nearly invisible on MRI.
Platinum is an increasingly popular alternative. It’s equally detectable on X-ray imaging but significantly easier to see on MRI scans. That’s because platinum is weakly paramagnetic (it interacts slightly with the magnetic field), giving it a distinct signal that gold lacks. This makes platinum especially useful when doctors need to fuse CT and MRI images together for planning.
Newer polymer markers, made from biocompatible plastics mixed with radiopaque materials, produce almost no artifact on CT scans while remaining visible enough for daily treatment alignment. A clinical comparison in prostate cancer patients found polymer fiducials preferable to gold for both CT and cone-beam CT imaging because of their minimal artifact and good visibility. Other materials in use include stainless steel, titanium, carbon, and ceramic substances, each with its own trade-offs between visibility, artifact, and tissue compatibility.
Risks of Fiducial Placement
Implanting fiducial markers is a minor procedure, but it’s not risk-free. A study of 357 implantation procedures found that about 77% went smoothly with no complications or marker migration. The overall complication rate was roughly 18%, though most of those (about 13%) were minor, like a small pneumothorax (air leak around the lung) that resolved on its own. Major complications requiring intervention occurred in about 5% of cases.
The risk depends heavily on where the markers are placed. Tumors that move with breathing, like those in the lung or liver, had a 32% complication rate, mostly because a needle has to pass through lung tissue to reach the target. Tumors in areas unaffected by breathing had a complication rate of just 1.2%. Marker migration, where the seed shifts from its original position after placement, happened in about 3.6% of cases.
Fiducials in Surgery
Surgeons use fiducials for a different but related purpose: linking a patient’s physical anatomy to their pre-operative scans. In image-guided neurosurgery, for example, the challenge is matching what the surgeon sees on the operating table to the 3D brain images taken beforehand. Small markers (either attached to the skin or anchored to bone) are placed before the imaging scan, then detected again in the operating room. Software matches the marker positions in both spaces, creating a precise map between the scan and the patient’s actual anatomy.
Bone-implanted markers are especially useful for patients who need multiple procedures or follow-up imaging over time, since they stay in a fixed position and can be relocated using ultrasound without additional imaging scans.
Fiducials in Computer Vision and Robotics
Outside of medicine, fiducials play a central role in how robots and cameras understand physical space. If you’ve seen a square black-and-white pattern printed on a wall in a robotics lab, that’s likely an ArUco or AprilTag fiducial marker. These are standardized visual patterns that a camera can instantly recognize, identify by their unique code, and use to calculate exactly where the camera is relative to the marker.
The process works because each marker has known dimensions and a unique pattern. When a camera detects the marker, software calculates the marker’s position and orientation in 3D space from a single image. This allows robots to navigate rooms, drones to land on platforms, and augmented reality apps to anchor virtual objects to real surfaces. More advanced versions encode 3D shape information into the marker design, letting both standard cameras and depth sensors use the same marker for precise alignment between 2D images and 3D models.
Fiducials in Electronics Manufacturing
Circuit board assembly relies on fiducials in a way that’s conceptually identical to their medical use: providing fixed reference points for a machine that needs to place things with extreme accuracy. On a printed circuit board (PCB), a fiducial marker is a small round copper dot placed on the board’s surface. Pick-and-place machines, which position tiny electronic components onto the board, use cameras to detect these dots and determine the board’s exact orientation before placing anything.
There are two types. Global fiducials sit at the corners of the board (typically three, to give two angular measurements) and tell the machine the board’s overall position and rotation on the assembly line. Local fiducials are placed near individual high-precision components, like processors with hundreds of tiny connection points, allowing the machine to verify alignment right at the spot where accuracy matters most. If the board is slightly skewed when it’s clamped into the machine, the fiducials let the system compensate automatically rather than placing every component slightly off-target.
The Common Thread
Whether it’s a gold seed inside a tumor, a patterned square on a warehouse wall, or a copper dot on a circuit board, a fiducial serves the same fundamental purpose. It’s a known, detectable point that lets a system answer the question: “Where exactly am I, and where exactly is the thing I need to act on?” The precision requirements differ (sub-millimeter in surgery, a few millimeters in robotics), but the principle is always the same. You place a trusted reference point so the system doesn’t have to guess.