What this is
A working set of notes on CT-guided fiducial marker placement into deep organs: what the markers are for, how they get there, and the quirks of doing this in the lung, liver, and pancreas. Our cases feed CyberKnife stereotactic radiotherapy, so the marker choice and geometry rules below are CyberKnife’s.
📊 Slide deck: Teaching deck for the team
The Problem These Solve
Picture trying to hit a moving dot on a piece of paper with a laser pointer, except the paper is also being slowly waved up and down by someone breathing on it. That’s the problem the radiotherapy team is trying to solve when they treat a tumour in the lung, liver, or pancreas.
CyberKnife is the radiotherapy system our patients are heading to, a robotic arm that swings around the patient and fires hundreds of tiny x-ray beams converging on the tumour. It’s one of several stereotactic body radiotherapy (SBRT) systems (the MRIdian linac, proton beam units, and gated linacs are the others), all built around the same idea: a very high, very focused dose to a small target. It only works if the machine knows exactly where the tumour is, in real time, while the patient is on the treatment couch breathing.
The trouble is that soft-tissue tumours in deep organs don’t show up well on the daily imaging the radiotherapy machine takes. A tumour in the lung blurs against air. A tumour in the liver sits inside a sea of similarly-coloured tissue. And both move 1 to 3 cm with every breath.
The whole logic, in one line
If you can’t see the tumour clearly enough to track it, put something there that you can see, and let the machine track that instead.
That something is a fiducial marker, a tiny radio-opaque object placed inside or right next to the tumour. CyberKnife takes x-ray images of the patient many times during treatment, picks the markers out by eye (well, by software), and adjusts the robotic arm so the beams stay aimed at the tumour. The tumour is locked to the markers, and the markers are locked to the machine.
The patients sent for this are usually one of three groups: lung tumours (early-stage cancers in people too unfit for surgery, or oligometastases), liver tumours (HCC or metastases not suitable for resection or embolisation), and pancreatic tumours (locally advanced, where chemo and radiotherapy are the only options).
What a Fiducial Actually Is
Most fiducials are about the size of a grain of rice or smaller. Two main shapes turn up in percutaneous practice.
Gold seeds are short cylinders, typically 0.8 to 1.1 mm thick and 3 to 5 mm long. The traditional workhorse. Made of gold because gold is dense (high atomic number), shows up sharply on x-ray, doesn’t react with body tissues, and is safe in an MRI scanner. The downside is gold also scatters x-rays heavily, which creates streaks on the planning CT that can obscure the very tumour you’re trying to treat.
Anchor markers (the Gold Anchor is the brand name you’ll hear) are very thin gold filaments that fold up into a tight ball once they’re pushed out of the needle. They go in through a much thinner needle (25-gauge or 22-gauge) and the folded-up shape catches on the surrounding tissue, which makes them harder to dislodge. Particularly popular for soft, bleed-prone organs like the liver.
Other shapes exist for niche cases (helical gold coils like Visicoil and FlexiCoil that reduce streak artefact, polymer markers like PolyMark and NOVA used in MRI-guided radiotherapy centres), but the two above are what you’ll see day-to-day.
How These Get In — Just the Percutaneous Route, For Now
There are technically a few different ways to get a fiducial into a deep organ. The one our practice does is the percutaneous CT-guided route: a needle goes through the skin under CT guidance, straight into the organ, and the marker is pushed out of the tip. That’s the only route in scope for these cases at the moment, and the rest of this note is built around it.
The other routes — for context, not for now
The radiotherapy literature also describes two other approaches that don’t happen at Radiology Victoria today. Transarterial placement threads a microcatheter up through the femoral artery and deploys platinum micro-coils as fiducials from inside the vessel feeding the tumour, same plumbing as a Liver Tumour Embolisation case. Endoscopic placement uses an EUS scope into the stomach or duodenum for pancreatic markers, or a bronchoscope down the airway for peripheral lung markers. These are worth knowing exist, they’re the obvious directions the service could expand into, but you won’t see them on our list.
The percutaneous route works for all three organs we’d realistically be asked about: lung, liver, and pancreas. The room setup is essentially a more deliberate version of a CT-guided biopsy.
How a Percutaneous Case Unfolds
The room looks much like a CT biopsy. The patient is positioned on the CT table (supine, prone, or on their side depending on where the tumour is) and a planning scan is done. The doctor marks the skin where the needle will go in. Local anaesthetic into the skin and down to the deeper tissues. Light sedation through the cannula is common (usually small doses of midazolam for anxiety and fentanyl for pain), but plenty of these cases are done with just local. Patients need to be able to hold their breath on command, so they can’t be deeply sedated.
The needle setup is the coaxial technique. Imagine threading a long straw through the body wall until its tip is sitting next to the tumour. That’s the introducer needle (usually 17 to 19 gauge for liver, 19 to 21 gauge for lung). Inside that introducer is a thinner stiff wire called a stylet that keeps the needle from clogging while it’s being pushed in. Once the tip is in position, the stylet comes out, leaving an open channel from the skin down to the tumour. The fiducial gets dropped into the hub of the introducer, and a slightly thinner pusher needle slides in behind it and pushes the marker out of the tip. Then the introducer is pulled back a centimetre, repositioned, and the next marker goes in.
Most cases need at least three markers, and usually four to six. The radiotherapy team has geometry rules they need the markers to meet: at least 18 mm apart, with at least 15 to 30 degrees of angular separation between any three markers, and not lined up in a row. The doctor is mentally building a small triangle or tetrahedron inside or around the tumour while placing them. That’s the shape the radiotherapy machine will use to track.
What the doctor is solving for
Three markers in a row gives the tracking software almost no information about rotation. Three markers spread out in a triangle gives it everything it needs. The geometry is doing more work than the markers themselves.
Between placements the doctor takes another CT scan to confirm the marker is where they thought it was, and to plan the next puncture. The contrast agent at this practice for any contrast-enhanced runs is Omnipaque (iohexol).
After the last marker is in, a final CT confirms position and checks for the obvious complications: a pneumothorax in the chest, a bleed in the liver. The needle comes out, a small dressing goes on, and the patient is moved off the table to recovery.
Lung-Specific Quirks
Lung is the most common organ for fiducial placement and the most complication-prone.
The big one is pneumothorax, air leaking into the space between the lung and the chest wall, partially collapsing the lung. Pneumothorax rates after lung fiducial placement run anywhere from 20% to over 50% in published series, depending on technique, needle size, and patient anatomy. About 5 to 10% of patients end up needing a small drain (an intercostal catheter or pigtail). Most of the smaller ones resolve on their own with observation.
Standard post-procedure observation involves chest x-rays, typically supine or prone at the 1-hour mark while the patient is still flat, then upright at 3 hours before discharge. Watching the patient’s oxygen saturation and respiratory rate on the screens is the bedside version of the same surveillance. New shortness of breath or oxygen requirement after a lung case is a pneumothorax until proven otherwise.
What to flag during recovery
Sudden chest pain, dropping sats, increasing respiratory rate, asymmetrical chest movement on inspiration. These mean the doctor needs to be in the room and a follow-up chest x-ray needs to happen now, not at the 3-hour mark.
The lung is also the organ where marker migration is most common. A marker placed near a small airway can be coughed up days later. A marker that ends up in the pleural space (sometimes from the procedure itself, sometimes after a small pneumothorax) won’t track with the tumour any more. The radiotherapy team takes its own planning CT a week or two later and checks the markers are still where they were put. If too many have moved, the patient comes back for more.
Patients are usually fit to go home the same day after a lung case, provided the chest x-rays look clean and they’re comfortable. The discharge advice is the same as for a lung biopsy: rest at home, avoid air travel for a few days, come back urgently if shortness of breath appears.
Liver-Specific Quirks
Liver fiducial placement has a much lower complication rate than lung, around 2 to 5% in most series, mostly small bleeds and pain.
The technical choice is whether to go through the chest wall (intercostal, between the ribs) or below the ribs (subcostal). Intercostal is the most direct route to the right lobe of the liver but risks crossing the pleura, which means a small pneumothorax is possible even though you’re aiming below the lungs. Subcostal avoids the pleura entirely and is preferred where the anatomy allows.
The biggest specific worry is marker migration into the vascular system. The liver is built like a sponge with vessels running through it, and a marker placed too close to a hepatic vein or the inferior vena cava can drift in and travel. There are case reports of markers ending up in the right atrium of the heart after liver placement, usually causing no harm, but obviously not useful for radiotherapy and occasionally requiring retrieval. The way around it is to place markers at the opposite edge of the tumour from the nearest big vessel, and to use thinner-needle anchor markers in vessel-rich areas.
Post-procedure observation is usually 2 hours of bedrest with obs (blood pressure, heart rate, respiratory rate) every 15 minutes for the first hour, every 30 minutes thereafter. If a haemoglobin or coagulation profile was borderline beforehand, a repeat blood test on the way home isn’t unreasonable. Most patients are home the same day.
Pancreas-Specific Quirks
The pancreas is the most awkward organ on the list. It’s deep, surrounded by bowel and major blood vessels, and the tissue itself reacts badly to being poked. Acute pancreatitis is a real risk after any pancreatic intervention. For percutaneous CT-guided placement, the access has to thread carefully between the stomach and bowel loops to reach the tumour. Doable, but fiddly, which is why most international centres lean on the EUS route, and one of the reasons this is the trickiest of the three organs to add to a service.
Antibiotics are routine here, not optional
Pancreatic cases are the one fiducial placement category where antibiotic prophylaxis is standard, usually a single dose before the procedure. The bowel and biliary tract are close enough that a needle track risks introducing infection into the bile system.
Other complications to watch for are mild post-procedure pancreatitis (a small bump in amylase or lipase, mild abdominal pain settling over a couple of days), minor bleeding (rarely needs anything more than observation), and migration of around 4%. Marker visibility on the SBRT planning scan also tends to be slightly worse than for liver or lung because the pancreas sits so deep, which is one reason centres often place six markers rather than three or four.
What the Geometry Has to Look Like
Worth knowing because it’s the reason the case sometimes takes longer than expected. CyberKnife has hard rules for what counts as a usable marker set: minimum 3 markers visible (preferably 4 to 6), at least 18 mm between any two markers, angular separation of at least 15° (often 30°) between any three markers, and no markers lined up in a row. The doctor is mentally building a small irregular triangle (or tetrahedron for four markers) around the tumour while placing them. If a marker drifts or falls out, the CyberKnife planning team flags it and the patient may need an extra one inserted before treatment can start.
What Follows the Procedure
Most patients go home the same day. The follow-up is on the radiotherapy side: a CyberKnife planning CT a week or two later, then the first treatment fraction a few weeks after that. The planning CT is also the moment of truth for marker migration. If too many have shifted, the patient comes back for another set before treatment can proceed.
The markers stay in the body permanently. They don’t dissolve, don’t get rejected, don’t set off airport metal detectors. They show up on future imaging as small dense dots, worth a line in the report at the time of placement so anyone reading old imaging years later knows what the dots are.
Discharge advice is organ-shaped. Rest in all cases. For lung, avoid flying for a few days and return urgently for shortness of breath or chest pain. For liver, no heavy lifting for 24 to 48 hours and return for severe abdominal or right-shoulder pain. For pancreas, complete the antibiotic course and return for severe abdominal pain, fever, or vomiting.
Numbers Worth Knowing
| Stat | Value | Why it matters |
|---|---|---|
| Fiducials commonly placed per case | 3 to 6 | Below 3, tracking fails for rotation; above 6, diminishing returns |
| Minimum inter-marker distance | 18 mm | Closer markers can’t be told apart by the tracker |
| Minimum angular separation between three markers | 15 to 30° | Closer to co-linear and rotation tracking fails |
| Gold seed dimensions | ~0.8–1.1 × 3–5 mm | Size of a grain of rice or smaller |
| Pneumothorax rate after lung fiducials | ~20–50%+ | Most resolve on their own; ~5–10% need a drain |
| Liver fiducial complication rate | ~2–5% | Mostly small bleeds; migration into vessels is the rare standout |
| Marker migration (overall) | ~3–5% | Highest in lung, lowest in liver with anchor markers |
| Time markers stay in the body | Permanent | Gold and platinum are inert — they’re not removed after treatment |
Related
- CT-Guided Spinal Injections — same CT-guided percutaneous setup, different target
- Liver Tumour Embolisation — related liver IR procedure, some patient overlap
Sources
- Fiducial Marker Placement — RadiologyInfo.org
- The fundamentals of fiducial marker placement for image-guided radiation therapy — JVIR
- Percutaneous Implantation of Fiducial Markers for Imaging-Guided Radiation Therapy — AJR
- CT-guided fiducial marker placement for stereotactic radiosurgery — Applied Radiation Oncology
- Fiducial markers for stereotactic lung radiation therapy: review of approaches — PMC
- Percutaneous fiducial marker placement under CT fluoroscopic guidance for SBRT of the lung — PMC
- Preoperative CT-guided Fiducial Marker Placement for Pulmonary Nodules — Radiology: Cardiothoracic Imaging
- Complications associated with percutaneous insertion of fiducial markers in the thorax — PubMed
- Percutaneous fiducial marker placement prior to SBRT for malignant liver tumours — PMC
- CT-Guided Implantation of Intrahepatic Fiducial Markers for Proton Beam Therapy — AJR
- Fiducial marker migration following CT-guided placement in the liver — AME Case Reports
- Cardiac embolisation of an implanted fiducial marker for hepatic SBRT — PMC
- Transarterial Fiducial Marker Placement for Image-guided Proton Therapy for Malignant Liver Tumors — CVIR
- EUS-guided placement of fiducial markers for treatment of pancreatic cancer — PMC
- EUS-guided fiducial marker placement in pancreatic cancer: systematic review and meta-analysis — PMC
- EUS-guided fiducial placement for pancreatobiliary malignancies: safety and antibiotic use — PMC
- Transarterial fiducial marker implantation for CyberKnife radiotherapy in pancreatic cancer — PMC
- Fiducial marker placement for gated radiotherapy in pancreatic cancer: transarterial vs percutaneous — JVIR
- Safety and efficacy of fiducial marker implantation for robotic SBRT with fiducial tracking — Radiation Oncology
- Evaluation of CyberKnife Fiducial Tracking Limitations — PMC
- A phantom study comparing polymer and gold fiducial markers — Scientific Reports
- MRI visibility of gold fiducial markers for image-guided radiotherapy — Radiotherapy and Oncology
Last updated May 18, 2026.