Y90 mapping is a planning procedure performed one to three weeks before Y90 radioembolization, a targeted radiation treatment for liver tumors. During mapping, a doctor threads a thin catheter through an artery to your liver, maps out the blood vessel anatomy, and injects a tracer particle that mimics how the actual radiation microspheres will travel. The results determine whether you’re a safe candidate for treatment and how your radiation dose should be calculated.
Think of it as a detailed dress rehearsal. The mapping session identifies potential problems, like blood vessels that could carry radiation to your stomach or lungs, so those issues can be fixed before the real treatment takes place.
What Happens During the Procedure
Y90 mapping is performed in an interventional radiology suite, typically under moderate sedation. For most patients, the doctor accesses the arterial system through the femoral artery in the groin, though wrist access through the radial artery is an emerging alternative. A catheter is guided through your blood vessels and into the hepatic artery, the main artery supplying the liver.
Once the catheter is in position, the doctor injects contrast dye and takes detailed X-ray images (called an angiogram) of every artery feeding the liver and surrounding organs. This vascular roadmap serves three purposes: it shows the doctor exactly where to deliver the Y90 microspheres on treatment day, it reveals unusual anatomy that could complicate delivery, and it identifies small branch arteries that feed nearby organs like the stomach or intestines.
If the doctor spots arteries that could accidentally carry microspheres to the gastrointestinal tract, they may block those vessels during the same session using tiny metal coils. This preventive step, called coil embolization, is one of the most important safety measures in the entire process. Without it, stray microspheres could cause serious ulcers in the stomach or duodenum.
The Tracer Injection and Scan
After the vascular anatomy is mapped and any problematic arteries are coiled, the final step is injecting a radioactive tracer called Tc-99m macroaggregated albumin (commonly referred to as MAA). These are tiny protein particles labeled with a low-dose radioactive tag. They’re roughly the same size as Y90 microspheres, so they flow through your liver’s blood vessels in a similar pattern.
The key assumption behind the entire mapping procedure is that the MAA particles will distribute the same way the Y90 microspheres will on treatment day. This isn’t always a perfect match, but it gives doctors the best available preview of where radiation will end up. Within about 30 minutes of the MAA injection, you’ll undergo imaging of your chest and abdomen so the medical team can see exactly where the tracer landed.
Why Lung Shunt Fraction Matters
One of the most critical measurements from the mapping scan is the lung shunt fraction, which tells doctors what percentage of the tracer particles passed through the liver and ended up in the lungs. Some degree of shunting is normal because the liver’s blood vessels connect to the lung circulation. But if too many microspheres reach the lungs during the actual treatment, they can cause radiation-induced lung inflammation, a condition called radiation pneumonitis.
The safety threshold generally used is a lung dose of 30 Gy per treatment session, or 50 Gy over a patient’s lifetime. When the lung shunt fraction exceeds roughly 20%, the estimated lung dose often crosses into unsafe territory. At that point, the treatment team may recommend a procedure to reduce the shunt fraction before moving forward, or they may suggest an alternative treatment entirely, such as chemoembolization or surgery.
Interestingly, how the imaging is done affects the numbers. Traditional flat (planar) imaging tends to overestimate lung shunting compared to more advanced 3D imaging called SPECT/CT. In one study, the average lung shunt fraction measured by planar imaging was about 8.3%, while SPECT/CT put it at 3.3% for the same patients. The discrepancy was even larger in patients whose planar scans showed shunt fractions above 20%. This matters because an overestimated shunt fraction could unnecessarily disqualify someone from treatment or lead to an unnecessarily reduced radiation dose.
How Mapping Data Shapes Your Dose
The images from your mapping scan directly feed into the math used to calculate how much Y90 activity you’ll receive. Two main dosing approaches exist, and they use the mapping data quite differently.
The simpler method, called the body surface area (BSA) model, calculates your dose based primarily on your body size and the percentage of liver occupied by tumor. It can account for lung shunting by reducing the dose, but it doesn’t distinguish between how much radiation the tumor receives versus how much healthy liver tissue absorbs. This limits how personalized the treatment can be.
The more detailed approach, called the partition model, relies heavily on the SPECT/CT images from your MAA scan. It separates the liver into three compartments: tumor tissue, normal liver tissue, and lungs. By analyzing how the tracer distributed across these zones, doctors can calculate the expected radiation dose to each one independently. This allows them to push the tumor dose higher while keeping healthy liver and lung exposure within safe limits. The partition model is more complex, but it offers a meaningfully more tailored treatment plan.
What the Results Can Mean for Treatment
Mapping findings don’t just fine-tune the treatment. They can change the plan entirely. A lung shunt fraction that pushes the estimated lung dose above 30 Gy may delay or cancel the procedure. Complex arterial anatomy, such as arteries feeding the liver that branch off in unusual locations, requires the doctor to adjust the catheter position for both the MAA injection and the eventual treatment. In some cases, aberrant vessels that can’t be safely coiled may make radioembolization too risky.
The MAA scan also reveals whether the tracer concentrated in the tumor or spread diffusely through healthy liver tissue. A scan showing strong tumor uptake is encouraging because it suggests the Y90 microspheres will do the same, delivering a high radiation dose precisely where it’s needed. Diffuse uptake in non-tumor tissue, on the other hand, raises concern about liver toxicity and may prompt dose adjustments.
Timeline From Mapping to Treatment
The standard gap between mapping and the Y90 treatment session is one to two weeks, though institutional practices can stretch this to three to five weeks. This window gives the medical team time to analyze imaging results, calculate the dose, and order the Y90 microspheres, which are manufactured to a patient-specific activity level and have a short usable life due to radioactive decay.
Some centers now perform same-day Y90, completing the mapping angiogram, coil embolization, MAA scan, and microsphere delivery all in a single session. This approach reduces the total number of procedures and eliminates the waiting period, but it requires rapid image analysis and on-site microsphere availability. It’s becoming more common for straightforward cases but isn’t yet standard practice everywhere.