Tc-99m MAA Lung Shunt Fraction for Y-90

Introduction

Before a single microsphere of Y-90 is delivered to a liver tumor, a Tc-99m MAA scan answers one safety-critical question: how much of that dose would end up in the lungs instead? The answer is the lung shunt fraction (LSF), and it directly sets the predicted lung dose, drives whether the prescribed activity is reduced or the treatment is held, and — if measured poorly — can wrongly deny a treatable patient or expose the lungs to radiation pneumonitis. ^{1, 2}

Yttrium-90 radioembolization (selective internal radiation therapy, SIRT) treats primary and metastatic liver cancer by delivering millions of radioactive microspheres through the hepatic artery. Because liver tumors are highly vascular, some fraction of any injected particle crosses arteriovenous connections in the tumor bed and travels to the lungs. Y-90 microspheres are permanent; there is no way to retrieve a dose that shunts. So the workflow uses technetium-99m macroaggregated albumin (Tc-99m MAA) — particles of a similar size to the therapy microspheres — as a pre-treatment surrogate to map where the dose will go. ^{1, 5}

This guide explains what the LSF is, how it is measured, why the choice between planar and SPECT/CT imaging materially changes the number, how the LSF converts into a predicted lung dose and an activity reduction, and where the medical physicist fits. DRPS provides this support as part of its PET/CT and nuclear medicine physics services across Florida, Maryland, Virginia, Washington DC, California, and Nevada.

Topic Explanation

Why MAA is the surrogate

Tc-99m MAA consists of albumin aggregates, most in the 10–90 μm range, which lodge in the first capillary or arteriolar bed they reach. Injected into the hepatic artery, MAA distributes much like the Y-90 microspheres will: most particles trap in the tumor and liver, while a fraction passes through tumor shunts into the pulmonary circulation. Resin microspheres (SIR-Spheres) are roughly 20–60 μm and glass microspheres (TheraSphere) roughly 20–30 μm, so MAA is an imperfect but practical size analog. ^{1, 5}

The MAA scan is not only about the lungs. It is a mapping study that also confirms catheter position and looks for extrahepatic deposition — for example, particles refluxing to the stomach or bowel — that would need to be corrected before therapy. The lung shunt measurement is the dosimetric half of that same study. For the therapy-side counterpart to this planning scan, see our guide to Y-90 radioembolization dosimetry and, on the safety side, Y-90 radioembolization radiation safety.

What the lung shunt fraction is, numerically

The LSF is simply the fraction of total counted activity that appears in the lungs:

On planar imaging, the counts in each region are taken as the geometric mean of the anterior and posterior views, which partially compensates for attenuation:

On SPECT/CT, the counts are summed within lung and liver volumes segmented on the co-registered CT. The output is a percentage — for example, an LSF of 0.05 means 5% of the counted activity is in the lungs. ^{1, 4, 5}

Key Technical Principles

From lung shunt to lung dose

The reason the LSF matters is that it, together with the administered activity and lung mass, determines the lung mean dose (LMD). For a beta emitter like Y-90 that deposits essentially all of its energy locally, the absorbed dose for complete decay is governed by the mean beta energy per decay (0.9337 MeV for Y-90). Working the MIRD arithmetic through for Y-90 gives the widely used constant of about 49.7 J·g⁻¹ per GBq·kg⁻¹, so: ^{1, 2, 9}

Worked lung-dose example

Consider a planned administered activity of 1.5 GBq, an LSF of 0.10, and the conventional 1 kg lung-mass assumption:

That is comfortably below the ~30 Gy single-treatment limit. Now hold the activity and lung mass fixed but let the LSF rise to 0.20:

The lung dose scales linearly with the LSF, which is exactly why a measurement error in the shunt fraction propagates directly into the dose estimate — and why overestimating the LSF can push a patient over a labeling threshold they never actually exceeded. ^{1, 4, 5}

Planar versus SPECT/CT: the number depends on the method

Here is the central practical point of this article. The same patient will usually show a lower LSF on SPECT/CT than on planar imaging, because planar projection superimposes the dome of the liver, cardiac blood pool, and background into the lung region of interest, inflating lung counts. SPECT/CT resolves lung from liver in three dimensions. ^{3, 4, 5}

In one 293-patient HCC series, mean planar LSF was 8.27% versus 3.27% on SPECT/CT, with the largest discrepancies in exactly the patients near decision thresholds. ³ A separate study modeling lung mean dose found planar imaging overestimated LSF by about 63% and lung dose by about 53% relative to a SPECT/CT-plus-diagnostic-CT method. ⁴ The clinical stakes are asymmetric: overestimating the LSF does not endanger the lungs, but it can strip dose from the tumor or deny therapy to a patient who was in fact eligible. ^{3, 4, 5}

Sources of error to control

• MAA quality and free pertechnetate. Breakdown of the MAA or free Tc-99m pertechnetate distributes to the thyroid, stomach, and soft tissue and can bias the shunt estimate; image promptly after injection and check radiochemical purity.
• Injection technique. Too many particles or forceful injection can cause stasis and reflux, distorting distribution.
• ROI/segmentation. On planar, including the liver dome in the lung ROI inflates the LSF; on SPECT/CT, careful lung and liver segmentation on the CT is essential.
• Lung mass assumption. The default 1 kg lung mass is a convention; using a patient-specific lung mass from the CT changes the dose estimate. ⁴

Clinical Impact

The LSF is one of a small number of pre-treatment measurements that can change a patient's therapy from "full dose" to "reduced" to "not a candidate," so its accuracy is a patient-selection issue, not just a dosimetry detail.

• Protecting the lungs. The dominant lung toxicity is radiation pneumonitis. The lung-dose limits of roughly 30 Gy single and 50 Gy cumulative come from Ho and colleagues' partition-model work, in which patients exceeding those thresholds developed pneumonitis. ²
• Not under-treating the tumor. Because Y-90 is permanent and tumors need a high absorbed dose for response, unnecessarily reducing activity from an inflated LSF can compromise the treatment. Correct LSF measurement supports the tumor dose while still respecting the lung limit. ^{3, 4}
• Patient eligibility. With resin microspheres, an LSF above 20% is a contraindication. A patient wrongly measured at 22% on planar imaging who is actually 12% on SPECT/CT could be denied a potentially effective therapy. ^{3, 5}
• A Florida-relevant example. In a University of Miami series, SPECT/CT LSF averaged 4.6% versus 8.5% on planar, and phantom work showed planar overestimation of about 26% versus 6% for SPECT/CT — a concrete demonstration that the imaging method, not the patient, often drives the reported shunt. ⁵

Practical Optimization Tips

Standardize the acquisition

Fix the MAA activity, particle number, camera energy window (140 keV for Tc-99m), collimator (low-energy high-resolution), and imaging time post-injection, and document them. A standardized protocol is what makes serial and cross-patient comparison meaningful and keeps the LSF from drifting with technologist or camera.

Prefer SPECT/CT where it changes management

When a planar LSF lands near a decision threshold (around 10%, 15%, or 20% for resin microspheres, or near the lung-dose limit for glass), acquire or reprocess with SPECT/CT before reducing or withholding therapy. The three-dimensional separation of lung and liver is most valuable exactly where the planar number is most likely to be falsely high. ^{3, 4, 5}

Verify the calculation and the dose

Have the medical physicist independently confirm the LSF arithmetic, the predicted lung dose, and the resulting activity reduction against the specific device labeling. Small ROI or segmentation choices move the number, and the therapy is not reversible.

Use patient-specific lung mass when feasible

The 1 kg lung-mass convention is a simplification. Deriving lung mass from the planning CT can refine the lung-dose estimate and is worth doing when the LSF is borderline. ⁴

Common pitfalls to avoid

• Treating the planar LSF as ground truth when it sits near a threshold.
• Including the liver dome in the lung ROI, which systematically inflates the LSF.
• Ignoring free pertechnetate, which redistributes activity and biases the estimate.
• Applying glass-microsphere logic to resin, or vice versa — the two products use different rules (lung-dose limit versus percentage reduction).
• Skipping independent physics verification of a non-reversible therapy calculation.

Regulatory Considerations

A Y-90 microsphere administration is a written-directive procedure, and the LSF-driven activity reduction is part of getting that directive right — so the lung shunt workup sits squarely inside the facility's radioactive-material program. Y-90 microspheres are a medical use of byproduct material regulated under NRC (or Agreement State) rules, and the manufacturers' labeling defines the LSF thresholds.

• 10 CFR Part 35 — Medical Use of Byproduct Material governs the authorized user, the written directive, and the dose determination for the microsphere therapy; the LSF-based activity is part of the prescribed activity that must be documented and delivered as directed. ⁶
• Device labeling. Resin microspheres (SIR-Spheres, FDA PMA P990065) specify LSF-based percentage reductions; glass microspheres (TheraSphere) specify lung-dose limits of about 30 Gy per treatment and 50 Gy cumulative. The physics workup must match the product actually being used. ^{7, 8}
• EANM and society guidance describe the MAA workup, LSF calculation, and dosimetry expectations and are the reference framework for a defensible procedure. ^{1, 10}

Of the states DRPS serves, Florida, Maryland, Virginia, California, and Nevada are NRC Agreement States that regulate medical use under their own radiation-control rules, while Washington, DC and Delaware are regulated directly by the NRC. A facility should confirm which authority licenses its Y-90 program and align the written directive, dose determination, and recordkeeping accordingly. This workup should be coordinated with medical physics consulting and the facility's authorized users. For the internal-dose framework behind the lung-dose arithmetic, see the MIRD schema for internal dosimetry.

Frequently Asked Questions (FAQs)

What is the lung shunt fraction?

The lung shunt fraction (LSF) is the percentage of injected Tc-99m macroaggregated albumin (MAA) that passes through the liver tumor vasculature and lodges in the lungs instead of the liver. Because MAA is a surrogate for Y-90 microspheres, the LSF estimates how much of the therapy dose would irradiate the lungs, and it is used to reduce the prescribed activity or hold treatment.

Why is a Tc-99m MAA scan done before Y-90 therapy?

The pre-treatment MAA scan is a mapping and safety step. It confirms catheter position, checks for extrahepatic deposition (for example gastrointestinal), and measures the lung shunt fraction so the lung dose from Y-90 can be predicted and kept within tolerance before any therapeutic activity is administered.

How is the lung shunt fraction calculated?

LSF equals lung counts divided by the sum of lung plus liver counts. On planar imaging it uses the geometric mean of anterior and posterior counts in lung and liver regions of interest; on SPECT/CT it uses counts in segmented lung and liver volumes. The result is expressed as a percentage.

Why does planar imaging overestimate the lung shunt fraction?

Planar images superimpose overlapping structures, so counts from the dome of the liver, cardiac blood pool, and background are projected into the lung region, inflating lung counts. SPECT/CT separates lung from liver in three dimensions, so it typically yields a lower and more accurate LSF, which can be the difference between denying and delivering treatment.

What lung dose limits apply to Y-90 radioembolization?

The commonly used limits, originating from the partition-model work of Ho and colleagues, are approximately 30 Gy to the lungs from a single treatment and 50 Gy cumulative from multiple treatments, above which radiation pneumonitis risk rises. Glass-microsphere labeling uses these lung-dose limits, while resin-microsphere labeling uses an LSF-based percentage reduction of the administered activity.

How does the lung shunt fraction change the prescribed activity?

For resin microspheres (SIR-Spheres), a lung shunt under 10 percent allows the full activity, 10 to 15 percent requires a 20 percent reduction, 15 to 20 percent requires a 40 percent reduction, and above 20 percent the treatment is contraindicated. For glass microspheres (TheraSphere), activity is reduced as needed to keep the lung dose under about 30 Gy per treatment and 50 Gy cumulative.

Does the medical physicist have a role in the MAA lung shunt workup?

Yes. The physicist helps standardize the acquisition, choose planar versus SPECT/CT, verify the LSF calculation, compute the predicted lung dose, and confirm the activity reduction against the device labeling and the written directive, which is important for both patient safety and regulatory compliance under 10 CFR Part 35.

Key Takeaways

• The LSF predicts lung dose. MAA is a size surrogate for Y-90 microspheres, and the shunt fraction estimates how much therapy dose would irradiate the lungs.
• Lung dose scales linearly with LSF. Using the Y-90 constant, $D_{\text{lung}}\approx 49.7\times A_{\text{inj}}\times \text{LSF}/m_{\text{lung}}$, so a measurement error becomes a dose error.
• Planar overestimates; SPECT/CT is more accurate. Reported planar-versus-SPECT/CT LSF differences (for example ~8% vs ~3–5%) can flip a patient across a treatment threshold. ^{3, 5}
• Two products, two rules. Resin microspheres reduce activity by LSF band (full under 10%, −20% at 10–15%, −40% at 15–20%, contraindicated over 20%); glass microspheres cap lung dose near 30 Gy single / 50 Gy cumulative. ^{2, 7, 8}
• Radiation pneumonitis is the toxicity the whole workup exists to prevent.
• Physics verification matters because the therapy is permanent and non-reversible.

Conclusion

The Tc-99m MAA lung shunt fraction is a small number with outsized consequences. It decides how much Y-90 the lungs will see, whether the prescribed activity should be cut, and whether a patient is a candidate at all. Because the therapy cannot be undone, the measurement deserves the same rigor as the treatment itself — a standardized acquisition, the right choice between planar and SPECT/CT, an independently verified calculation, and an activity reduction matched to the specific device labeling.

The recurring lesson from the literature is that the reported shunt often reflects the imaging method as much as the patient's physiology. A planar LSF near a threshold should prompt SPECT/CT before therapy is reduced or withheld — protecting the lungs without needlessly under-treating the tumor.

How DRPS Can Help

Diagnostic Radiation Physics Services helps nuclear medicine and interventional programs build defensible Y-90 workflows. This may include standardizing the MAA acquisition, advising on planar versus SPECT/CT LSF, independently verifying the lung shunt calculation and predicted lung dose, checking the activity reduction against device labeling and the written directive, and integrating the workup with PET/CT and nuclear medicine physics and medical physics consulting.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware.

A safe radioembolization program makes the correct lung-shunt workup a routine, documented step — not an afterthought discovered at inspection.

Related Resources

• Y-90 radioembolization dosimetry
• Y-90 bremsstrahlung SPECT imaging
• The MIRD schema for internal dosimetry
• SPECT/CT quality control
• Y-90 radioembolization radiation safety
• PET/CT and nuclear medicine physics
• Medical physicist consulting

References

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2. Ho S, Lau WY, Leung TW, et al. Clinical evaluation of the partition model for estimating radiation doses from yttrium-90 microspheres in the treatment of hepatic cancer. Eur J Nucl Med. 1997;24(3):293-298. doi:10.1007/BF01728766. PubMed
3. Elsayed M, Cheng B, Xing M, et al. Comparison of Tc-99m MAA planar versus SPECT/CT imaging for lung shunt fraction evaluation prior to Y-90 radioembolization: are we overestimating lung shunt fraction? Cardiovasc Intervent Radiol. 2021;44(2):254-260. doi:10.1007/s00270-020-02638-8. PubMed
4. Lopez B, Mahvash A, Lam MGEH, Kappadath SC. Calculation of lung mean dose and quantification of error for 90Y-microsphere radioembolization using 99mTc-MAA SPECT/CT and diagnostic chest CT. Med Phys. 2019;46(9):3929-3940. doi:10.1002/mp.13575. PubMed
5. Georgiou MF, Kuker RA, Studenski MT, et al. Lung shunt fraction calculation using 99mTc-MAA SPECT/CT imaging for 90Y microsphere selective internal radiation therapy of liver tumors. EJNMMI Res. 2021;11(1):96. doi:10.1186/s13550-021-00837-z. PubMed
6. U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. ecfr.gov
7. U.S. Food and Drug Administration. SIR-Spheres (Yttrium-90 resin microspheres) — PMA P990065, summary of safety and effectiveness and instructions for use. accessdata.fda.gov
8. Boston Scientific. TheraSphere Yttrium-90 Glass Microspheres — Instructions for Use. bostonscientific.com
9. National Nuclear Data Center, Brookhaven National Laboratory. NuDat: Yttrium-90 decay data (mean beta energy 0.9337 MeV). nndc.bnl.gov
10. Giammarile F, Bodei L, Chiesa C, et al. EANM procedure guideline for the treatment of liver cancer and liver metastases with intra-arterial radioactive compounds. Eur J Nucl Med Mol Imaging. 2011;38(7):1393-1406. doi:10.1007/s00259-011-1812-2. PubMed