A Magseed is a tiny magnetic marker, about the size of a grain of rice, that surgeons use to pinpoint breast tumors too small to feel by hand. Made by Endomag, it is placed directly into the breast at the site of a suspicious lesion so the surgeon can find and remove it precisely during a lumpectomy. The seed measures just 5 mm long and 1 mm wide, making it one of the smallest localization devices available.
Why a Marker Is Needed
Many breast cancers are detected on imaging long before they form a lump you or a doctor can feel. These are called impalpable lesions. Without some kind of internal marker, a surgeon operating on an impalpable lesion would have no reliable way to find it. The traditional solution was to insert a thin wire into the breast on the morning of surgery, threading it through a needle until its tip sat right at the tumor. That wire would then stick out of the skin until the patient was wheeled into the operating room.
A Magseed replaces that wire. Because it can be implanted days or even weeks before surgery, it eliminates the need for a same-day wire placement and the logistical pressure that comes with it. Patients no longer need to start their surgical day in the radiology suite with a wire protruding from their breast, and hospitals gain more flexibility in scheduling.
How the Seed Is Placed
A radiologist places the Magseed during a short outpatient procedure using local anesthetic. The seed is loaded inside a needle, and the radiologist guides that needle to the exact location of the lesion using either ultrasound or stereotactic X-ray imaging. Once the needle tip is confirmed to be at the target, the seed is deployed and the needle is withdrawn. The entire process is similar to a core needle biopsy and typically takes only a few minutes.
After placement, a mammogram or ultrasound confirms the seed is sitting in the right spot. In a large European study published in Clinical Breast Cancer, 97.5% of seeds were correctly placed into the target lesion on the first attempt, with only 2.5% showing any displacement from the intended position.
What the Seed Is Made Of
The Magseed is a cylindrical device made of medical-grade stainless steel and iron oxide. It is paramagnetic, meaning it responds to an external magnetic field but does not generate one on its own, and it contains no radioactive material. The newer version, Magseed Pro, uses nitinol (a nickel-titanium alloy common in medical implants) and is designed to self-expand slightly after deployment to help anchor itself in tissue.
One important limitation comes from the steel composition: the seed creates a signal void of roughly 2 to 6 cm on MRI scans. That artifact can obscure the surrounding tissue, which means MRI is generally not recommended for monitoring the area while the seed is in place. Mammography and ultrasound remain usable for follow-up imaging.
How Surgeons Find It During Surgery
In the operating room, the surgeon uses a handheld magnetic probe called the Sentimag to locate the seed through the skin. The probe emits sounds of varying pitch and displays a numerical reading that tells the surgeon how close the instrument is to the marker. As the probe gets nearer to the Magseed, the pitch changes and the reading increases, allowing the surgeon to home in on the exact location. The system works reliably across all breast sizes and at all tissue depths. In the same European study, surgeons achieved a 100% detection rate across more than 200 cases.
Once the surgeon reaches the seed, they remove it along with a margin of surrounding tissue. The excised tissue is then sent to a pathologist who checks that the edges are free of cancer cells. The reoperation rate (cases where a second surgery is needed because margins were not clear) was 5.1% in that study, which is comparable to or better than rates reported for traditional wire-guided surgery.
How Long It Can Stay in the Body
The original Magseed was commonly placed up to 30 days before surgery, giving hospitals a wide scheduling window. The newer Magseed Pro has FDA clearance for implantation beyond 30 days, which is particularly useful for patients undergoing chemotherapy before surgery. In those cases, the seed can be placed at the tumor site before treatment begins, then left in place for months while chemotherapy shrinks the tumor. This ensures the original tumor location is still marked even if the lesion becomes difficult to see on imaging after a good treatment response.
What It Feels Like for the Patient
The placement procedure feels much like a needle biopsy. You will receive a local anesthetic injection, which stings briefly, and then feel pressure as the deployment needle is positioned. Most patients describe mild soreness afterward, similar to post-biopsy tenderness, that resolves within a day or two.
Once the seed is in place, you should not be able to feel it. At 5 mm long, it is too small to create a noticeable lump or sensation under the skin. There is no wire extending from the body, no external dressing to manage, and no restriction on normal daily activities between placement and surgery. You can shower, exercise, and go about your routine.
On the day of surgery, the Magseed means you skip the early-morning radiology appointment that wire localization requires. You arrive for your scheduled surgical time, and the localization step happens in the operating room itself using the magnetic probe. For many patients, this streamlined process reduces both the length of the day and the anxiety that comes with having a wire in place for hours before an operation.
Limitations to Know About
The MRI artifact is the most clinically relevant drawback. If your treatment plan calls for breast MRI monitoring (common during neoadjuvant chemotherapy or for high-risk screening), the 2 to 6 cm signal void around the seed can make interpretation difficult. Your care team may rely on mammography and ultrasound instead, or factor the artifact into their imaging strategy before placing the seed.
Migration is possible but uncommon. In most reported series, the seed stays within a few millimeters of where it was placed. The 2.5% dislocation rate from the European study reflects cases where the seed shifted enough to be noted, though surgeons were still able to detect and retrieve it in every case. The self-expanding design of the newer Magseed Pro is intended to reduce this risk further by gripping the surrounding tissue more firmly after deployment.