What Are Negative Margins in SFT Surgery?

When your surgeon says they need to get “negative margins,” they mean they must remove the solitary fibrous tumor (SFT) along with a continuous border (or “margin”) of healthy, normal tissue surrounding it. Achieving a negative margin means the entire tumor and a protective border of healthy tissue have been removed, significantly reducing the chance that microscopic tumor cells remain at the surgical site. This is the most critical factor in preventing the tumor from growing back in the same spot, which is known as local recurrence.

How Do They Know the Margins Are Negative?

During surgery, the tumor and the surrounding rim of normal tissue are removed in one piece. The width of this healthy border varies greatly depending on where the tumor is located; in some areas, surgeons can take a wide margin, while in others, they can only take a few millimeters to protect vital organs.

Once the tissue is removed, a doctor called a pathologist examines it. Processing the tissue and reviewing it takes time, so you generally will not know your margin status immediately after waking up from surgery; it usually takes several days to a week to get the pathology report.

The pathologist coats the outer edges of the removed tissue with a special medical ink ^[1][2]. They then slice the tissue into thin sections and look at representative slices under a microscope. If the tumor cells are entirely inside the healthy tissue and do not touch the ink, it is called a negative margin or an R0 resection ^[1][2].

If tumor cells are found touching the ink, it means the tumor went right up to the edge of what was removed. This is called a positive margin (or R1 if only seen under a microscope), which suggests that microscopic tumor cells may have been left behind in the body ^[2][1]. If a surgeon can clearly see that part of the tumor was left behind during surgery, it is known as a macroscopic positive margin or R2 ^[2][1].

Why Achieving Negative Margins Matters

A complete surgical removal with negative margins is the gold standard and most important treatment for an SFT ^[3][4]. When tumor cells are left behind (a positive margin), there is a significantly higher risk that the SFT will grow back in that same location ^[2][5].

Your margin status is also a key piece of information that doctors use, along with factors like the size of the tumor and how fast the cells are dividing (often called the mitotic rate or mitotic count), to predict how your SFT will behave and to create your long-term care plan ^[2][6][7].

What Happens if Margins Are Positive?

Sometimes, a tumor is located too close to vital blood vessels, nerves, or organs, making it impossible for the surgeon to remove a wide border of healthy tissue safely. If your margins are positive, your care team may discuss additional treatments, such as radiation therapy, to help lower the risk of the tumor returning ^[5][8][9].

Even when surgeons achieve perfect negative margins, SFTs are known to be unpredictable and can sometimes recur ^[10][11][12]. Furthermore, because pathologists look at representative slices rather than every single microscopic piece of tissue, micrometastases could potentially remain. Because of this, long-term monitoring with regular imaging scans is necessary for everyone who has had SFT surgery, regardless of their margin status ^[13][14].