If only our furry friends could talk, as pet parents we could rest a little easier knowing exactly how they feel and what's working for them and what's not. The reality is that we are charged with the responsibility to advocate on their behalf, particularly when they are faced with a dreaded cancer diagnosis. That is why it is so important to know what good tumor margins in dogs are and how they affect your dog's cancer care.
If surgery seems to be the best option to remove a tumor, you should be armed with the right information to give your beloved companion the best chance of a successful recovery from canine cancer.
What Is a Tumor Margin?
A tumor margin is the edge or border of tissue surrounding a tumor that is removed during cancer surgery along with the tumor itself. It is critically important to have the tumor and margin retained for analysis rather than discarded after surgery—this examination is the only way to know exactly what you're dealing with.
After surgery, pathologists use a special type of ink to draw a line along the outer edge of the entire tissue sample. They then slice it into thin sections and examine it under a microscope to determine whether cancer cells remain at the edges (Liptak & Forrest, 2013).
How Margins Are Classified
Understanding the pathology report is essential for knowing whether the surgery was successful:
Negative, clear, or clean margins mean no cancer cells were found at the edge of the tissue, suggesting that all of the cancer has been removed. In some cases, the pathology report will also tell you how wide the clear margin is—the distance between the outer edge of the surrounding tissue that was removed and the edge of the cancer (Pratschke et al., 2013).
Positive or involved margins indicate that cancer cells extend to the edge of the tissue, suggesting that not all of the cancer has been removed and further surgery or another form of treatment is required.
Close margins fall somewhere between clear and positive. This means the cancer cells are close to the edge of the tissue but not right at the edge, and more surgery may be needed. In fact, if cancer cells are found anywhere between the tumor and the outer edge of the margin, additional surgery may be recommended (Dernell et al., 2007).
The Critical Mistake: Assuming "Clean" Means Adequate
Here's where many pet owners—and even some veterinarians—get tripped up. A pathology report might say "margins were clean," which sounds like good news. But when you look at the microscopic description, the measurement might read less than 1 millimeter. For a malignant tumor, this is NOT considered truly clean or adequate.
Veterinary oncologists often see dogs and cats months after a tumor was removed, only to have it grow back. When reviewing the original biopsy report, it frequently reads "margins were clean," but the actual measurement shows margins of less than 1mm. Sadly, it's not surprising that these malignant tumors removed with narrow margins recurred.
This important detail is overlooked far too often. The term "clean" can be misleading if you don't know the actual measurements and whether those measurements are appropriate for the specific tumor type your dog has.
What Are Good Tumor Margins and How Wide Should They Be?
Unfortunately, there is no one-size-fits-all guideline for tumor margins in dogs—it depends entirely on the tumor type. For some doctors, 2 millimeters or more of normal tissue is considered to be enough, while other doctors consider a 1 millimeter rim of healthy tissue, or maybe less, to be a healthy margin (Simpson et al., 2004).
However, tumor type makes all the difference:
For benign skin tumors like sebaceous adenomas or benign liver tumors, margins of a few millimeters are fine. But for malignant tumors, much wider margins are typically needed.
For many malignant tumors, 2 to 3 centimeters (20-30 millimeters) of clean tissue are recommended. For example, soft tissue sarcomas with incomplete or "dirty" margins are ten times more likely to recur (Bacon et al., 2007; Kuntz et al., 1997). Mast cell tumors also require wider surgical margins—often 2-3 centimeters of healthy tissue—due to their invasive growth patterns.
Remember: Size matters! The determination of what constitutes a clear margin can mean the difference between a full recovery or eventual recurrence of cancer. It has been said that malignant cancer will come back if you don't have wider clean margins rather than margins that are too narrow. How torturous it would be to think that all is well only to be devastated all over again.
Why Tumor Type Matters
Having an accurate diagnosis of the type and grade of the tumor is very important for the surgeon when planning the surgical margins (Selting et al., 2018). Different types of tumors behave differently. Some tend to grow invasively, like a plant shooting out long roots in every direction. Other types of tumors tend to metastasize, spreading to distant spots and organs, rather than growing invasively. This behavior influences how much surrounding tissue needs to be removed to achieve a successful outcome.
Why Good Margins Are So Critical
Reducing Recurrence Risk: The primary goal of achieving clean margins is to prevent the tumor from growing back. If cancer cells remain after surgery, they can multiply and cause the tumor to recur—and tumors with incomplete margins are significantly more likely to come back (Seguin et al., 2001).
Avoiding Additional Surgeries: When margins aren't clean, your veterinarian may recommend a second surgery to remove more tissue from the area. This means another anesthesia event, more healing time, and additional expense.
Guiding Treatment Decisions: Margin status helps determine whether additional therapy is needed. Clean margins might mean surgery alone is sufficient, while incomplete margins may require radiation therapy, chemotherapy, or other treatments (LaRue & Gillette, 2007).
The Questions You MUST Ask Your Vet
You know your pet through and through. Combine that knowledge with your love for your pooch and trust your instincts so you'll make the right treatment decisions when your pet has been diagnosed with cancer. Do your research and ask as many questions as you need to feel comfortable that you understand the diagnosis, prognosis, and medication and treatment options thoroughly.
Most importantly, if your pet is going to have surgery, make it unmistakably clear that you want the tumor to be analyzed after the operation to ensure good tumor margins. Don't just accept "clean margins" as the final answer. Ask your veterinarian:
- How wide were those margins? Get actual numbers in millimeters or centimeters.
- Are those margins wide enough for this specific tumor type?
- What margins are they planning to achieve during surgery?
- What does "clear" mean in their practice—1mm, 2mm, or more?
- What type of tumor does my dog have, and how does that affect the margins needed?
- What happens if the margins come back positive or close?
- If your vet isn't sure whether the margins are adequate, ask to see an oncologist.
Remember, your canine pal can't ask these important questions, so you'll need to do it for them. After you get the pathologist's report, it can't hurt to get a second opinion.
This Information Isn't Told to All
The average pet owner has no idea how important it is to be educated about the implications of tumor margins. Your best pal's life may depend on understanding this critical aspect of cancer surgery. There are so many options for treatment, and selecting the right one starts with being informed and asking the right questions.
The key takeaway: Don't assume "clean" surgical margins were actually adequate. Always ask how wide those margins were and whether they're wide enough for your dog's specific tumor type. A margin that's perfectly fine for a benign tumor could be dangerously inadequate for a malignant one.
Good tumor margins represent the difference between a tumor that's truly gone and one that may return. When your veterinarian discusses margins, they're talking about giving your dog the best possible chance for a cancer-free future. Be your pet's advocate—they're counting on you to speak up on their behalf.
References
Bacon, N. J., Dernell, W. S., Ehrhart, N., Powers, B. E., & Withrow, S. J. (2007). Evaluation of primary re-excision after recent inadequate resection of soft tissue sarcomas in dogs: 41 cases (1999-2004). Journal of the American Veterinary Medical Association, 230(4), 548-554. https://doi.org/10.2460/javma.230.4.548
Dernell, W. S., Ehrhart, N. P., Straw, R. C., & Vail, D. M. (2007). Tumors of the skeletal system. In S. J. Withrow & D. M. Vail (Eds.), Withrow and MacEwen's small animal clinical oncology (4th ed., pp. 540-582). Saunders Elsevier.
Kuntz, C. A., Dernell, W. S., Powers, B. E., & Withrow, S. J. (1997). Prognostic factors for surgical treatment of soft-tissue sarcomas in dogs: 75 cases (1986-1996). Journal of the American Veterinary Medical Association, 211(9), 1147-1151.
LaRue, S. M., & Gillette, E. L. (2007). Radiation therapy. In S. J. Withrow & D. M. Vail (Eds.), Withrow and MacEwen's small animal clinical oncology (4th ed., pp. 193-210). Saunders Elsevier.
Liptak, J. M., & Forrest, L. J. (2013). Soft tissue sarcomas. In S. J. Withrow, D. M. Vail, & R. L. Page (Eds.), Withrow and MacEwen's small animal clinical oncology (5th ed., pp. 356-380). Elsevier Saunders.
Pratschke, K. M., Atherton, M. J., Sillito, J. A., & Lamm, C. G. (2013). Evaluation of a modified proportional margins approach for surgical resection of mast cell tumors in dogs: 40 cases (2008-2012). Journal of the American Veterinary Medical Association, 243(10), 1436-1441. https://doi.org/10.2460/javma.243.10.1436
Seguin, B., Leibman, N. F., Bregazzi, V. S., Ogilvie, G. K., Powers, B. E., Dernell, W. S., Fettman, M. J., & Withrow, S. J. (2001). Clinical outcome of dogs with grade-II mast cell tumors treated with surgery alone: 55 cases (1996-1999). Journal of the American Veterinary Medical Association, 218(7), 1120-1123. https://doi.org/10.2460/javma.2001.218.1120
Selting, K. A., Powers, B. E., Thompson, L. J., Mittleman, E., Tyler, J. W., Lafferty, M. H., & Withrow, S. J. (2018). Outcome of dogs with high-grade soft tissue sarcomas treated with and without adjuvant doxorubicin chemotherapy: 39 cases (2001-2004). Journal of the American Veterinary Medical Association, 246(8), 877-882. https://doi.org/10.2460/javma.246.8.877
Simpson, A. M., Ludwig, L. L., Newman, S. J., Bergman, P. J., Hottinger, H. A., & Patnaik, A. K. (2004). Evaluation of surgical margins required for complete excision of cutaneous mast cell tumors in dogs. Journal of the American Veterinary Medical Association, 224(2), 236-240. https://doi.org/10.2460/javma.2004.224.236