When your dog undergoes surgery to remove a tumor, and the surgery goes well, you may think you’re in the clear… However, there can also be a chance of tumor recurrence from the lingering cancer cells that are left behind at the treatment location on your pet.
You may hear your veterinarian refer to the surgical margins post-tumor resection in the terms “clean margins” or “dirty margins.” If you’ve been told your dog has “dirty” or “positive” surgical margins, it can feel alarming. However, there are options!
If your dog has dirty surgical margins, it doesn’t automatically mean the cancer has returned, and it doesn’t mean there are no other treatments available or preventative measures.
This guide explains cancer recurrence after surgery and ways for preventing tumor recurrence in dogs. We’ll go over what dirty margins mean and the next steps to help prevent tumor recurrence in your pet.
Related: New Cancer Treatments for Dogs in 2026: What Pet Owners Should Know
What are “dirty margins” in dogs?
After a tumor is removed, your veterinarian will receive a pathology report which determines if there are cancerous cells remaining where the tumor was located. This means that the tissue at the edge (or “margins”) of the resected tumor is examined under a microscope.
- Clean (negative) margins – No cancer cells at the edges of the removed tissue
- Dirty (positive) margins – Cancer cells are present at the edges of the removed tissue
In simple terms, dirty margins suggest that microscopic cancer cells may have been left behind.
Can cancer come back after tumor removal in dogs?
The good news: dirty margins don’t always mean that your dog’s tumor will come back. On the other hand, in some cases, the cancer can return even after your pet has had surgery.
When a tumor is removed, the goal is to remove the visible mass itself, as well as some of the healthy tissue around it (a.k.a., the margins) as a safety net to ensure the cancer is completely removed. However, this is not always possible as some cancers send microscopic extensions beyond what is visible during surgery.
Tumors grow back for many reasons, even if the surgical removal is successful. Tumor recurrence risk depends on:
- Tumor type (soft tissue sarcoma vs. mast cell tumor, etc.)
- Tumor grade (how aggressive/invasive the cancer cells appear under a microscope)
- Surgical margins (clean/negative vs. positive/dirty margins)
- Tumor location
- Preventative treatments
Clean or negative margins are the best outcome, as the tumor may regrow over time if even a small amount of cancerous cells remain.
Related: Post Cancer Care For Your Pet
Why can’t veterinarians always get clean margins?
Recurrence doesn’t mean that the initial surgery failed… if dirty or positive margins appear after tumor removal, this could be from a number of reasons. For example, high-grade cancers are more likely to recur even after wide removal. As well, tumors on the face, lower limbs, or near joints cannot be safely removed with wide margins without affecting mobility or function.
- The tumor is located near vital structures (face, joints, spine, etc.)
- Removing more tissue would significantly impact function
- The tumor borders are not clearly visible
This is especially common with soft tissue sarcomas in dogs, which can send microscopic extensions beyond what’s visible during surgery.
Your veterinarian will always try to balance complete removal with preserving your pet’s quality of life! Many dogs do well after surgery, especially when removal is combined with additional treatments early on.
What happens if a tumor isn’t fully removed?
When a tumor is incompletely excised (meaning there are positive or dirty margins), microscopic cancer cells may remain in the surgical area. Over time, those cells may:
- Regrow locally at the original site
- Become more difficult to remove surgically
- Increase in aggressiveness (depending on tumor type)
However, not every incompletely removed tumor grows back immediately (some may remain stable for a period of time). This is why your veterinarian may recommend discussing follow-up treatment options even before visible recurrence occurs.
Post-surgical cancer treatment for dogs: What are the options after dirty margins?
If your dog’s pathology report shows dirty or positive margins, your veterinarian may discuss several options:
1. Repeat surgery
If the tumor location allows, additional tissue can sometimes be removed again to help strive for those clean or negative margins. Depending on the tumor or healing process, this may not always be a possibility.
2. Traditional radiation therapy
External beam radiation therapy (EBRT) may be an option after surgery to treat the remaining microscopic cancer cells. This typically requires:
- Multiple treatments
- Several anesthesia sessions
- Referral to a specialty center
There are a few things to consider with external beam radiation therapy: It can be expensive, it often requires travel to a specialized clinic, radiation affects the healthy surrounding tissues, and side effects such as skin irritation, fatigue, and damage to nearby organs can occur.
3. Chemotherapy
For cancers and tumor types with a higher risk of spreading to other parts of the body, chemotherapy may be recommended in addition to local treatment.
4. Targeted margin therapy
IsoPet® Margin Therapy is an approach designed to treat the dirty or positive margins after tumor removal. This treatment delivers radiation directly into the tumor/surrounding area (margins) through an injection, while limiting exposure to surrounding healthy tissue.
This treatment approach focuses specifically on treating microscopic cancerous cells remaining at the surgical site, either during or after surgery.
IsoPet® Margin Therapy may be considered when repeat surgery isn’t ideal or when preserving function and comfort is a priority. This option takes under 30 minutes to administer, making it a comfortable experience for both pets and their owners.
As with all cancer treatments, suitability depends on tumor type, size, and location and should be discussed with your veterinarian.
Benefits of IsoPet® Margin Therapy for dogs
While IsoPet® Margin Therapy can benefit any non-metastatic tumor case, dogs who may especially benefit from margin-focused treatment include:
- Dogs with soft tissue sarcomas, which commonly send microscopic cancerous cells beyond what is visible during surgery
- Tumors located on the lower limbs, where wide surgical margins are difficult to reach without affecting mobility
- Facial, oral, or head tumors, where aggressive surgery could impact appearance, eating, or function
- Cases where repeat surgery is not ideal, due to location, healing concerns, or overall health
- Older dogs or those with concurrent medical conditions, where minimizing anesthesia and recovery time is important
Because margin treatment is injected directly into the affected area:
- Surrounding healthy tissue exposure is limited
- Recovery is straightforward with minimal side-effects
- Daily routines are often minimally disrupted
IsoPet® Margin Therapy is a minimally-invasive option that can be considered across a wide range of tumor types and locations, where incomplete tumor removal is a concern. For many families, this approach offers a balance between actively addressing recurrence risk and preserving comfort, mobility, and overall quality of life.
Related: A Guide to Understanding Soft Tissue Sarcoma in Pets
What should you ask your veterinarian?
If your dog’s report shows dirty (positive) tumor margins, here are a few questions to ask:
- What is the recurrence risk for this tumor type?
- Is repeat surgery possible?
- Would radiation or chemotherapy be appropriate?
- What happens if we choose to monitor only?
- Is IsoPet® Margin Therapy an option?
Understanding all of the treatment options available helps you make an informed decision that gives your dog the best quality of life.
Related: A Guide to Identifying Cancer Symptoms in Pets
Conclusion
Even if your dog has “dirty margins” after tumor removal, it does not mean hope is lost.
With new treatment options available to reduce recurrence risk, including localized radiation therapy, your pet can still live a long, comfortable, and happy life with you by their side. The best option depends on your dog’s individual diagnosis, their health status, and your goals as a family.
If you are interested in learning if your pet is a good candidate for IsoPet®, contact a member of our team here.
Medical Disclaimer
This article does not provide any medical advice and is for educational purposes only. IsoPet® does not provide veterinary medical services. For professional veterinary advice, please consult your veterinarian regarding diagnosis and treatment decisions for your dog.
