Should Your Dog or Cat Have a Biopsy Before Surgery? How Veterinarians Decide Whether to Biopsy or Remove a Mass

By Steve Mehler, DVM, DACVS

Finding a lump on your dog or cat can be alarming. One of the first questions pet owners ask is:

"Can you just remove it?"

Sometimes the answer is yes.

Other times, removing the mass without first knowing what it is can actually reduce the chances of completely curing the disease.

As a veterinary surgeon, one of the most important decisions I make isn't how to remove a tumor—it's whether we should biopsy it before surgery at all.

Surprisingly, the best approach depends less on whether the lump is cancerous and more on whether the diagnosis will change the surgical plan.

The First Step Is Almost Never Surgery

Before discussing biopsies, it's important to understand that most skin and subcutaneous masses should first be evaluated with a fine-needle aspiration (FNA).

An FNA uses a small needle to collect cells from the mass. It usually doesn't require anesthesia, takes only a few minutes, and is far less invasive than surgery.

For many tumors, an FNA can provide valuable information, including whether the mass is likely:

• A lipoma (fatty tumor)

• A mast cell tumor

• An abscess

• A cyst

• Lymphoma

• Another round-cell tumor

However, not every tumor sheds enough cells for diagnosis. Soft tissue sarcomas, peripheral nerve sheath tumors, fibromas, and many spindle-cell tumors often produce inconclusive aspirates.

When that happens, the next decision becomes critical.

Excisional Biopsy vs. Incisional Biopsy

These terms sound similar but represent two very different surgical strategies.

Excisional Biopsy

An excisional biopsy means the entire mass is removed and then submitted to the pathologist for diagnosis.

In many cases, this surgery serves both as the diagnostic procedure and the treatment.

Incisional Biopsy

An incisional biopsy removes only a small piece of the mass.

The purpose is not treatment.

The goal is to identify exactly what the tumor is before planning the definitive surgery.

When Is It Reasonable to Remove the Entire Mass?

If a mass is:

• Small

• Freely movable

• Located in an area where wide margins can easily be obtained

• Unlikely to require complex reconstruction

An excisional biopsy is often an excellent choice.

For example, a small skin mass on the trunk may be removed with appropriate margins during a relatively straightforward procedure.

If pathology later identifies a benign tumor, the pet is already cured.

If pathology identifies a low-grade malignant tumor that was completely removed, treatment may also be complete.

When Should You Biopsy First?

Sometimes the diagnosis changes everything.

Imagine a large mass on a dog's thigh.

If the veterinarian simply "shells out" the lump because it appears removable, the pathology report may later identify a soft tissue sarcoma.

Unlike benign tumors, soft tissue sarcomas frequently send microscopic cancer cells several centimeters beyond the visible edges of the mass.

The original surgery may leave behind cancer cells that cannot be seen with the naked eye.

Now the surgeon faces several challenges:

• Scar tissue has formed.

• Normal tissue planes have been disrupted.

• The original tumor boundaries are no longer obvious.

• Achieving clean margins becomes much more difficult.

The opportunity for the best surgery has already been lost.

Why Surgical Planning Matters

Veterinary surgical oncologists often repeat a simple phrase:

"The first surgery is the best opportunity for cure."

Cancer surgery isn't just about removing what you can see.

It's about removing the microscopic disease that surrounds many tumors.

Different cancers require dramatically different surgical margins.

For example:

• A benign lipoma may require only simple removal.

• A low-grade soft tissue sarcoma often requires margins of 2–3 cm plus one deep fascial plane.

• A feline injection-site sarcoma may require margins of 5 cm or more, often including underlying muscle or bone.

• An oral melanoma may require partial removal of the jaw.

• A peripheral nerve sheath tumor may require limb amputation or extensive nerve resection.

Without knowing what type of tumor is present, it's impossible to know which surgery offers the best chance of cure.

Large or Difficult Tumors Often Deserve a Biopsy First

Veterinarians are more likely to recommend an incisional biopsy when surgery could involve:

• Limb amputation

• Chest wall reconstruction

• Large skin flaps

• Facial reconstruction

• Perineal reconstruction

• Body wall surgery

In these situations, the pathology results help determine:

• Whether surgery is appropriate

• How aggressive surgery should be

• Whether advanced imaging is needed

• Whether referral to a surgical specialist is recommended

• Whether radiation or chemotherapy should be included in the treatment plan

What About Mast Cell Tumors?

Mast cell tumors deserve special mention because they are among the most common skin cancers in dogs.

Fortunately, many mast cell tumors can be diagnosed with a simple fine-needle aspiration.

If the tumor is:

• Small

• Located where adequate margins are achievable

• Easily removable

Many surgeons proceed directly to definitive surgery.

However, very large mast cell tumors, tumors in difficult locations, or tumors suspected to be high grade may benefit from additional planning before surgery.

Cats Often Require More Aggressive Planning

Cats behave differently than dogs when it comes to certain cancers.

Some feline tumors, particularly injection-site sarcomas, are extremely invasive.

Research has consistently shown that cats undergoing aggressive first surgery have significantly better long-term outcomes than cats undergoing multiple surgeries after incomplete removal.

This is one reason veterinarians are often more cautious about recommending biopsy before definitive surgery in cats.

The Role of Advanced Imaging

For larger tumors, your veterinarian may recommend CT or MRI before surgery.

Advanced imaging can reveal:

• How deeply the tumor extends

• Whether muscles, bones, or nerves are involved

• The relationship to important blood vessels

• The best surgical approach

This information is particularly valuable when limb-sparing surgery or reconstruction is being considered.

The Bottom Line

Not every lump should be biopsied before removal.

And not every lump should simply be removed.

The decision depends on one simple question:

Will knowing the diagnosis change the surgery?

If the answer is no, removing the mass may be the best option.

If the answer is yes, a biopsy first can dramatically improve surgical planning and increase the chances of completely curing the disease.

In veterinary surgical oncology, success isn't measured by whether the lump comes out.

It's measured by whether it comes out once—and with the best possible chance of never coming back.

If You Find a Lump on Your Pet

Remember these four steps:

1. Don't ignore it. Small lumps can still be malignant.

2. Ask about a fine-needle aspiration. It is often the quickest and least invasive first test.

3. Discuss whether the diagnosis would change the surgical plan.

4. When in doubt, ask if referral to a veterinary surgeon or oncologist is appropriate before surgery.

Thoughtful planning before the first incision often gives your pet the greatest opportunity for a successful outcome.