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Tumors In Dogs And Cats
What's the difference between a tumor and a cancer?

One of the most memorable but frightening events in a pet owner's life is the discovery of a lump on the pet.  It wasn't there before, was it!  And  now you need to know what it is before you can determine if the lump is serious or nothing to worry about.  Does it need to be removed?
 Look for the answers here at

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key words: dog... cat... tumors, growths, lumps, bumps, masses 

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I'm confused.  No one has explained to me what the difference is between a growth, a tumor, or a cancer.  My dog has these bumps of different sizes under her skin but my vet says not to work, they're just fat deposits.

When is a lump a cancer?  Is a mass a cancer?  Are all tumors cancerous? 

Oh, yes... how can you tell when a lump or whatever needs to be surgically removed?

A histiocytoma on foreleg of a dog
A histiocytoma... not a tumor

  A biopsy procedure entails obtaining a small tissue sample from a larger mass (not totally removing the mass) for microscopic evaluation by a pathologist.  Biopsies can be taken via needle penetration of the mass or by scalpel excision or even by a pinch or punch technique.

  An impression smear utilizes a glass microscope slide that is pressed onto an open lesion which imparts cells onto the slide.  These cells are then stained and analyzed.

      You have some excellent questions and we'll try to simplify, keeping in mind that in medicine there are often grey areas of knowledge as we strive to clarify information as black or white.  For example a group of cells in the skin may appear under microscopic examination to be nearly normal...or not quite abnormal... or precancerous... or display early changes from normal.  And that's the best we can do for now in some situations.

Question:  When does a lump need to be removed?
1.)  When there is evidence from a biopsy that the lump is a type of cancer that could continue to grow or spread.
2.)  When its presence creates a comfort, movement, infection or cosmetic problem.
3.)   When it is a cyst filled with dead skin or fluid and poses a threat of infection or significant tissue damage or disfiguration.  An example would be an enlarging sebaceous cyst on an eyelid margin.

Look at the definitions in the column to the right.  Each term (word) could have several descriptions of a definition, plus... there are several "medical dictionaries". 

Fact:  A Specialist in Veterinary Pathology needs to look st the cells before a definitive diagnosis can A large dermoid cyst in a dogbe made regarding any tumor.  Your veterinarian may elect to make an informed presumptive diagnosis, but a definitive diagnosis rests with the pathology specialist after looking at microscopic specimens of the tissue. Based upon established protocols, cells will fall into certain classifications ranging from normal through highly malignant.  By taking a sample of the lump or bump, called doing a biopsy, your veterinarian will be best able to tell you what a lump is and what, if anything, needs to be done.

A human dermatologist told me "The only way we know
 if we need to do a biopsy is to do a biopsy".

Doctor's Notes

Let's look over a few short definitions...
  A lump is is an abnormal enlargement of tissue

   A growth is tissue that has enlarged by excess cell reproduction.  To find out what the growth is, a tissue sample (biopsy) needs microscopic analysis by a veterinary pathologist.  So the term "growth" is very general.

  A mass is a grouping of individual parts or elements that compose a unified body of unspecified size or quantity.

  A tumor is a. any abnormal swelling; b. a mass of tissue formed by a new growth of cells, normally independent of the surrounding structures

  A cancer is a.) a malignant and invasive growth or tumor, esp. one originating in epithelium, tending to recur after excision and to metastasize to other sites.
any disease characterized by such growths.

  Benign refers to a tissue mass that stays confined to the immediate area, does not metastasize, and may or may no do harm depending upon where it originates and how large it becomes.

  Malignant means rapidly growing, invasive and capable of spreading cancerous cells through the blood or lymph to other areas of the body.
A tumor characterized
by uncontrolled growth; cancerous, invasive, or metastatic.

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Some small growths must have wide excisions done to improve the chances of eliminating any infiltrating tumor cells. This huge tumor of the dog spleen has potential to release cells into the blood and lymph and is a risk for intra-abdominal hemorrhage. This benign tumor arising from the skin of a dog's thigh took years to grow to this size. Even innocuous appearing small skin nodules have potential to be very serious tumors and should be evaluated by a veterinary pathologist.

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