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What’s That Lump? Four Common Growths In Dogs

Lumps and bumps and skin growths are one of the most common complaints among our canine patients, both young and old.  While most growths turn out to be benign, new growths should always be checked out.

These four types of tumors are commonly seen in dogs.

Type of Tumor: Benign

This is a type of tumor that comes from fat cells.  A normal fat cell is capable of enlarging to store fat.  A tumorous fat cell grows at a higher rate than normal fat cells, forming a lump under the skin.  These fatty tumors are seen more commonly in overweight animals and a lipoma will shrink as an overweight patient loses fat, although the tumor will usually not disappear completely.

A lipoma is diagnosed by taking a sample of the cells using a needle.  The cells appear as fat droplets on a microscope slide.  Lipomas are usually a cosmetic issue, but your vet may recommend removal if the tumor is very large, interferes with the patient’s mobility or if it is in an area that makes the patient uncomfortable.  Once removed, lipomas don’t tend to recur, but a patient can develop new ones elsewhere on the body.

Type of Tumor: Benign

This type of skin growth is usually seen in young dogs and usually on the front half of the body.  A histiocytoma comes from a type of cell in the skin that is part of the immune system and if left alone, will usually go away on its own.  A histiocytoma is round and red with an eroded appearance.  Since this type of tumor can look similar to more dangerous growths, your vet will usually recommend removing the growth and sending it to the pathology lab.

Sebaceous Cyst
Type of Tumor: Benign

This type of lump comes from the sebaceous glands in the skin, which are responsible for excreting an oily substance called sebum that lubricates the hair follicle and keeps the skin and coat healthy.  If inflammation results or the follicle becomes clogged, a sac forms under the skin and fills up with sebum, creating a lump.

A sebaceous cyst usually does not bother the dog.  Sometimes they will rupture on their own, oozing a thick, waxy substance that is white, yellow or sometimes black.  Some cysts will resolve on their own, while others will continue to rupture and refill.  Your vet may recommend surgical removal of the cyst if it becomes chronically infected or if it is very large. A diagnosis is made using a needle biopsy to obtain cells from the lump to examine under the microscope.

Mast Cell Tumor
Type of Tumor: Malignant

Mast cells are found within tissues that are exposed to invaders from the outside world, namely the skin, intestinal tract and respiratory tract.  Part of the immune system, they are responsible for defending the body against parasites and do so by releasing toxic biochemicals.  In the absence of adequate parasites to fight, mast cells will sometimes overreact to less harmful antigens like pollen, releasing the biochemicals that will then cause localized redness, itching and inflammation.  Mast cells are capable of forming tumors made of many mast cells congregated together.  When this happens, simple contact is enough to trigger release of the toxic biochemicals.

Mast cells are most commonly seen in the skin, although they can appear in any tissue that contains mast cells.  Mast cell tumors are red and angry looking, very invasive and difficult to treat.

A mast cell tumor can be diagnosed with a needle aspirate, but a tissue biopsy is needed to grade the stage of the tumor to determine treatment.  Lower grade tumors can be cured with surgical removal, taking wide margins all the way around the growth.  Higher grade tumors will need additional treatment with radiation or chemotherapy to prevent recurrence.

If you notice a new growth on your pet, or if an existing growth has changed size drastically or is bleeding or oozing, an exam with your vet is needed.  The only definitive way to differentiate between a benign growth and a malignant one is with biopsy and cytology.

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