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I Found a Lump on My Pet — Is it Cancerous? 

Finding a new lump or bump on your beloved pet can be distressing. Many new lumps will prove to be benign, but any new skin mass or mass under the skin should be evaluated by your family veterinarian. 

In general, it is best to sample any new mass via fine-needle aspiration. This is a procedure where a small needle is used to draw material out of a mass. We then spray that material onto a slide, stain the slide, and look at it under a microscope (cytology). This is a quick and easy test that can be performed to differentiate potentially malignant masses from benign masses. In many cases, if malignancy is suspected, a full biopsy may be recommended. A biopsy differs from a fine needle aspirate in that a biopsy entails the removal of a piece of tissue from the mass where an aspirate removes only some cells. A biopsy is a more thorough way of diagnosing masses but often requires heavy sedation or anesthesia and is more invasive. For that reason, we often start with the less invasive aspirate then move on to biopsy if necessary. Often if a mass is found to be benign, a recommendation will be made to monitor the mass for changes over time. Changes that would be concerning include: a rapid change in size; changes in character such as becoming more firm or becoming attached to the underlying structures; or ulceration or redness of the overlying skin or rupture of the mass.

Such changes could indicate a more malignant cause for the mass and your family vet should be notified.

Some bumps and lumps are benign masses that are commonly seen. Here are descriptions of their characteristics as well as options to consider when such masses are found.

Lipoma: Lipomas are benign accumulations of fatty tissue. These are benign growths and neither metastasis nor invasive growth is expected. Lipomas can grow anywhere, and some dogs will develop multiple lipomas over time. Lipomas can grow to a very large size in some cases. Lipomas are generally easy to diagnose by fine needle aspirate and cytologic review as noted above. In most cases, lipomas do not need to be removed, but in some cases, they can limit mobility when they become large and that would be a reason to consider removal. There is a malignant version of a fatty tumor called liposarcoma but that is a rare form of cancer. Generally, the cells would look more aggressive when cytology is performed, which would tip the vet off that a malignant process is present. There is also a rare, somewhat strange version of a fatty tumor called infiltrative lipoma. This is a benign tumor-like the typical lipoma but it grows invasively into the nearby muscles. These can be difficult to diagnose as cytology resembles a lipoma but the growth pattern is different in that it grows more invasively and the tumor has much less discrete borders. Often, referral to an oncologist is sought if liposarcoma or infiltrative lipoma are suspected.

Sebaceous Hyperplasia/Adenoma/Epithelioma (Wart): Benign sebaceous tumors are common in older dogs. They generally occur on the skin and resemble warts or pink frond-like growths. Some can become pigmented (darkly colored). Sometimes they will crust or bleed, and now and again, they fall off (or pieces of them do). These are benign growths and neither metastasis nor invasive growth is expected. If the masses grow to a large size or irritate nearby structures (such as the eye), they can be removed, but removal is not necessary in many cases. It is best to sample them when first noted to rule out malignancy such as sebaceous carcinoma. If they are too small to be sampled, they can be monitored or a small biopsy can be performed to clarify the diagnosis.

Cysts: Various types of cysts can develop on the skin. Cysts appear as soft or firm bumps on the skin. As with lipomas and sebaceous growths, they should be sampled to clarify their origin. Often when cysts are sampled, we see clear to yellow fluid come from them. Cysts typically are benign. That said, they can grow to large sizes sometimes and occasionally they will rupture and ooze or become infected. If rupture or infection occurs, surgical removal is often considered. Otherwise, cysts do not need to be removed unless we are concerned that they may represent a more aggressive, malignant growth.

Jason Kidd, DVM, DACVIM (Oncology) practices at SAGE Concord. He attended veterinary school at the University of California, Davis and received internship training at Colorado State University, and residency training at North Carolina State University. His clinical interests include lymphoma, histiocytic cancers, and metronomic chemotherapy. Born in the Bay Area, he lives in Berkeley with his wife, Katherine (also a veterinarian), daughter, and pointer mix, Fintan. 

Above: A veterinarian prepares a golden retriever for surgery to remove a growth.

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Main article photo by: Chalabala-iStock