Classification of Tumors in Dogs

Written by rachael garrison
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Classification of Tumors in Dogs
Tumours can occur in any dog at any life stage. (Dog image by CiberZoom from

Tumours are defined as an abnormal growth of tissue, and they fall into two broad categories: malignant, or cancerous; or benign, or non-cancerous. Classification of your dog's tumour can be done only by a veterinarian or a veterinary pathologist. This may be done by either an aspirate or biopsy. Treatment of the tumour is almost always performed once a diagnosis has been made, and treatment can include doing nothing, surgery or chemotherapy.


If you ever suspect that your dog has a skin tumour, even if it feels like a small benign mass, never hesitate to take him to the veterinarian to identify the mass as benign and set your mind at ease. Malignant masses can often feel like benign fatty lumps, so removing of any suspicious mass is recommended. Tumours don't only occur on the skin; many tumours grow in areas that aren't so apparent, such as the abdominal cavity, chest, bone and other areas. If your dog is having trouble breathing, is losing weight or displaying other abnormal behaviour, take him to your veterinarian to have him checked for an underlying disease.

Benign Skin Tumors

Skin tumours are often benign. Older dogs are more likely to develop them, and there are many different types.

Sebaceous cysts are a fluid-filled sebaceous gland that can be drained or removed if necessary, and there are a number of other benign sebaceous gland tumours as well. Warts or "skin tags" grow over time, are benign and can be removed.

Lipomas are a common mass that are typically located under the skin in any area of the body and usually feel slightly soft on palpation. These occur more often in overweight, older dogs and can grow quite large.

A hematomas is a blood-filled area that often occurs in the ear due to mites or infection. It is benign and can typically be treated easily by your veterinarian.

Malignant Skin Tumors

Any skin tumour that appears to you to be benign may actually be a malignant tumour and must be diagnosed by your veterinarian by either aspirate or biopsy.

One of the most common malignant skin tumours is a mast cell tumour, which comes in many forms, sizes and locations. They can be difficult to remove and treat due to their contents--mast cells--which can lead to systemic issues even if removed. Other malignant skin tumours include melanomas, carcinomas and mammary tumours, which can sometimes be benign. Removal isn't always an option with malignant skin tumours, because entire tumour may not be able to be removed and because systemic issues, such as metastasis, could result. Sometimes chemotherapy is the preferred treatment.

Abdominal and Thoracic Tumors

Abdominal tumours in dogs are identified by their location and classification. Tumours can occur on any organ, such as the liver, spleen, kidney, adrenal gland, lymph nodes, stomach, intestinal wall, bladder, prostate and more. The type of tumour must be diagnosed by aspirate or biopsy, often requiring sedation or surgery. Chest tumours are almost always malignant and typically are spread from another tumour in a separate location.

Other Tumors

Other tumours can occur in the brain, and even if benign, they can cause some serious issues due to their location. Bone tumours, such as osteosarcoma, often require surgery, such as amputation, in addition to chemotherapy. Nasal or thoracic tumours are often first noticed when there is trouble breathing. They can lead to a number of issues; most are difficult or impossible to remove and, may require chemotherapy or radiation.

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