10 Common Skin Tumors in Dogs and Cats

Finding a lump or bump on your fur baby can be a typical experience for a pet parent. The skin is the most common site of tumor growth in dogs and second most common in cats. However, finding a new bump is not necessarily a reason to panic. There are many benign growths visible on the skin. It is important to have any new lumps or bumps checked by your veterinarian. They may do a fine needle aspiration to look at cells under a microscope, recommend a biopsy to remove the mass, or take a larger piece of the mass to get a diagnosis.

Below we will review the 10 most common skin tumors in dogs and cats.

1. Papillomas. Papillomas are true warts caused by a papillomavirus. They can be solitary, but there are generally many present on the skin. The canine oral form is the most common variance and affects younger animals. A papilloma is usually a pink or grey cauliflower pedunculated nodule. The papillomavirus is contagious, but the disease is very treatable. Papillomas are much rarer in cats.
2. Squamous cell carcinomas (SCC). SCCs are a common solar-induced neoplasia in dogs and cats. They appear as firm, fleshy nodules which may ulcerate or have crusting associated with them. They are malignant, but are generally locally invasive and less commonly metastasize to other organ systems. SCC are typically seen in older dogs. Lightly pigmented areas of skin are more prone to develop this type of cancer, and surgical excision is generally the treatment of choice. The location of the mass may affect the prognosis.
3. Hair follicle tumors. This category is made up of five different tumors that arise from different portions of the hair follicle. They generally appear as solitary, rounded, alopecic, firm nodules. Most of these tumors have benign behavior and are cured with surgical removal. These are fairly common in dogs and rare in cats.
4. Sebaceous adenomas. Sebaceous adenomas are often referred to as “old dog warts.” While they appear similar as pink-grey, waxy, lobulated nodules they are not viral in origin, but proliferations of the oil glands (sebaceous glands). They are benign, but often are cosmetically displeasing to owners or they are easily traumatized during grooming or by the pet. These are very common in dogs and rare in cats. There are many options from cryosurgery to surgical removal of these nodules.
5. Hemangiomas/cutaneous hemangiosarcomas. These are tumors of vascular origin and often appear as blood-filled nodules. They may be red to bluish-black. They are typically solar-induced neoplasia, which are common in light coat dogs. Most hemangiomas are benign, but can cause problems as they grow, break open, and bleed. Malignant cutaneous hemangiosarcomas are less common, but may spread to other organ systems. Biopsy of these nodules is important to determine if they are benign or malignant. There are non-surgical options like cryosurgery for more benign lesions.
6. Lipomas. Lipomas are benign accumulations of lipocytes (fat cells). They are one of the most common tumors in dogs and generally feel like soft movable nodules under the skin. They are not painful and slow growing, and can generally be diagnosed with a fine needle aspiration. Many owners will opt for monitoring of these nodules. Surgical excision is curative, but new nodules may occur elsewhere on the body.
7. Mast cell tumors. Mast cell tumor behavior is dependent on their grade. Grade I or low-grade mast cell tumors are more benign, while high grade II and grade III have malignant behavior which requires aggressive therapy. Mast cell tumors most commonly appear as ulcerated, alopecic nodules which may wax and wane in size. They can generally be diagnosed. Although they are fairly easy to diagnose with a fine needle aspiration, surgical excision with wide margins is important for the management of mast cell tumors.
8. Histiocytomas. Histiocytomas are common benign growths mostly seen on young dogs. They are hairless pink nodules ,which are most commonly solitary in nature and found on the head, limbs, or ears. They can appear and grow rapidly over 1-4 weeks. Diagnosis is commonly obtained from fine needle aspiration. Many of these will be involute, meaning they will resolve on their own, and do not require surgery.
9. Melanocytomas/Melanomas. While melanomas have a strong negative connotation on the human side, in most cases, melanocytic neoplasias in dogs and cats have more benign behavior. Unlike humans, they are not related to solar exposure. They usually appear as pigmented nodules and are most commonly solitary in nature. The neoplastic behavior is often dependent on the location of the tumor. Dermal melanocytic tumors are more benign, while oral or digital melanomas are more malignant and aggressive. Surgical excision is the most common recommendation for melanocytic neoplasms.
10. Keratinizing acanthomas. These are large cystic nodules that are filled with thick material made of keratin and sebaceous material. Individuals that develop these tumors are prone to developing numerous tumors. While they are benign in nature they become problematic because they can be painful, rupture, and become infected. While surgical excision can help treat existing lesions, there is no way to prevent new growths.

Ultimately, many of the growths we find on our animals are benign in nature, but it is important to have any new lumps or bumps evaluated by your veterinarian. Growths that rapidly increase in size, change in appearance, or are bothersome to your pet should be checked out as soon as possible.