Cutaneous tumors are the most frequently diagnosed neoplastic disorders in domestic animals, in part because they can be identified easily and in part because the constant exposure of the skin to the external environment predisposes this organ to neoplastic transformation. Chemical carcinogens, ionizing radiation, and viruses all have been implicated, but hormonal and genetic factors may also play a role in development of cutaneous neoplasms.

The skin is a complex structure composed of various epithelial (epidermis, adnexa), mesenchymal (fibrous connective tissues, blood vessels, adipose tissue), and neural and neuroectodermal tissues (peripheral nerve, Merkel cells, melanocytes), all with the potential of developing distinctive tumors. Because cutaneous tumors are so diverse, their classification is difficult and often controversial. There is also controversy regarding the criteria used to define whether a lesion that arises in the skin or soft tissues is neoplastic and, if so, whether it is benign or malignant. To avoid confusion, the following terms are used in this discussion: A hamartoma (nevus) is a localized developmental defect associated with enlargement of one or more elements of the skin. A sebaceous hamartoma, for example, refers to a localized region of the skin where sebaceous glands are extremely prominent and sometimes malformed. Although by strict definition, hamartomas are present at birth, they may occasionally take a long time to reach a size when they are clinically apparent and may not be diagnosed until an animal is mature. To confuse matters further, some lesions with clinical and histologic features of congenital hamartomas may develop in adult animals. Such “acquired” hamartomas are difficult to separate from benign epithelial and mesenchymal neoplasms. In human medical literature and some veterinary texts, the term “nevus” is used synonymously with hamartoma. A benign neoplasm is localized, noninfiltrative, and because it is surrounded by a capsule, easily excisable. A neoplasm of intermediate malignancy is locally infiltrative and difficult to excise but does not metastasize. A malignant neoplasm is infiltrative with metastatic potential.

Although cutaneous neoplasms characteristically are nodular or papular, they also can occur as localized or generalized alopecic plaques, erythematous and pigmented patches and plaques, wheals, or nonhealing ulcers. The variability in clinical presentation can make distinguishing a neoplasm from an inflammatory disease difficult; furthermore, distinguishing a benign tumor from a malignant tumor is even more subjective because sarcomas or carcinomas early in their development may palpate as discrete, encapsulated masses. To establish a definitive diagnosis, histopathology is generally required. Cytologic evaluation can also be useful and, for some neoplasms (eg, round cell tumors), can rival or even surpass the value of histologic examination.

Therapy depends largely on the type of tumor, its location and size, and signalment of the animal. For benign neoplasms associated with neither ulceration nor clinical dysfunction, no therapy may be the most prudent option, especially in aged dogs.

For more aggressive neoplastic diseases or benign tumors that inhibit normal function or are cosmetically unpleasant, there are several therapeutic options. For most, surgical intervention with complete excision provides the best chance of a cure at least cost and often with the fewest side effects. Lumpectomy is adequate for benign lesions, but if a malignancy is suspected, the lesion should be removed with wide (3 cm) surgical margins. For tumors that cannot be completely excised, partial removal or debulking may prolong the life of the animal and increase the effectiveness of radiation or chemotherapy. Cryosurgery is also an option, although it is more effective for benign, superficial lesions than for malignant cutaneous neoplasms. Radiation therapy is of most value for infiltrative neoplasms that are not surgically resectable, or when surgical intervention would cause unacceptable physical impairment. Chemotherapy can be used either as a primary method for treatment of malignant neoplasms or as an adjunct to surgery or radiotherapy. In the skin, it is most commonly used to treat round cell tumors (eg, lymphosarcomas, mast cell tumors, transmissible venereal tumors, etc) or solid tumors that cannot be excised completely. Although generally palliative, long remissions may sometimes be obtained. Other forms of therapy include hyperthermia, laser therapy, photodynamic therapy, antiangiogenic therapy, gene therapy, and immunotherapy.

Fig. 1
Diagram showing the octopus-like structure of many malignant skin tumors and why some are not completely or adequately removed at surgery. Complete margins are often not “adequate” or wide enough to prevent recurrence due to residual cells left in and around the tumor bed. Courtesy of Dr. Alice Villalobos.

Last full review/revision July 2011 by Alice Villalobos, DVM, DPNAP; Margaret Finlay, BVMS, MRCVS