Tumors of the skin and soft tissues 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 to develop 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 clinically apparent size 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 so-called acquired hamartomas are difficult to differentiate 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.
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 older companion animals. For more aggressive neoplastic diseases or for 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 with the least cost and often with the fewest adverse events.
In most cases, cytologic examination of fine-needle aspirates of skin and soft tissue tumors should be performed to allow determination of tumor type and appropriate treatment planning. Fine-needle aspiration can save many companion animals unnecessary surgery for benign masses. If the tumor is malignant, then the surgeon can plan for wide, deep surgical margins to achieve adequate removal at the first surgery. For some neoplasms (eg, round cell tumors), cytologic examination can rival or even surpass the value of histologic examination. Cytologic examination can be nondiagnostic due to poor technique, and some tumor types are poorly exfoliative. When aspirating a mass, care should be taken not to introduce the needle too deep and come out the other side into the surrounding tissues as needle biopsies can dislodge and seed neoplastic cells along needle track or lead to metastasis. Appropriate clinical judgment and owner input may suggest a need to proceed directly to excisional biopsy (ie, removal of the mass without prior histologic evaluation) in some cases.
To establish a definitive diagnosis, histologic examination is generally required along with immunohistochemical staining to detect expression of specific cellular markers that determine phenotype. Histologic assessment of margin status is useful to predict local recurrence of cutaneous malignant tumors treated by means of excision alone. However, method accuracy varies among tumor types and grades. Recurrence times suggest postsurgical follow-up should continue for ≥2 years. Careful postsurgical management is recommended for animals with both infiltrated and close tumor margins.
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. Electrochemotherapy (electroporation) applied to tumor margins and the tumor bed at the time of surgery, before closure, or applied to the tumor bed incision after surgery may extend the tumor-free interval.
Cryosurgery is also an option, although it is more effective for benign, superficial lesions than for malignant, cutaneous neoplasms. Radiation therapy, including stereotactic and palliative radiation, is of most value for infiltrative neoplasms not surgically resectable or when surgical intervention would cause unacceptable physical impairment. Chemotherapy can be used either systemically (and if appropriate, intralesionally or via electroporation) as a primary method for treatment of malignant neoplasms or as an adjunct to surgery or radiation therapy.
In the skin, radiation is most commonly used to treat round cell tumors (eg, lymphosarcomas, mast cell tumors, transmissible venereal tumors) or solid tumors that cannot be excised completely. Although generally palliative, long remissions may sometimes be obtained with radiation therapy. Other forms of therapy include hyperthermia therapy, laser therapy, photodynamic therapy, antiangiogenic therapy, metronomic therapy, gene therapy, immunotherapy, and multimodal therapy using a combination or sequencing of various therapies.
Biller B, Berg J, Garrett L, et al. 2016 AAHA Oncology Guidelines for Dogs and Cats. J Am Anim Hosp Assoc. 2016 Jul-Aug;52(4):181-204.
Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition.
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