Skin cancer in cats: what are the most common neoplasms?
Introduction
The skin and subcutaneous cell tissue are the primary locations of tumours in cats; between 29.6 and 41.5% of all neoplasms develop in these tissues.1 As it is the largest and most exposed organ, the skin is especially prone to external lesions, but is also highly visible and palpable.1 Malignancy rates reported for skin tumours vary between 52.7%1 and 76.1%2 depending on the study.
Although there is some discrepancy in the literature regarding the prevalence, the general consensus is that the four most common skin neoplasms are squamous cell carcinomas, fibrosarcomas, basal cell carcinomas and mast cell tumours.1,3 Although clinically relevant, lymphomas, lymphangiosarcomas, adenocarcinomas of ceruminous glands and digital tumours are less common.3
Main skin cancers in cats: basal cell carcinomas
Basal cell carcinomas (BCC) originate from pluripotent basal cells in the skin appendages or epidermis. They account for between 11% and 28% of all skin neoplasms and several studies describe them as the most common skin cancer in cats.1 The mean age of affected cats is 7–10 years and the most predisposed breeds are Siamese, Persian, Himalayan and the domestic long-haired cat.3
These neoplasms are mainly located in the head and neck, although they can be found anywhere on the body.4 Most are benign, slow-growing and rarely metastasise.3,4 They develop as well-defined, sometimes pigmented, isolated masses that usually measure around 0.5–2.5 cm, although they can reach diameters of 10 cm. They are compact tumours that move when palpated as they not attached to deep layers.3
If a subsequent histological study confirms that surgical excision removed the entire mass, treatment is usually curative.4 BCCs are very radiosensitive and radiation therapy may be a suitable alternative to excision. In cats with a high risk of surgical complications or for whom the outcome would be unaesthetic, conservative therapy may suffice as the risk of metastasis is very low.3
Fibrosarcomas
Fibrosarcomas can appear anywhere on the body and develop from fibroblasts in the subcutaneous tissue and occasionally the dermis. They are the most prevalent mesenchymal tumour and represent 20–43% of all skin cancers in cats. They are usually described as the second leading skin cancer in cats, although some studies place them as the most prevalent.1,2 There is no apparent predisposition for breed or sex.
Fibrosarcomas are usually divided into three distinct clinical entities. Multiple fibrosarcomas in young cats, although considered very rare, appear more frequently in cats under 5 years who are also infected with either feline leukaemia virus (FeLV) or feline sarcoma virus, which is considered the causal agent.3 They manifest as firm, rapid-growing masses that are poorly differentiated and cause adhesions in adjacent tissues.3 While they can be removed surgically in early stages, their recurrence and the appearance of new masses is common, so the prognosis is serious.
Solitary fibrosarcomas, of unknown aetiology, usually occur in isolation in cats aged 8–10 years who are free from viral infections. They have a firm consistency, irregular outline, are well-differentiated, slow-growing and poorly delimited. Metastasis is rare and it usually affects the lungs. The standard of care is surgical removal with wide resection margins as they are susceptible to a high rate of recurrence.3
Feline injection site fibrosarcomas are invasive neoplasms of unknown aetiology. Recent data on their pathogenesis indicate there is a genetic predisposition whereby a chronic inflammatory reaction can lead to malignant transformations in fibroblasts and myofibroblasts. This has been associated with vaccines in cats, amongst other causes.3 Although fibrosarcomas have a low incidence, when they appear, they grow rapidly, are infiltrating and very rarely metastasise. Although surgical removal can be curative, recurrence is very typical, even when the histological study reveals wide, tumour-free margins.3
Squamous cell carcinomas
Squamous cell carcinomas (SCC) are a malignant neoplasm that originates in the squamous epithelium and accounts for 15% of skin tumours in cats.5 Various studies place SCC as the third-most common skin cancer in cats.1,2 These tumours usually affect geriatric cats, with an average age of 10–12 years.5
Their main cause is exposure to ultraviolet (UV) light and they tend to be located almost exclusively on the head. Cats with white fur or white areas on their coat are at greatest risk. The coat is a physical barrier to UV radiation, so SCC appears in unpigmented regions without much hair, such as the ears, eyelids or the nasal planum.4,5
Before SCC evolves into invasive lesions, cats develop erythematous lesions with some scabs that do not heal. Histologically, these lesions may be observed as actinic keratosis or carcinoma in situ. If left untreated, the lesions will ulcerate before invading and destroying adjacent tissues.4
Diagnosis is reached by means of a biopsy, as most lesions are too superficial or inflamed to determine a reliable diagnosis via a fine-needle aspiration. Cutaneous SCC metastasise very slowly and, if they do metastasise, they usually compromise the lymph nodes and lungs.5
The treatment options depend heavily on the location and extent of lesions,5 although early and complete excision with wide surgical margins is recommended in most cases.3 Superficial lesions of the nasal planum, small isolated palpebral lesions and lesions on the ear tips are relatively easy to treat, but those at a more advanced stage are more difficult and can greatly affect the animal’s aesthetics.5
Mast cell tumours
Cutaneous mast cell tumours usually appear in older cats, although the age of presentation varies significantly from kittens under 12 months up to 19-year-old elderly cats. The specific aetiology of feline mast cell tumours is unknown, although some studies have described a genetic predisposition in Siamese cats.6 They are reported to represent 2–21% of all feline skin neoplasms and are the fourth-most common skin tumour.1
Cutaneous mast cell tumours (MCTs) are usually found in the head, neck and trunk and are histologically classified into two forms: mastocytic and atypical (formerly known as histiocytic). The mastocytic form is the more common and is subdivided into two forms: well differentiated (previously known as compact) or poorly differentiated (previously known as pleomorphic or diffuse).7 Well-differentiated, mastocytic MCTs are circumscribed, non-enveloped masses formed by uniform, round cells with little mitotic activity. This is the main histological type with one review reporting that they account for 60% of all cutaneous MCTs in cats.7
A diagnosis is usually made through a cytological study from a fine-needle aspiration of the mass.7 Surgery, if feasible, is the treatment of choice for mast cell tumours. Surgical excision of the mastocytic form is usually curative, with a low rate of recurrence and metastasis, even when minimal surgical margins are noted in histopathology. Poorly differentiated, mastocytic MCTs with a high mitotic index have an increased risk of metastasis.
Conclusions
Skin cancers in cats are among the most common neoplasms diagnosed in the feline species, while a high percentage are malignant. The four most prevalent neoplasms are squamous cell carcinomas, fibrosarcomas, basal cell carcinomas and mastocytomas. Surgical resection is usually the treatment of choice in most cats.
