Principally, we can differentiate two types of skin tumours. The first kind presents as a nodule and includes tumours such as lipomas, fibromas, fibrosarcomas, etc. The approach to any nodule in the skin includes aspiration cytology and subsequent biopsy. Classically, the nodular tumours may be again subdivided into benign neoplasias (such as the vast majority of the tumours derived from follicular epithelium as well as lipomas or sebaceous adenomas) and malignant tumours such as fibro-sarcomas. The treatment of choice for benign neoplasias is surgical excision and the prognosis is excellent. The prognosis for malignant neoplasias presenting as nodules is variable and depends on tumour type and site of occurrence. However, the second type of skin tumours which more frequently present to dermatologists than to oncologists includes neoplasias which may truly be misdiagnosed as an inflammatory dermatitis. I firmly believe that wherever possible, oncologists should be consulted for the most up-to-date treatment options available as, although dermatologists may diagnose many of these cases, oncologists are the best trained in the subtle management of most cancers, and therapeutic options change rapidly in the world of oncology. From a prognostic point of view, a general rule of thumb used when discussing nodular tumours is that 80% of skin tumours in cats are malignant as opposed to the majority of those in dogs which are benign. The definition of malignant is frequently mistaken to define 'metastatic'. Many 'malignant' tumours result in euthanasia not because of metastasis, but because of local invasion resulting in an inoperable tumour. Skin tumours covered in this lecture include those which present as scaling/erythema and alopecia and as miliary dermatitis of cats.

Skin Tumours Which Present as Scaling, Erythema And Alopecia

Epitheliotropic (T-Cell) Cutaneous Lymphoma (Mycosis Fungoides)

Epitheliotropic lymphoma or mycosis fungoides (MF) is a special variant of cutaneous lymphoma. It is dominantly a T-cell tumour of atypical lymphocytes. This tumour has a strong affinity for the epithelium so that the epidermis and follicular epithelium are typically heavily infiltrated by lymphocytes where the underlying/surrounding dermis may be only sparsely involved. In very early stages, the lymphocytic infiltrate can be extremely difficult to differentiate from that of a non-specific allergic reaction or a mild, subtle Malassezia dermatitis. Later in the development of the disease the characteristic intraepidermal microabscesses (Pautrier's micropustules) may be found. The tumour development is typically slow: it may be present for several years from start to finish in some animals.

The clinical appearance of MF varies enormously in animals. Initially, however, it presents as a subtle, erythematous, scaling and mildly alopecic lesion. These may be solitary or multifocal. In humans this stage may last for more than a decade before developing into plaques and nodules. The later stage--which clinically resembles a mycosis--is the source of the name. The scaly, erythematous lesions are frequently associated with pruritus and very easy to confuse clinically with allergies and other inflammatory dermatoses. The scales and erythema may even encourage a mild secondary overgrowth of Malassezia or bacteria, further confusing the diagnosis as they will then partially respond to the appropriately given antimicrobial therapy. The average survival time following a confirmed diagnosis ranges from 4-7 months. Nodules occur in the end stage, at which time, in animals, a clinical cure is not to be expected.

When many different stages are present on the same animal, the clinical appearance can be highly suggestive of this diagnosis. Biopsy at that late stage should be diagnostic. However, when the lesions are early and there is pruritus with the erythema and scale, the diagnosis can be tricky. At this clinical stage there may be a diagnostic biopsy finding. However, there may also be mild interface dermatitis, a lymphocytic exocytosis or a mild superficial perivascular dermatitis with exocytosis. These are all relatively non-specific findings, often taken in an itchy animal as representing a likely hypersensitivity. In humans there are documented cases of MF arising in sites of chronic allergy. It is possible this is also the case in animals. It is therefore more than theoretically possible that an original diagnosis of allergies may indeed transform into this tumour!

The aim of treatment is not to achieve a clinical remission or even to prolong life, but to improve the quality of life. A number of treatments have been reviewed. In practice, glucocorticoids administered with concurrent retinoids (synthetic Vitamin A derivatives, which are safer, but significantly more expensive) are commonly recommended. The use of essential fatty acid supplementation at the rate of 3 ml/kg twice a week has been reported to be associated with clinical success. In practice, for a number of dermatologists, this has not been successful.

Feline Multicentric Squamous Cell Carcinoma In situ (Bowens In situ Carcinoma)

This is an uncommon skin neoplasia of older cats (>10 years). Solitary lesions are reported, but the typical presentation is multifocal. The scaling lesions are preceded by hyperpigmented macules with alopecia, these lesions are subtle and frequently not observed by the owners. The alopecic, scaly to crusted lesions are mostly found on the face, neck, shoulders and legs. This syndrome is not well understood, but 45% of tested lesions have been positive for papillomavirus on immunohistochemistry. A significant number of cats are feline immunodeficiency virus (FIV) positive and these are reportedly more difficult to manage. In addition lesions have been associated with localised demodicosis. Although lesions are typically confined to the skin, metastasis following transformation to malignant squamous cell carcinoma has been reported in 17% of cases.

Reportedly successful therapies have included excisional surgery and acitretin 2-3 mg/kg q24h. Local therapy with imiquimod (Aldara 5%, 3M pharmaceuticals, a local human antiviral topical medication) has been anecdotally successful in some cases. In cats, the oral ingestion of the cream must be avoided through the use of baby suits or Elizabethan collars. 5-fluorouracil has been used with success for dogs; it is, however, contraindicated in cats due to neurotoxicity.

Actinic Keratoses

This dysplastic skin lesion presents with scaling and alopecia in the sparsely haired areas of the body. These lesions have the definite potential to develop into squamous cell carcinoma with repeated, ongoing ultraviolet exposure.

Early Squamous Cell Carcinoma

Although this is an extension of a dysplastic epidermal change, it may occur acutely and not necessarily arise from a chronic lesion of actinic keratosis. Many squamous cell carcinomas are eroded or ulcerated focal lesions which typically develop serocellular crusts, heal partially, lose the crusts and then the cycle begins again. If they move past the clinical stage of erythema and scaling to erosion and ulceration, then they are typically transforming into an invasive squamous cell carcinoma.

Skin Tumours Which Present as Miliary Dermatitis (in Cats)

Cutaneous Mast Cell Tumours of Cats

These are common (8%, 15%, 21% of all skin tumours, depending on the report) and in comparison to dogs are typically behaviourally benign. One presentation is that of either military dermatitis (multifocal crusted papules) or of multiple small papules to nodules (typically less than 1 cm diameter). In cats, 'well differentiated' and 'poorly differentiated' tumour categories are used commonly. One 'poorly differentiated type' that has also been referred to as histiocytic mast cell tumour, has a reported age and breed predisposition for young Siamese.

Selected Paraneoplastic Syndromes

Internal neoplasia may also be associated with skin disease and the syndromes listed below require a further work-up and treatment beyond just a 'skin-deep' approach.

Feline Paraneoplastic Alopecia

The typical clinical presentation is of an older cat with hair loss on the abdomen and ventral chest and a 'shiny' skin. Early cases present with marked scaling and a characteristic feature is of an adherent brown seborrhoea which frequently affects the claw folds, toes and feet/lower legs, but may also occur on the trunk. Malassezia are frequently found in large numbers within this exudate.

Paraneoplastic Exfoliative Dermatitis (Thymoma-Associated Dermatitis)

Cats present with a severe scaling and alopecia which typically begins on the head and neck and may extend over the entire body. Pruritus is a common feature, although the grade of pruritus is lower than the grade of the scale-crust formation. Many of these cases were initially reported in older cats with thymoma.

Metabolic Epidermal Necrosis/ Hepatocutaneous Syndrome/Superficial Necrolytic Dermatitis

Although this was originally reported in dogs with diabetes mellitus, it has since been recognised to be more commonly associated with chronic end-stage liver disease and occasionally with pancreatic tumours. The most common presenting complaint in typically old dogs is a severe, painful, crusted and cracked/fissured dermatitis affecting the footpads and scrotum. Skin changes precede systemic signs such as depression and lethargy. Secondary infections are frequent, severe and superficial, and typically involve yeast, bacteria or both. Diagnosis is made by skin biopsies and a characteristic echolucid and echodense honeycomb pattern on hepatic ultrasonography.