This is a common tumour in the dog and deserves special mention as they can often be misdiagnosed, along with the other common subungual tumour, melanoma, as infection. SCC is the most common subungual neoplasm and accounts for between 30 and 50% of all subungual tumours (Henry et al 2005, Marino et al 1995, O’Brien et al 1992, Wobeser et al 2007) and typically presents as a nail bed (paronychia) infection that does not respond to antibiotic therapy. It is seen most frequently in large breed dogs (~75%), with black dogs accounting for two-thirds of the cases (Vail & Withrow 2007). It arises from the subungual epithelium and often causes lysis of third phalanx (Figure 18.2).

Typically SCC is locally invasive but the majority of these tumours do not metastasize (Marino et al 1995). Multiple digits can be involved and unfortunately if digital amputation is not possible, this warrants a poor prognosis. Liptak et al (2005) reported good function and tumour control with amputation of both central weight-bearing digits (digits 2 and 3). In the authors’ experience, small dogs can function with amputation of most digits on a single limb.

The role of chemotherapy has not been established in the management of canine SCC and is currently not recommended. For superficial solar-induced and pre-neoplastic skin lesions, retinoids (etretinate) have been used (Marks et al 1992). Etretinate is a valid option for treatment of solar-induced pre-neoplastic lesions; however, the cost of treatment can be considerable. It may be considered for multifocal superficial disease not easily managed by other means of local therapy. PDT should be considered for canine patients with superficial disease (see Chapter 8).

Cutaneous lymphoma has always presented a treatment challenge because of the general poor response to standard chemotherapy protocols. The poorest responses were seen in patients with cutaneous T-cell lymphoma (CTCL). Immunophenotyping is recommended for all malignant cutaneous round cell tumours because it can be difficult to distinguish lymphomas from histiocytic tumours based on morphology alone, and with lymphoma immunophenotype is a prognostic indicator. CCNU is the drug of choice for cutaneous lymphomas. It will induce good partial to complete remission in many cases, although the response is transient with progression of disease inevitable (Figure 18.3). For a full discussion of lymphoma, see Chapter 22.

ELSA, although an uncommon clinical disease, is the most common variant of CTCL. Historically it has been referred to as ‘mycosis fungoides’ (MF) because of its similarities to MF in humans. CTCL typically expresses the T-cell markers CD3 and CD8, and ELSA is characterized by lymphocyte epitheliotropism throughout all stages of the disease.

The clinical presentation and progression of ELSA can be extremely variable. In some patients the diagnosis of ELSA may take many months as the patients present with ‘dandruff’ and are treated with medicated baths for dry skin. Eventually, these may progress to solitary patches (Figure 18.4), plaques or nodules, generalized erythema and scaling with mucocutaneous involvement. Most dogs affected with ELSA will become extremely pruritic. Rarely, patients may present with solitary oral lesions.

In patients with these clinical signs a biopsy is required for diagnosis. The histopathological changes seen in a biopsy are characteristic for ELSA and distinct from other cutaneous lymphomas. The unique characteristic of ELSA is the presence of Pautrier’s microabscesses (collections of neoplastic lymphocytes around cutaneous dendritic cells); the neoplastic cells are confined to the epidermis and show tropism for the hair follicles and apocrine sweat glands. Advanced disease is characterized by lymphadenopathy or circulating T cells in the peripheral blood (Sézary syndrome); the latter is extremely uncommon.

The variable clinical behaviour of ELSA is well documented, with a range from mild indolent disease to a rapidly progressive and fatal condition with survival times of only a few months. The more advanced the disease at the time of diagnosis, especially with lymph node or systemic involvement, the more guarded the prognosis. A number of treatment options are available for patients with ELSA and include surgical excision or external beam radiation for isolated lesions. The latter would be considered the treatment of choice for solitary oral lesions and the authors currently recommend three to four treatments at 500 cGy/treatment (6 MV) at weekly intervals.

For those patients with more significant disease and not suitable for localized treatment, a number of approaches have been tried and although the gold standard of treatment has not been established, CCNU is currently the drug of choice. To compare CCNU with other treatment options a number of studies have evaluated this drug (Risbon et al 2006, Williams et al 2006). In these studies the response rate was from 78 to 83% compared to <50% with other earlier treatments including retinoids (White et al 1993), ciclosporin, fatty acids, dacarbazine, PEG l-asparaginase and pegylated doxorubicin (Vail & Young 2007). The duration of response was variable, as were the number of cycles of drug required to induce and maintain the response. The authors currently recommend three cycles at 3-weekly intervals at a dose of 60 mg/m² (this may need to be adjusted depending on the degree of myelosuppression). After three cycles the patient is evaluated and treatment modified according to response.