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72 CLINICAL INSIGHTS CLINICAL INSIGHTS I personally prefer and start with fenestration as I find it easier and consistently get diagnostic samples. Aspiration is more useful for fluid-filled masses. Unless the sample is composed exclusively of fat, clear cystic fluid, or acellular debris, it should be submitted to a trained cytopathologist. When in doubt, send it out. Including an adequate history helps the pathologist make an accurate diagnosis. Biopsy If cytology is nondiagnostic, a pretreatment biopsy is recommended before complete tumor removal. This biopsy will help determine the optimal treatment plan. A practical recommendation in these cases is if the lesion fits in an 8-mm biopsy punch, punch it out. If the mass is larger than an 8-mm biopsy punch, an incisional biopsy (wedge, Tru-cut, punch) is required for diagnostic confirmation before tumor removal. Staging diagnostics are also often indicated before curative- intent surgery. Consultation with a veterinary oncologist is recommended to help in these diagnostic decisions. 3. Make the First Surgery the Only Surgery It is tempting to remove a mass right away, and owners often say they want it removed as soon as possible. An excisional biopsy establishes a diagnosis and removes the tumor at the same time. However, this approach is not recommended for undiagnosed skin and superficial masses because surgical approaches vary with tumor type. For benign masses, marginal excision may be adequate for long-term control. In contrast, malignant tumors often require 2 to 3 cm margins. 1,4–6 When an excisional biopsy (or debulking surgery) leads to incomplete margins for malignant tumors, more treatment, more morbidity, and more expense ensue. Thus, removing the mass entirely is not recommended without a cellular diagnosis before definitive excision. Research confirms that the first surgery is the best chance for a cure. 2 WHAT WILL WE FIND? Primary skin and subcutaneous tumors are common in dogs and cats. While the overall incidence is difficult to determine, approximately 25% to 43% of submitted canine and feline biopsy samples are of the skin. Of submitted samples, 20% to 40% are reported to be malignant. 7 The most common malignant skin tumors in dogs are mast cell tumors, soft tissue sarcomas, and squamous cell carcinomas ( Table 1 ). The most common benign canine skin and subcutaneous benign tumors include lipomas, histiocytomas, and perianal gland adenomas. 7 In cats, the most common superficial tumors are basal cell tumors, mast cell tumors, squamous cell carcinomas, and fibrosarcomas. These 4 tumor types make up about 70% of all skin tumors in cats. Sebaceous gland adenomas are much less common. If basal cell tumors are excluded, the percentage of malignant skin tumors in cats is higher than in dogs, with studies reporting 70% to 80%. 7 TABLE 1 Most Common Skin Tumors in Dogs and Cats 1,7 DOGS CATS MALIGNANT BENIGN MALIGNANT BENIGN Mast cell tumors: 10%–17% Soft tissue sarcomas: Fibrosarcomas: 2%–6% Malignant nerve sheath tumors: 4%–7% Squamous cell carcinomas: 2%–6% Lipomas: 8% Histiocytomas: 8%–12% Perianal gland adenomas: 8%–12% Sebaceous gland adenomas/ hyperplasia: 4%–6% Trichoepitheliomas: 4% Papillomas: 3% Basal cell tumors: 4%–5% Melanomas 4%–6% Hemangioma 4% Hemangiosarcoma 23% Melanoma a 1%–2% Squamous cell carcinomas: 10%–15% Fibrosarcomas: 15%–17% Mast cell tumors: 13%–21% Basal cell tumors: 15%–26% Sebaceous gland adenomas: 2%–4% Apocrine adenoma 4%–5% Lipoma 3% a The biologic behavior of melanoma in cats is less predictable than in dogs.