Pathology: Skin Neoplastic + Non-Neoplastic Tumors

Neoplastic Tumors

• Skin masses are commonly seen in companion animals and pose both diagnostic challenges and economic opportunities for veterinarians.
• Primary skin neoplasms are more prevalent than metastatic neoplasms from other body sites.
• Definitive diagnosis is crucial for selecting appropriate treatment, which can include observation, surgical excision, or chemotherapy.
• Cytology is the primary diagnostic test for skin tumors, often using fine needle aspiration.
• For surface-oriented masses, such as squamous cell carcinomas, tissue scrapes may be more effective than aspiration.
• Cytology is cost-effective, requires no sedation, and provides rapid results, typically within 24 hours if sent to a laboratory.
• Differentiating between inflammatory and neoplastic processes is important for treatment planning and client communication.
• The most common benign skin tumors in dogs include histiocytomas, lipomas, follicular cysts, sebaceous hyperplasia/adenomas, and basal cell tumors.
• Malignant potential in skin tumors is primarily associated with mast cell tumors, which rank sixth among common canine skin tumors.

Non-Neoplastic Tumors

• Non-neoplastic tumors arise from non-neoplastic cells and can either resolve spontaneously or require surgical excision.
• Inflammatory tumors may result from bacterial, viral, or fungal infections, or trauma, and can often be treated conservatively.
• Nevus (focal adnexal hyperplasia) consists of benign collagen or adnexa and requires excision only if symptomatic.
• Sebaceous gland hyperplasia frequently occurs in certain dog breeds and appears as wart-like growths; excision is curative but multiple lesions may develop over time.
• Follicular cysts (dermal cysts) form from malformed hair follicles that trap keratin, often leading to inflammation and possible rupture.
• Calcinosis circumscripta develops as a nodular mass, predominantly in young large breed dogs, resulting from dystrophic calcification due to trauma.

Possibly Neoplastic Tumors

• Canine cutaneous histiocytomas are the most common benign skin tumors in dogs, characterized by localized histiocyte proliferation with spontaneous resolution in weeks.
• Lipomas, subcutaneous fat deposits, are the second most common skin tumor, typically mobile and benign, requiring excision only if they interfere with function.
• Hemangiomas are well-differentiated vascular tumors, often associated with solar exposure; complete excision is curative.

Primary Skin Neoplasms

• Epithelial neoplasms arise from skin adnexal structures and are classified as adenomas (benign) or adenocarcinomas (malignant).
• Benign sebaceous and apocrine gland adenomas account for 10% of canine skin neoplasms and are commonly found on eyelids, limbs, and trunk.
• Basal cell tumors, common in dogs, cats, and horses, arise from follicular basal cells and can be pigmented; excision is curative.
• Follicular neoplasms include trichoepithelioma, pilomatricoma, and intracutaneous cornifying epithelioma and are mainly benign.
• Squamous cell carcinoma is the most common malignant epithelial neoplasm, particularly in cats, linked to UV exposure and presenting as ulcerated lesions.
• Hepatoid gland adenomas, testosterone-responsive skin tumors in dogs, resemble hepatocytes on cytology and are most common in uncastrated males.### Hepatoid Gland Adenomas vs. Apocrine Gland Adenocarcinomas
• Hepatoid gland adenomas develop externally around the anus, detectable by touch.
• Apocrine gland adenocarcinomas arise within the anal sacs, presenting as diffuse perineal swelling.
• Hepatoid gland adenomas are benign, testosterone-responsive, with no systemic effects.
• Apocrine gland adenocarcinomas can produce PTH-rp, are infiltrative, metastatic, and often fatal.
• Castration reduces the likelihood of hepatoid gland adenomas, prostatic disorders, and testicular tumors.

Mammary Gland Tumors in Dogs and Cats

• Common in both dogs (50-80% benign) and cats (almost always malignant).
• Early spaying significantly lowers the risk of mammary neoplasms in dogs.
• Canine mammary tumors often feature glandular structures with anaplastic cells, and bone and cartilage may be present.
• Tumors that are harder and more nodular tend to be benign.
• Cytology is often inadequate for differentiating mammary adenomas from adenocarcinomas.
• Feline mammary adenocarcinomas metastasize quickly and may spread to skin.
• Tumor size is critical for prognosis in cats; larger tumors indicate worse outcomes.
• Early spaying does not impact mammary cancer risk in cats.

Round Cell Neoplasms

• Round cells originate in the bone marrow and travel to skin, losing mobility upon settling.
• Low interaction between these cells leads to distinct cytological features.

Mast Cell Tumors (MCTs)

• MCTs are the 6th most common tumors in dogs and 21% of feline skin tumors.
• Nearly all feline MCTs are benign, while canine MCTs can be malignant with variable behavior.
• Diagnosis typically via cytology, revealing round cells with dark purple granules.
• Canine MCTs can be low or high grade based on histological invasion and differentiation.
• Low-grade canine MCT removal is often curative; however, recurrence occurs in 5-15%.
• High-grade MCTs have poorer prognoses and can metastasize early.
• Feline cutaneous MCTs are usually benign and localized; surgical excision is curative.

Cutaneous Plasmacytoma

• Occur in dogs mainly in the mouth, feet, and pinnae; rare in cats.
• Cytology shows non-cohesive round cells with a characteristic ‘clock-face’ nucleus.
• Generally do not recur locally, but new plasmacytomas may arise.
• Cats are more likely to develop plasmacytomas linked to multiple myeloma, which affects prognosis.

Spindle Cell Tumors

• Develop from connective tissue; soft tissue sarcomas are a common type in dogs.
• Cytology often shows spindle-shaped cells within visible bundles.
• Can be infiltrative despite seeming encapsulated; local recurrence is about 30%.
• Complete excision attempts should prioritize wide margins to reduce recurrence risks.

Dermal Melanoma

• Common in dogs, its behavior varies depending on skin location.
• Non-haired skin melanomas (e.g., nail-bed, mucocutaneous junction) are generally malignant.
• Canine melanomas on haired skin are often benign; complete excision is typically curative.
• Feline and equine melanomas are often malignant and affirmatively require careful monitoring.

Cutaneous Hemangiosarcoma

• Primarily affects older dogs, induced by sunlight; often found on the ventrum.
• Typically, only metastasize late in progression; multiple lesions can develop.
• Excision is usually effective, but preventive measures against sun exposure are imperative.

Equine Sarcoid

• Most common skin tumor in horses, associated with bovine papillomavirus (BPV).
• Develops variably in clinical presentation, usually in trauma-prone areas.
• Cytology reveals spindle-shaped cells, while histology shows fibroblast proliferation without metastasis.
• Local recurrence is common, and treatment remains inconsistent.

This quiz focuses on the identification and implications of neoplastic and non-neoplastic tumors, particularly those that develop on the skin of companion animals. Understanding these masses is crucial for veterinarians to provide appropriate treatment plans and effectively address client concerns. Test your knowledge on this important topic in veterinary medicine.