GENERAL CONSIDERATIONS

General Considerations

•digit tumors are rare in cats and common in dogs

•digit masses: 61% are malignant tumors, 20% are benign tumors, and 19% are pyogranulomatous inflammation

•12% of nail and nail bed diseases are tumors

•digit masses include SCC (33%-54%), malignant melanoma (11%), STS (9%), and OSA (6%)

•others digit masses include MCT, epidermoid cyst, plasmacytoma, HSA, and calcinosis circumscripta

•black the most common colour coat for dogs with digit masses

Survey Radiographs

•osteolysis is more commonly associated with tumors, but it is difficult to differentiate malignant tumors from pododermatitis and benign tumors

•bone lysis can be caused by malignant tumor (83%) or benign tumor or pyogranulomatous inflammation (17%)

•benign tumors causing bone lysis include intraosseous epidermoid cyst and infiltrating lipoma

•tumors and non-neoplastic masses associated with soft tissue swelling and no bone involvement include MCT, solitary plasmacytoma, STS, sebaceous gland ADC, histiocytoma, papilloma, and calcinosis circumscripta

•inflammatory changes tend to be proliferative whereas neoplastic changes are usually destructive

•pressure erosion and infarcts common in adjacent bone

MALIGNANT TUMORS

General Considerations

•single digit: solitary plasmacytoma, STS, OSA, and carcinoma

•digits and metatarsus: synovial cell sarcoma and ADC

•multiple digits: solitary plasmacytoma, synovial cell sarcoma, ADC and carcinoma

Squamous Cell Carcinoma

General Considerations

•digit SCC arise from the subungual epithelium

•digit SCC are usually solitary, ulcerative, and expansile with occasional hemorrhage

•multiple digit SCC have been reported in 3 related Giant Schnauzers

•biologic behaviour: locally invasive with low-to-moderate metastatic potential

•bone invasion detected in 80% of digit SCC

Clinical Features

•mean age: 9 years

•no sex predilection

•75% are large breed dogs

•67%-71% occur in dogs with black coats (i.e., Labrador Retriever and Standard Poodle)

•treatment: digit amputation (to metacarpophalangeal, metatarsophalangeal, or proximal interphalangeal level)

Prognosis

•prognosis is guarded with reports on digit SCC varying widely

•5%-29% metastatic rate to regional lymph nodes and lungs, including 13% metastatic at presentation

•1-year survival rate 60%-95%

•2-year survival rate 20%-74%

•however, tumor-related deaths are rare (4.7%, 1/21)

•prognostic factors include region of digit involved and surgical dose:

•SCC arising from subungual has a significantly better survival rate than other areas of the digit (1-year survival rate 95% v 60%, and 2-year survival rate 74% v 44%)

   •marginal resection is associated with an increased local tumor recurrence rate

Melanoma

•50% are benign melanocytic nevi and 50% malignant melanoma

•melanocytic nevi are not associated with local tumor recurrence or metastasis following surgical excision

•bone lysis is detected in 5% of digit melanoma

•prognosis is guarded because of a high distant metastatic rate

•local recurrence rate 14%-30%

•distant metastatic rate 58%-71% to regional lymph node and lungs, including 32% at diagnosis

•MST 12 months with:

•1-year survival rate 42%-66%

•2-year survival rate 12%-22%

Soft Tissue Sarcoma

•biologic behaviour similar to other sites

•treatment: digit amputation

Mast Cell Tumor

•subungual MCT are usually grade III MCT with a similar prognosis to other mucocutaneous junction MCT

•digit sites other than subungual have a similar biologic behaviour to other cutaneous MCT

•prognosis: MST 20 months with 1-year survival rate 88% and 2-year survival rate 50%

Osteosarcoma Distal to the Carpus and Tarsus

•treatment: digit amputation and postoperative chemotherapy

•MST 466 days with euthanasia because of metastatic disease

•prognosis better than OSA in other appendicular sites

•however, digit OSA is still a malignant tumor with high metastatic potential

•surgical excision of OSA distal to the carpus or tarsus in humans usually results in an excellent prognosis

OTHER DIGIT TUMORS

Feline Digit Tumors

•subungual tumors are rare

•acrometastasis is relatively common with multiple digit involvement with primary bronchoalveolar ADC or SCC ± cutaneous SCC

Footpad Tumors

•eccrine tumors are rare

•eccrine tumors are usually malignant in cats, but can be benign or malignant in dogs

•clinical signs: lameness and swelling of multiple digits

•gross appearance: poorly defined swelling of the footpad or digit with ulceration and may involve multiple digits

•survey radiographs: bone lysis is common

•eccrine ADC is aggressive with early metastasis to regional lymph node and lungs

•DDx: SCC, keratoacanthoma, malignant melanoma, OSA, LSA, and HSA

Benign Tumors

•histiocytoma, lipoma (infiltrative and non-infiltrative), osteochondroma, and papilloma

Non-Neoplastic Masses

•epidermal inclusion cyst

•calcinosis circumscripta

•pododermatitis