digit tumors

BACKGROUND

Digit tumors are relatively common in dogs, but rare in cats. The majority of digit tumors are malignant and the most common tumor types are squamous cell carcinoma (SCC) and malignant melanoma. Others include mast cell tumor, soft tissue sarcoma, and osteosarcoma. The risk of metastasis is dependent on tumor type, but metastasis is relatively common in dogs with malignant melanoma.

DIAGNOSIS

Digit tumors are diagnosed with an incisional or excisional biopsy. Fine-needle aspirates are often non-diagnostic for tumors other than mast cell tumors because of significant ulceration and inflammation within the tumor. Radiographs of the affected paw are recommended to determine the extent of the tumor and for surgical planning.

CLINICAL STAGING

The two most common sites for metastasis are the regional lymph nodes and lungs. The regional lymph nodes should be palpated and aspirated. Sentinel lymph node mapping should be considered because the sentinel lymph node (which is defined as the first draining lymph node) may be different to the regional lymph node. Sentinel lymph node mapping involves peritumoral injection of a lipid-soluble contrast agent 24 hours prior to surgery (usually under sedation), regional radiographs immediately prior to surgery to identify the sentinel lymph node, and then peritumoral blue dye injection during surgery to aide in intraoperative identification and excision of the sentinel lymph node. Thoracic radiographs or CT scans are recommended to assess the lungs for metastasis.

SLN Mapping for a Digit SCC

Intraoperative Blue Dye SLN Mapping

TREATMENT

Digit amputation is recommended for removal of the affected digit. Digit amputation rarely causes long-term lameness and recovery is often quick. In one study of 33 dogs treated with amputation of one or two digits, incisional dehiscence was the most common complication with lameness reported in 39% of dogs in the first 14 days and 25% of dogs after 14 days (but this lameness was mild and/or intermittent in the majority of the affected dogs). There was no difference in the risk of complications between amputation of one or two digits, although complications in the first 14 days were more likely following amputation of digit(s) on the pelvic limb. Chemotherapy or immunotherapy is recommended for dogs with malignant melanoma, osteosarcoma, and perhaps SCC.

Single Digit Amputation

Partial Foot Amputation

PROGNOSIS

Orthopedic Outcome

In a 2013 clinical study, the surgical complications and orthopedic outcome were assessed retrospectively in 33 dogs treated with either digit amputation or partial foot amputation (two adjacent digits). Short-term complications (0-14 days) included lameness (39%) and dehiscence with or without infection (27%). Short-term complications were significantly more likely to occur following amputation of a pelvic limb digit. Mild or intermittent lameness (25%) was the only long-term complication (> 14 days; mean followup 43.1 months, range 2 months to 11.5 years). Partial foot amputation was not associated with an increased risk of complications. A survey of owner satsifaction showed that 96% of owners were satisfied following either single digit or partial foot amputation in their dogs.

Oncologic Outcome

The prognosis is good to guarded for dogs with malignant digit tumors treated with surgical excision, depending on the tumor type.

Squamous Cell Carcinoma

For SCC, a metastatic rate up to 29% has been reported with 1- and 2-year survival rates of 95% and 74%, respectively. A median survival time was not reached because > 50% of dogs died of reasons unrelated to their digit tumor. Digit SCC has been reported to involve multiple digits, although this is rare.

Malignant Melanoma

For malignant melanoma, local recurrence is reported in up to 30% dogs and metastasis in up to 71% dogs with a median survival time of 365 days and 1- and 2-year survival rates of 43%-66% and 22%, respectively. The prognosis is better for dogs with digit malignant melanoma when digit amputation is combined with immunotherapy (melanoma vaccine). Overall, when treated with surgery and the melanoma vaccine, the median survival time was 476 days with a 1-year survival rate of 63%. However, survival times were also dependent on clinical stage with median survival times of >952 days, >1093 days, 321 days, and 76 days reported for dogs with stage I (< 2cm diameter or superficial), stage II (2-5cm diameter with minimal invasion), stage III (>5cm diameter or invading into subcutaneous tissue), and stage IV (invasive into bone) disease, respectively. The 2- and 3-year survival rates for dogs with stage I and II disease was 48% at both time points.

Mast Cell Tumor

The median survival time for digit mast cell tumor is 20 months with 1- and 2-year survival rates of 88% and 50%, respectively.

Osteosarcoma

The median survival time for dogs with digit osteosarcoma treated with surgery and chemotherapy is 466 days.