Robyn Elmslie, DVM DACVIM (Oncology)
Tumors of the digits (toes) are most common in dogs over 10 years of age. Clinical signs of toe tumors include limping, swelling of the toe joint or nail-bed and loss of the toe nail. It can be challenging to differentiate a benign process in the toe from a malignant process without doing a deep tissue biopsy, which often requires amputation of the affected toe.
Wobeser BK et al (Vet Pathol 2007) and Marino DJ et al (JAVMA 1995) have both reported, in retrospective studies consisting of over 500 amputated digit submissions to pathology laboratories, that 60% were determined to be cancerous, whereas the remaining 40% were benign, divided equally between benign tumors and inflammatory processes. Of those patients that had x-rays performed prior to digit amputation, only 23% had evidence of bone lysis (bone destruction). When bone lysis was present, malignant tumors were diagnosed in 83% of patients however 17% of patients with radiographic evidence of bone lysis were diagnosed with a benign process. Squamous cell carcinoma (SCC) represents approximately half of all cancers of the digits. 10% of patients diagnosed with SCC will develop multiple tumors affecting two or more digits (Belluco S et al Vet Pathol 2013; Henry CJ JVIM 2005). Large breed dogs with a dark coat (eg, Rottweilers, black Labradors, giant Schnauzers, and standard poodles) have an increased risk of developing digital SCC compared to other breeds.
As most digital SCC are well differentiated (low grade) or small at the time of diagnosis, amputation resulting in a clean microscopic margin is generally curative. However, large tumors are associated with poor prognosis due to either the inability to achieve a complete surgical removal and/ or due to metastasis to lungs and lymph nodes by the time of diagnosis. When complete excision is not possible, local control is significantly improved with radiation therapy to the surgery site.
Malignant melanoma is the second most common malignant tumor of the digits in dogs. This tumor is typically more aggressive, more likely to invade into underlying bone and to metastasize to regional lymph nodes and lungs than the more common digital SCC. Retrospective studies (Marino et al. JAVMA 1995 and Henry CJ et al JVIM 2005) report that 33-40% of dogs with digital melanoma had metastasis to the lungs or lymph nodes at the time of diagnosis. In these studies, in which none of the patients were treated with systemic therapy (eg, Merial melanoma vaccine, metronomic chemotherapy), only 42-44% of patients were alive at one year post surgery and only 11-13% were alive 2 years post surgery. In a study of 58 dogs with digital melanoma, treated with digit amputation and the melanoma vaccine (Manley CA et al JVIM 2011), 63% of dogs were alive at one-year post surgery, and 48% alive at 2- and 3-years post surgery. While digital SCC is typically cured with surgery alone, digital melanomas are typically more aggressive requiring both surgery and systemic therapy to achieve long-term tumor control.
Biopsy of the digit swelling is necessary to definitively differentiate between a benign and malignant process. In some cases, amputation of the affected digit is required both for diagnosis and treatment. This may seem to be a radical recommendation for some pet owners, concerned that permanent lameness will be a consequence for theira pet. A recently published pet-owner survey (Kaufman KL et al JAVMA 2013) reported that of 33 digit amputation procedures, dogs tolerated the treatment very well regardless of the digit amputated. Short-term complications were more common in their study, related to delayed healing and infection. In my experience, short term complications are rare in our VRCC patients when post-operative care consists of strict rest and bandage placement to avoid licking or excessive use of the affected foot.
Sudden lameness, loss of a toe-nail or swelling of a digit are all signs your pet should be evaluated by your family veterinarian or a veterinary oncologist. X-rays of the foot are helpful but do not definitively differentiate between a diagnosis of cancer and a benign process. When a diagnosis of a cancer of the digit is made, survival time and quality of life are significantly improved with amputation of the digit, but staging and histopathology results are critical for determining if additional therapy post digit amputation is indicated.