Tumors of the digits (toes) most commonly include squamous cell carcinoma, melanoma, osteosarcoma, hemangiopericytoma, benign soft tissue tumors and malignant soft tissue tumors. Squamous cell carcinoma accounts for more than 50% of all digital tumors. This tumor originates from skin cells, is very locally invasive, and commonly destroys the bone in the digit.
Melanoma, the second most common tumor, accounts for 16% of all digital tumors. These tumors originate from the pigment-producing cells in the skin called melanocytes, which are responsible for giving humans a tan with sun exposure. Melanomas of the nail bed spread rapidly to other areas of the body. At the time of diagnosis of a digital melanoma, one-third of all affected dogs will have detectable spread (metastasis) to the lungs. Melanomas of the digits that do not involve the nail bed and are confirmed to be benign on the biopsy usually do not metastasize.
Signs and diagnosis
Squamous cell carcinoma is most commonly seen in large breed dogs with black coats. Overrepresented breeds include Labrador retrievers and standard poodles. Digital tumors cause the toe to swell see photo below right) and may cause lameness.
Initially, a tumor of the digit may mimic the appearance of an infected toe; however, treatment with antibiotics does not resolve the problem. If the tumor appears to be darkly pigmented, a melanoma is more probable; however, some melanomas lack pigment and may mimic the appearance of a squamous cell carcinoma or another type of tumor. Enlargement of lymph nodes in the area of the tumor may be a sign of spread of the tumor to these nodes. If the tumor has spread to the lungs, potential clinical signs may include breathing difficulty, coughing, weight loss and poor appetite.
The diagnosis of a digital tumor is based upon a fine needle biopsy or surgically collecting a piece of tissue from the mass. If the fine needle biopsy does not provide a definitive diagnosis, a core of tissue may be required. A complete blood count, chemistry profile, and urine testing are done to evaluate the health status of your companion’s internal organs prior to anesthesia and surgery. X-rays of the affected digit may show destruction of the bone, especially if the tumor is a squamous cell carcinoma. Chest X-rays and abdominal ultrasound are used to identify visible spread of the cancer; please note that microscopic spread of the tumor to other organs cannot be detected with X-rays or ultrasound. Abdominal ultrasound is also performed to rule out spread of tumor to the internal abdominal organs. Lymph nodes in the area of the tumor are aspirated to rule out spread of the cancer to the nodes, regardless of whether they are enlarged or not. If this test does not provide a clear-cut answer, removal of a regional lymph node and analysis of the node by a pathologist is recommended.
Surgery is essential to treat a digital tumor. If the tumor is located on the toenail bed, the entire toe must be amputated. In some cases, benign tumors of the skin of the digits may be removed without removing the digit.
Adjunctive therapy (chemotherapy or radiation) may be indicated for some malignant tumors. Usually surgery is all that is needed for a squamous cell carcinoma; however, melanomas should be treated more aggressively with a melanoma vaccination, chemotherapy and/or radiation.
The oncologist may recommend chemotherapy for your companion. Typically, one treatment is administered every three weeks for a total of four to six treatments. Most patients tolerate the chemotherapy medication with transient mild side effects.
Radiation therapy has been shown to prevent or delay the onset of tumor regrowth. Six to seven radiation treatments are administered to the tumor site and regional lymph nodes, starting two weeks after the tumor has been removed. Radiation treatments are administered twice weekly. A short anesthesia is required during administration of each radiation treatment.
Digital melanomas treated with surgical amputation of the digit result in a median survival time of one year. Digital melanomas that are not located on the nail bed and have benign characteristics (low mitotic index) on the biopsy are commonly cured with surgery alone.
Inoculation with the melanoma vaccine can provide excellent long-term control of the disease and increased survival times for melanomas. The best result is seen with intradermal vaccination that must be administered with a special injector system.
Squamous cell carcinoma has a very good chance to be cured with surgery alone.
Short-term complications following surgery are uncommon and may include temporary dehiscence (opening) of the incision and infection. Tumor recurrence and spread of cancer are other complications. Rarely, amputation of a digit will cause ongoing lameness on the limb that was operated on.
For more information on this subject, speak to the veterinarian who is treating your pet.