ORAL FIBROSARCOMA IN DOGS

Background

The third most common oral malignancy in the dog is fibrosarcoma. These tumors generally occur in older dogs; however, these tumors may occur in young dogs more commonly than melanoma or squamous cell carcinoma.

Clinical Parameters

Fibrosarcomas are just as likely to arise from the maxilla as from the mandible. These tumors most commonly originate from the gingival tissue. Tumors are usually large, with diameters of greater than 4 cm. Dogs may be asymptomatic, or they may have ptyalism, anorexia, oral bleeding, and facial deformity.

Clinical Work-up

Dogs with any oral tumor should be staged using blood work, thoracic radiographs, lymph node evaluation, and fine-detail skull radiographs. Fibrosarcomas frequently invade bone and may extend much farther than is obvious by external viewing. In addition, all soft tissue sarcomas have “tendrils” of tumor cells that extend deep into normal surrounding tissues, making complete excision very difficult without wide margins.

Before surgery, particularly when the tumor involves maxilla, high-detail skull radiographs or a CT scan should be obtained to gain a better appreciation of tumor borders. Due to the fact that radiographs usually underestimate tumor margins, CT scanning is a more accurate method of assess­ing fibrosarcoma margins. A CT scan provides a spatial assessment of the tumor that may be useful for planning surgery, as well as either presurgical or postsurgical radiation therapy. CT scanning may also indicate whether complete surgical removal is impossible, thereby protecting the patient from a poorly planned procedure.

Occasionally a tumor may be termed a fibroma, which implies that the process is benign. Fibromas of the oral cavity should be treated as aggressively as fibrosarcomas. A recent report described 25 dogs with tumors that were histolog­ically labeled as either fibromas, nodular fasciitis, or granulation tissue. Although all of these lesions were considered benign, they invaded bone and metastasized in 5 of the dogs. Bony invasion should be interpreted as a sign of malignancy regardless of the pathology report.

Fibrosarcoma is rarely metastatic at the time of diagnosis. However, mandibular lymph nodes should be palpated and always subjected to fine- needle aspiration or biopsy. Young dogs apparently have more aggressive tumors than old dogs. In eight series totaling 107 dogs, metastasis was reported in 23 (21%) dogs. In most cases metastasis was to regional lymph nodes; it is rare for oral fibrosarcoma to metastasize to lungs. Thoracic radiographs should, however, be performed before definitive surgery. Metastases often appear many months after surgery, and it is possible that earlier reports with less effective therapies may have underestimated the metastatic rate, because dogs died from inadequate local tumor control before metastasis occurred.

Therapeutic Approach

Complete surgical excision is the treatment of choice for fibrosarcoma of the oral cavity. Tumor­present margins lead to rapid recurrence. Radical surgical techniques such as maxillectomy and mandibulectomy are well tolerated by dogs and are necessary to obtain adequate surgical margins. In five series, totaling 54 dogs treated for oral fibrosar­coma with aggressive surgery, the median survival was 12 months and ranged from 1.5 weeks to 33 months. Early and aggressive management of maxil­lary and mandibular fibrosarcoma in large purebred dogs with histologically low-grade yet biologically high-grade tumors should be a standard approach.

Even after mandibulectomy or maxillectomy, local recurrence was a problem in 20 of 54 dogs (37%).

In one study, pretreatment with 50 to 56 Gy of radiation seemed to improve control rates, although few dogs were involved in this study. Control of fibrosarcoma improves only at high doses of 50 Gy or more. Of 17 dogs treated with 40.0 to 54.5 Gy of orthovoltage, 4 died during or soon after radiation therapy. Survival times in the remaining 13 dogs ranged from 2 months to more than 27 months, with a median survival of 6 months. Tumors recurred in 12 dogs at a mean time of 3.9 months after radiation was complete. In another study, radiation therapy without sur­gery was able to control tumor growth in 3 of 13 dogs. Megavoltage radiation may be more effi­cacious than orthovoltage in controlling oral fibrosarcomas. Twenty-eight dogs with fibrosar­coma were treated with 48 Gy of ⁶⁰Co teletherapy. Nine dogs had local recurrence as the first cause of failure, and 4 dogs developed distant metastasis as the first cause of failure. Dogs with rostrally located tumors and dogs with smaller tumors had longer remissions. Median progression-free survival was estimated to be 26.2 months. Clinical stage was important in predicting time to failure. Radiation alone, or combined with hyperthermia, results in a median survival of over 18 months for biologically high-grade, histologically low-grade, fibrosarcomas.

When used in combination with radiation, inter­stitial hyperthermia provides better local control rates than those achieved by radiation alone. Ten dogs that received between 32 and 48 Gy of ortho­voltage also received interstitial hyperthermia to a temperature of either 50° C or 43° C for 30 seconds. Complete remissions were obtained in 9 of 10 dogs, and overall median survival was 12.9 months, which is comparable to survival times for dogs that undergo surgery.

Chemotherapy has had little application in the treatment of oral cavity fibrosarcomas, although doxorubicin has been noted occasionally to pro­duce objective responses in soft tissue sarcomas. Low doses of doxorubicin (10 mg/m² intra­venously every 7 days) appear to act as a “radiation sensitizer” and improve tumor response at lower radiation therapy dosages.

Intratumoral injections of cisplatin and bovine collagen matrix were given every week during a 48-Gy course of ⁶⁰Co teletherapy to five dogs with oral fibrosarcoma. Complete remission was seen in three dogs, and partial remission was seen in one dog, for a median duration of 14 weeks.There was tumor recurrence in three of these dogs. In these and other dogs treated with radiochemotherapy, recurrences often took place at the periphery of the chemotherapy site but still within the radiation field, implying that the combination is synergistic or additive in its effect on the tumor.

The treatment of choice for oral fibrosarcoma probably involves combined surgery and radiation therapy to dosages that exceed 50 Gy. Intralesional chemotherapy is investigational but may improve tumor control.