A peculiar case of soft tissue sarcoma in a cat

Abstract

A 13-year-old cat was presented to a clinic in Texas with an open draining lesion and severe swelling of the left foreleg. Initial diagnosis was a brown recluse spider bite and treatment was undertaken accordingly. A few weeks later, the cat was returned to the clinic with further swelling of the left foreleg. A diagnosis of a high-grade soft tissue sarcoma with an increased risk of metastasis was made based on a histopathology report. The cat was euthanized following the diagnosis as the cat had a fair to poor prognosis.

In early May 2019, a 13-year-old spayed British shorthair cat was presented to the Gray County Veterinary Clinic in Pampa, Texas for severe swelling of the medial left foreleg. There was a draining lesion on the left foreleg, just distal to the elbow, and the leg was edematous. The patient was unable to bear weight on the leg. The cat was eating and drinking well, and all remaining components of the physical examination were within normal limits. A radiograph was taken to identify if a fracture was present, which could be causing the edema. The radiograph revealed no fracture, but soft tissue swelling could be seen surrounding the radius and ulna (Figure 1). Further diagnostic tests included a probe of the lesion and a fine-needle aspirate. The cat was given dexmedetomidine hydrochloride (Dexmedesed; Dechra, Leawood, Kansas, USA), 35 μg/kg body weight (BW), IM and butorphanol tartrate (Dolorex; Merck Animal Health, Madison, New Jersey, USA), 0.17 mg/kg BW, IM. The lesion was probed, and massive edema was identified, but there was no purulent material present. A fine-needle aspirate was also obtained but there were no significant cytologic changes. At this time, the appearance and characteristics of the lesion resulted in the top differential being a spider bite, specifically a brown recluse spider bite. In light of this differential, the cat was given cefovecin sodium (Convenia; Zoetis, Kalamazoo, Michigan, USA), 8 mg/kg BW, SQ, and was sent home. A few weeks later, on May 20, 2019, the patient returned to the clinic as the lesion had grown substantially since the last visit and a probe of the lesion revealed purulent material (Figure 2). The lesion was approximately 5 mm in height and 5 cm wide and there was a small abscess cranial to the mass. An additional fine-needle aspirate was obtained, but there were no significant cytologic findings. Due to non-diagnostic cytology, the lesion was biopsied and submitted for histopathology. For sedation, the patient was given dexmedetomidine hydrochloride (Dechra), 30 μg/kg BW, IM and butorphanol tartrate (Merck Animal Health), 0.17 mg/kg BW, IM. An incisional biopsy was completed, followed by a wide excisional biopsy to remove the mass completely. There was not enough skin to close the site so a bandage was applied and the cat was given dexamethasone (Phoenix, Saint Joseph, Missouri, USA), 0.09 mg/kg BW, SQ. The patient was hospitalized at the clinic so that the foreleg could be monitored, and treatment could be continued. During the patient’s stay of 4 d, she was given clindamycin phosphate (Clindamycin; Alvogen, Pine Brook, New Jersey, USA), 40 mg/kg BW, IM, q24h, ceftiofur sodium (Ceftilex; Aspen, Liberty, Missouri, USA), 10 mg/kg BW, SQ, q24h, and her bandage was changed daily. Over the next 3 d, the patient made good progress — eating and drinking normally, and cardiorespiratory parameters and temperature were within normal limits. However, within 2 d, a mass reappeared at the site of the original mass and was approximately the same size as it had been before removal. The mass was now more purulent and began draining as it became larger (Figure 2). On May 24, the histopathology results identified a high-grade soft tissue sarcoma. Specifically, the report indicated that there was extensive coagulation necrosis with purulent inflammatory cells and neoplastic spindle-shaped fibroblastic cells. There were also tumor cells with a collagenous stroma present and the mitotic index was 26. The prognosis was fair to poor as this high-grade soft tissue sarcoma was locally infiltrative and had an increased risk of metastasis. The owners were given the diagnosis, prognosis, and treatment options and they elected to have the patient euthanized. The cat was given dexmedetomidine hydrochloride (Dechra), 50 μg/kg BW, IM, butorphanol tartrate (Merck Animal Health), 0.17 mg/kg BW, IM, and pentobarbital (Euthanasia-III Solution; Med-Pharmex Incorporated, Pomona, California, USA), 0.22 mg/kg BW, IV.

Figure 1.

A lateral radiograph of the left foreleg. There was soft tissue swelling around the radius and ulna. No fracture was present.

Figure 2.

On the left is the appearance of the lesion when the patient returned to the clinic on May 20th, 2019. The lesion had some necrosis and little to no purulent material. On the right, is the left forearm as seen on May 22nd following biopsy. The original mass had been debrided down to the skin and this re-growth resulted a few days later.

Discussion

The primary differential diagnosis for this case was a brown recluse spider bite. The brown recluse spider (Loxosceles reclusa) is a nocturnal spider found in the southern regions of the United States (i.e., Texas, Georgia). The primary active ingredient in the spider venom is sphingomyelinase, which results in local necrosis as well as systemic effects. Initially, a brown recluse spider bite results in local signs of pain, redness shortly after being bitten, and necrosis around the bite (1). The bite becomes a blister, resembling a “bull’s eye,” which then expands and becomes a necrotizing wound (2). Although there was no “bull’s eye” or “target-like” lesion for this cat, the assumption was that the spider bite was several days old and so, the classic appearance had been missed. After a few days, the bite becomes a necrotic dermal lesion which can range in diameter from 1 to 25 cm. A typical case of a brown recluse spider bite is described for a cat in Tennessee which had a necrotic lesion on the flank. The lesion had sloughing of tissue over time (3). Systemic signs can also develop, including hemolysis, fever, hematuria, and arthralgia. A definitive diagnosis can be made by an assay that identifies circulating antibodies to the spider venom; however, this test is not available in the United States. A complete blood cell count and urinalysis will detect changes such as anemia, thrombocytopenia, and hemoglobinuria but these are uncommon and do not provide a definitive diagnosis. Treatment ideally includes an antivenom; however, antivenoms for spider bites are not currently available. Supportive therapy can be administered, which includes a cool compress, immobilization of the affected area, and in some cases with severe necrosis, surgical management (4). Because of the prevalence of brown recluse spiders in Texas, the acute onset, the appearance of the lesion initially, and the lack of significant cytologic findings, the most likely differential diagnosis initially was a brown recluse spider bite. Accordingly, supportive treatment was carried out to hopefully slow the necrosis and allow the bite to heal. However, after a few days it was clear that the treatment was not effective, and so other differential diagnoses were investigated.

Another differential diagnosis was an aggressive neoplasia, as the lesion was growing rapidly. Differentiating a brown recluse spider bite from a neoplasia requires histopathology. Consequently, a biopsy of the lesion was taken and submitted for histopathology — the report indicated a soft tissue sarcoma. Soft tissue sarcoma is a generalized term for common tumors such as histiocytoma, nerve sheath tumor, liposarcoma, fibrosarcoma, and lymphangiosarcoma (5). A fine-needle aspirate can help identify a tumor to be of soft tissue origin. However, soft tissue sarcomas often exfoliate poorly, particularly in effusions, which results in a non-diagnostic sample. In general, cytologic diagnosis is difficult; however, malignant sarcomas exfoliate better and are accordingly more evident on cytology (6). Special stains can help differentiate between the various types of tumors (4). Soft tissue sarcomas account for approximately 7% of skin and subcutaneous tumors in cats (7). Most soft tissue sarcomas are heterogenous and are solitary tumors that have no breed or sex predilection. They often occur in middle-to older-aged animals, which is consistent with the signalment of this case (4). In terms of appearance, soft tissue sarcomas often present as pseudoencapsulated tumors that feel soft to firm (3). The tumors are locally invasive, often on limbs, and are poorly marginated (5). An incisional or marginal excisional biopsy can be completed to provide a histological diagnosis. A marginal excisional biopsy should be completed if the mass is small (< 1 cm) and not on a limb, otherwise a wide excisional biopsy is needed (6). In this case, an incisional biopsy was completed for a histological diagnosis and was then followed with a wide excisional biopsy. If surgical excision is completed, there is a high chance of recurrence, particularly if the margins are not clean. A clean margin would be one in which there are no tumor cells in contact with the margins. There is up to a 20% chance of metastasis and the histological grade can indicate the likelihood of metastasis (7). The histopathological diagnosis in the present case was a high-grade soft tissue sarcoma, which had an increased risk of metastasis.

Due to financial constraints and the fair to poor prognosis, further diagnostic tests were not completed. An appropriate next-step diagnostic could have been thoracic radiographs to determine whether there was pulmonary metastasis. Without any identified metastasis, amputation of the affected limb could have been a treatment option. Amputation would be considered a radical resection as it would completely remove cancerous cells as well as surrounding tissue. Removing the limb leaves no chance for local recurrence on the affected limb. It also decreases the chance of hematogenous spread and thus, metastasis. Another local treatment would be radiation of the affected limb. If metastasis has occurred then systemic therapy is needed, which includes the options of chemotherapy, radiation, and immunotherapy. The grade of the tumor dictates whether chemotherapy is appropriate. Further studies on useful diagnostic tests for detecting brown recluse spider bites may help to quicken identification of a spider bite. In this case, it would have been less expensive and more timely if there was a way to rule out a spider bite from the list of differential diagnoses.