Foreign material injected in the subcutis or muscle of predisposed cats can induce a chronic inflammatory response that can lead to neoplastic transformation. The tumors are mesenchymal tumors, most commonly fibrosarcomas, but malignant fibrous histiocytoma, osteosarcoma, chondrosarcoma, rhabomyosarcoma and undifferentiated sarcoma have also been reported.
The most common inciting agent is vaccination, with an incidence of between 1/1000–1/10,000 vaccinated cats. Other reported inciting factors include injections of long acting antibiotics or steroids, pesticides, non-absorbable suture material, and microchip implants
The latency period can be as long as 10 years; however, periods as short as 3 months have been observed. Once the tumor appears, however, growth can be dramatic, often reaching several centimeters in diameter within a few weeks, and leading to central necrosis.
Biopsy is recommended for these lesions, as fine needle aspiration and cytology evaluation can be nondiagnostic in about 50% of cases. As the masses can be quite heterogeneous, incisional biopsy is preferred to avoid issues with sampling error (small tissue samples can lead to misdiagnosis as a granuloma).
The mainstay of therapy is wide surgical excision with macroscopically free margins of 3 cm – 5 cm, with overall median disease-free intervals of ~10 months. Adjuvant chemotherapy and radiation therapy has also been used.
Current guidelines recommend administration of FeLV vaccination in the left rear leg, as distal as possible, the rabies vaccine in the right rear leg, as distal as possible, and the four-in-one vaccine FVRCP ± C2 in the right shoulder.