Cat's Injection Site Tumor: Possible Causes & Concerns

皮下 cysts are pathological structures that occur beneath the dermis or in the subcutaneous tissue. They are characterized by a cystic cavity, lined with a wall, and filled with fluid or other substances. These cysts are typically round in shape and protrude from the skin surface as halfspheres. They usually develop in the dermis and subcutaneous tissue. Due to the encapsulating wall, their edges are smooth and welldefined, with minimal adhesion to surrounding tissues, giving them a smooth, elastic, and cystic feel. The skin surface overlying the cyst is usually free of inflammation and maintains a normal skin color. Their growth is usually localized and does not expand indefinitely, thus avoiding causing local compression symptoms.

Cat injection site sarcomas, also known as FISS (Feline Injection Site Sarcomas), were first identified in the 1990s. Initially, these tumors were believed to be associated with the aluminum adjuvant in rabies vaccines, hence the name "cat vaccineassociated sarcomas." However, recent research has shown that the presence of aluminum adjuvants in vaccines does not have a significant positive correlation with the development of this tumor, and it may also be caused by injections of other vaccines or medications. Therefore, it is now referred to as FISS. The exact pathogenesis is still unclear, but it may be related to adjuvants and the immune response to antigens, or chronic inflammatory reactions at the injection site. The common histopathological result is fibrosarcoma.

Cats with FISS may develop rapidly growing soft tissue tumors at the injection site within a few weeks or months after immunization or injection. These tumors can originate from the subcutaneous tissue or muscle, so any cat with superficial or deep tumors found in the scapular or thigh area should be suspected of having FISS. Diagnosis of FISS usually requires a surgical biopsy, and fine needle aspiration followed by cytology can also often confirm the diagnosis.

Clinical symptoms include soft tissue swelling at the site of previous vaccination or other injection sites, commonly in the hind limbs, abdominal flanks, lateral chest, interscapular space, or above the scapula. After a few weeks, the soft tissue swelling transforms into a very hard, painless mass. As time progresses, the subcutaneous mass becomes immobile and adheres to deep tissues, including bone. If left untreated, the mass may rapidly grow due to vascular distribution, reaching a size sufficient to cause surface necrosis.

Cytological examination shows a fine needle aspiration of a pale, thick yellow fluid, with a microscopic view revealing clusters or scattered cells, which are spindleshaped with slight variations in size; nuclei are oval with slight variations in size as well; cytoplasm is basophilic with a high nuclearcytoplasmic ratio; and nucleoli are clearly visible. The cytological features are consistent with mesenchymal cells.

Treatment recommendations for FISS include aggressive surgical removal, as early surgery after diagnosis can reduce the chance of tumor metastasis. Cats that undergo aggressive surgery have a longer diseasefree survival time, with those having tumors in the forelimbs living longer than those with tumors in the trunk. FISS with a diameter less than 2 cm can usually be longterm relieved after aggressive surgical removal.

1. Triple Therapy

Surgery: Surgical intervention is the foundation of treatment, requiring a wide and deep surgical incision due to the invasive nature of the tumor, which can extend beyond the palpable mass.

Radiation: Radiation helps control tumors that extend beyond the mass and into adjacent tissues and must be combined with surgical treatment.

Chemotherapy: Preoperative chemotherapy can reduce the tumor size and facilitate surgical removal. Various treatment regimens using doxorubicin, cyclophosphamide, vincristine, carboplatin, and mitoxantrone can be effective. For inoperable tumors, chemotherapy can alleviate symptoms and improve the comfort of the affected cat.

2. Adjunctive Therapy

Supportive Therapy: Buprenorphine can be administered at a dose of 0.005 to 0.01 mg/kg every 612 hours by intramuscular, intravenous, or subcutaneous injection to control pain. Cats can tolerate oral administration, with effects lasting up to 6 hours.

In this case, a cat with a tumor found in the thigh area, which was yellow upon incision, was suspected to have an injection site sarcoma. Diagnosis could be confirmed by performing a cytological examination on the yellow excised tissue.