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Injection-site sarcomas (ISS) are also referred to as fibrosarcomas. They are mesenchymal tumors that are locally invasive and have a low to moderate chance of spreading to other sites (metastases). These tumors develop in areas where cats have received injections, mainly vaccines. Common locations include the interscapular (between the shoulder blades) region, the flank region, and the rear legs. These tumors tend to be firm, subcutaneous masses that are not freely movable.
The main treatment recommendation is wide surgical excision. Because these tumors are locally invasive, it is often difficult to clearly identify the margins of the tumor on palpation alone. A CT scan is useful in identifying the extent of the mass, which then aids in the surgical planning. Radiation therapy may be indicated if the tumor was not completely excised and a more aggressive surgery cannot be performed due to its size or location. Although the majority of ISSs are only locally invasive, approximately 25% will metastasize to the lungs or other sites. Chest radiographs should be taken prior to surgery or radiation therapy. Chemotherapy may be indicated before surgery (neo-adjuvant) if the tumor is too large to remove, in the hopes of reducing its size. Adjuvant chemotherapy (after surgery or radiation therapy) may help to prevent or delay the onset of metastases.
Some cats may develop a postvaccinal lump that usually resolves within 4 to 6 weeks; however, any lump that is present after 6 weeks from the time of vaccination or that is larger than 2 cm in diameter should be removed; an incisional biopsy should be evaluated to confirm the diagnosis prior to an aggressive surgery.
If you have concerns or questions about a lump on your cat, we recommend that you consult with your veterinarian. If your cat has been diagnosed with an ISS, treatment options may include a combination of surgery, radiation therapy, and chemotherapy; consultation with an oncologist is advised.