injection-site sarcomaS

BACKGROUND

Injection-site sarcomas (ISSs), also known as vaccine-associated sarcomas, are uncommon tumors in cats. ISSs are  induced by vaccinations (especially adjuvant rabies and FeLV vaccines), but  other types of injections have also been associated with ISSs (such as microchips, long-acting corticosteroids, meloxicam, and luferenon). These tumors are locally aggressive and have a high risk for local recurrence, but the risk of metastasis is usually low.

DIAGNOSIS

The accuracy of fine-needle aspirates for the diagnosis of ISSs is only about 50%. A diagnosis of a ISS  is often suspected if the fine-needle aspirate cytology is inconclusive because most infections and other tumor types can be diagnosed with a high degree of accuracy (> 90%) with fine-needle aspirates. An incisional biopsy can be considered for those tumors not diagnosed with a fine-needle aspirate.

CLINICAL STAGING

CT scan or MRI of the tumor is recommended to determine the extent of the local tumor and assist in surgical planning. Contrast-enhanced advanced imaging will often show that ISSs are much more extensive than palpation alone would suggest. Thoracic radiographs or CT scans are recommended to assess the lungs for metastatic disease.

TREATMENT

The recommended treatment for cats with ISSs is aggressive surgery. Complete excision rates and lower local recurrence rates are more likely with 5cm lateral margins and two fascial layers for deep margins. Depending on the location of the tumor, this  may involve removal of segments of the vertebrae (spinal bones), scapula (shoulder blades), chest and/or abdominal wall, or limb amputation. Radiation therapy is recommended either before or after surgery to reduce the risk of local tumor recurrence, but the need for this is debatable considering the published outcome with 5cm surgical margins is far superior to surgical excision with 3cm lateral margins combined with radiation therapy. The metastatic rate is low in cats with ISS and most published studies show no beneficial effect with chemotherapy on either local tumor control or survival time.

Other considerations in the management of ISS in cats is the administration of preoperative chemotherapy, which results in lower local recurrence rates because of the effect of chemotherapy on satellite populations of tumor cells distant to the ISS; and immunotherapy, which is not recommended in my opinion because the results are inferior to wide excision and the use of this commercial product encourages under treatment and a worse prognosis.

PROGNOSIS

The prognosis for cats with ISS is dependent on treatment. For cats treated with 3cm lateral margins and one fascial layer for deep margins, the complete excision rate is only 50%, the overall local recurrence rate is 65%-91%, and the median disease-free interval is only 79 days for incompletely excised ISSs and 325-416 days for completely excised ISSs. When this surgical approach is combined with radiation therapy, the overall local recurrence rate and median disease-free interval are 28%-45% and 398-584 days, respectively; while the local recurrennce rates and median disease-free intervals for incompletely and completely excised ISSs following radiation therapy are 42% and 32%, respectively, and 112-292 days and 700-986 days, respectively. In comparison, when ISSs are excised with 5cm lateral margins and two fascial layers for deep margins, the complete excision rate is 97%, the 3-year local recurrence rate is 14%, and the 3-year survival rate is 80%. The risk of metastasis is low and is reported in 10%-25% cats. The median survival time following treatment with surgery and radiation therapy is 600-842 days, but 1,461 days when treated with 5cm surgical margins alone.

Last updated on 6th March 2017