A 14-year-old spayed female cat was presented to a referral center 1 month after a firm interscapular mass was discovered by the referring veterinarian. An incisional biopsy was diagnostic for fibrosarcoma. A microchip had been implanted in the interscapular area 3 years before the diagnosis. Vaccines, including feline parvovirus, feline herpesvirus-1, feline calicivirus, and rabies, had been given within the interscapular area 1 year before the diagnosis of fibrosarcoma. Feline leukemia virus vaccination had last been given 4 years before the diagnosis, also in the same region. Radiographs and computed tomography performed by the referral center showed the microchip to be cranial to the mass. The cat was treated with preoperative radiation therapy and aggressive surgical resection. Its last follow-up, 289 days after the surgery, revealed no evidence of tumor recurrence or metastasis. Dissection of the excised mass showed the microchip adjacent to the mass, but there was no gross or microscopic reaction around the chip. Although cases of fibrosarcoma and liposarcoma in the area of microchip implantation have been reported in dogs, this is the first known report in a cat. Veterinarians should be aware that, although rare, tumors at microchip sites are possible and owners should be educated to monitor the area. Following current vaccination recommendations will also prevent administration of vaccines at microchip implantation sites, avoid subsequent alterations in the microenvironment of the site that might increase risk for sarcoma, and help veterinarians better identify the cause of tumors should they develop.

COMMENTARY: Inflammation of any cause seems to be a risk factor for sarcoma in susceptible cats. This cat had been vaccinated yearly in the interscapular area with multiple vaccines, including rabies vaccine, and there was also a microchip at the site. Because there is no possibility to ascribe causality in this patient, this report should not discourage microchip implantation in animals, but it should remind veterinarians of the importance of careful and responsible vaccination practices and of following the American Association of Feline Practitioners guidelines for vaccination. (In this case, if the cat had been vaccinated at recommended sites, it may have been possible to establish which of the procedures was most likely to have caused the tumor formation).

Fibrosarcoma adjacent to the site of microchip implantation in a cat. Daly MK, Saba CF, Crochik SS. J FELINE MED SURG 10:202-205, 2008.