Fibrosarcoma adjacent to the site of microchip implantation in a cat

Abstract

A 14-year-old spayed female domestic shorthair cat presented with an interscapular mass. A computed tomography scan, biopsy, and histological examination revealed a fibrosarcoma adjacent to a pet identification microchip. Because the cat was previously vaccinated at this site, it is not possible to establish definitive causation of the fibrosarcoma, but this is the first report of a tumor in the vicinity of a microchip in a cat. Microchip-associated tumors have been reported in rodents and dogs. Veterinarians should be aware that because inflammation may predispose felines to tumor formation, separation and observation of vaccination and implantation sites are indicated. Adherence to American Association of Feline Practitioners (AAFP) vaccination guidelines and monitoring of microchip implantation sites are recommended.

A 14-year-old spayed female domestic shorthaired cat was referred to the University of Georgia, College of Veterinary Medicine (UGA-CVM) for evaluation of an interscapular fibrosarcoma. One month prior to referral, the cat presented for routine annual exam. On physical examination, a 2×2 cm multilobular, firm interscapular mass of unknown duration was noted. An incisional biopsy was diagnostic for fibrosarcoma. Feline leukemia virus and feline immunodeficiency virus tests were negative. Complete blood count, serum biochemical profile, thyroxine level, and urinalysis performed 6 days prior to presentation at UGA-CVM were normal except for hematuria.

Physical examination at UGA-CVM revealed a healthy overweight cat with a 5.4×4.5×2.2 cm firm subcutaneous interscapular mass. A urine sample was obtained via cystocentesis. The specific gravity was 1.020 and on microscopic examination, 6–12 white blood cells and many rod-shaped bacteria were noted per high power field. Based on culture and sensitivity results, the cat was treated with amoxicillin trihydrate/clavulanate potassium (Clavamox; Pfizer Animal Health) at 22 mg/kg every 12 h.

No intrathoracic abnormalities were present on radiographs but a 6×3 cm ovoid soft tissue mass was visualized dorsal to the thoracic vertebral spinous processes at the site of a microchip. Abdominal radiographs were normal except for a small right kidney. Helical computed tomography (CT; Somatom AR, Star, Siemens AG, Munich, Germany) was performed from the cervical (C3) to the lumbar (L2) area with reconstructed intervals of 1.5 mm. Images were obtained pre- and post-intravenous administration of 12.5 ml of sodium iothalamate (Conray 400LM, Mallinckrodt, St Louis, MO). A peripherally enhancing well-defined ovoid soft tissue density mass was seen in the subcutaneous tissues dorsal and to the right of the thoracic vertebrae from T6 to T11. A poorly defined wispy soft tissue density was seen extending caudal from the mass to the level of T12–T13. A metallic object was adjacent to the cranial aspect of the mass. This object corresponded to the microchip observed on thoracic radiographs and on scout images obtained during the CT (Fig 1).

Fig 1.

Post-contrast CT image at the level of T8: note the hyperattenuating material (white arrow) adjacent to the mass within the dorsal subcutaneous tissue. Beam-hardening artifact (black streaks) is also present due to the metal present in the microchip. Left is up, dorsal is to the right.

Multimodality therapy including preoperative radiation therapy, aggressive surgical resection, and consideration of adjunctive chemotherapy was recommended for the fibrosarcoma. The total prescribed minimum tumor radiation dose was 48 Gy administered in 12 equal fractions of 4 Gy on a Monday–Wednesday–Friday schedule. Manual treatment planning was performed. The prescribed treatment volume included the tumor visualized on the CT images plus a 3 cm margin of surrounding tissue, excluding the thoracic spinal cord. Treatments were delivered using a rotational telecobalt-60 unit (Model C-9, Picker Corporation) at an 80 cm source to skin distance. Equally weighted, parallel opposed lateral treatment portals were used and portal localization radiographs were obtained before each treatment. Five-millimeter tissue equivalent bolus material was used to insure prescribed dose to the tumor periphery.

Two weeks following radiation therapy, the cat was examined and a repeat CT scan was performed. The mass had decreased to 1.4 cm diameter and wide surgical excision was performed. No further treatment was elected and at the last follow-up, 289 days following surgery, the cat was healthy with no evidence of tumor recurrence or metastasis.

Histopathological evaluation of the mass removed post-radiation and review of the pre-radiation biopsy were performed. The pre-radiation mass was infiltrative, non-encapsulated, and composed of atypical spindle-shaped cells in swirling fascicles. Multifocal necrosis was present. Individual cells had oval pleomorphic nuclei with stippled chromatin and one or two large nucleoli and a moderate amount of amphophilic cytoplasm. Anisokaryosis was marked, numerous karyomegalic and multinucleated tumor cells were present, and the mitotic rate was high (17 mitotic figures per 10 high power fields [40×]). Multiple lymphoid aggregates and infiltrations of hemosiderin-laden macrophages were observed in adipose and connective tissues at the periphery of the mass. On immunohistochemical staining, all tumor cells were strongly positive for vimentin, approximately 50% of the tumor cells were strongly positive for smooth muscle actin, and lymphoid aggregates were primarily composed of CD3 positive cells. These findings were consistent with fibrosarcoma (grade II, as defined by Couto et al 2002) with a myofibroblastic phenotype (Fig 2). After radiation therapy, the mass was composed of a necrotic core surrounded by collagenous tissue that was sparsely populated with fibroblasts and had multiple small, mostly perivascular, infiltrations of lymphocytes and plasma cells. A band of collagenous tissue crossing the dorsal aspect of the mass had a small number of spindle cells with multiple hyperchromatic nuclei that varied in size and had prominent nuclei interpreted to be residual neoplastic fibroblastic cells. Surgical margins were free of neoplastic cells. Findings were consistent with post-radiation remnants of a spindle cell sarcoma. Dissection of the post-radiation excisional biopsy revealed a microchip adjacent to the mass, but no gross or microscopic reaction around the microchip. No aluminum deposits were detected in the pre- or post-radiation samples and this tumor was histologically similar to a fibrosarcoma reported at the site of microchip implantation in a dog (Vascellari et al 2006). In that report, as in this cat, the microchip was found adjacent to but not embedded in the tumor.

Fig 2.

Pre-radiation incisional biopsy from the subcutis of a cat. Approximately 50% of the neoplastic spindle-shaped cells were immunopositive for smooth muscle actin (left half of photomicrograph). Lymphoid aggregates (arrow) and infiltrations of hemosiderin-laden macrophages were observed in the adipose tissue at the periphery of the mass. Immunoperoxidase technique for smooth muscle actin. DAB chromagen with hematoxylin counterstain. Bar=100 μm.

Implantable microchips for identification of animals were introduced in the 1980s and have been used for >10 years in long-term rodent toxicity and carcinogenicity studies (Le Calvez et al 2006). Implantation of a microchip is considered a safe, relatively painless, and permanent means of identification. In small animal practice, commercially available microchips are used routinely to identify companion animals and have been shown to have excellent accuracy (Sorensen et al 1995). Rare complications have included migration, malfunction, loss, and infection or swelling (Swift 2002). There is one report of spinal cord injury immediately following microchip implantation due to incorrect placement (Platt et al 2006).

Pet identification microchips are sealed in antimigrational bioglass capsules composed primarily of silicon. Bioglass is a material that is insoluble and biocompatible. Although an initial inflammatory response follows implantation of the microchip, it has been documented in dogs and mice that within 3 months after implantation, inflammation has resolved and the microchip is surrounded by a connective tissue capsule composed of collagen fibers, elastic fibers, and few fibroblasts (Rao and Edmonson 1990, Murasugi et al 2003). Once enclosure in connective tissue is complete, a microchip is expected to function safely for the rest of the animal's life, however, tumors at the microchip site have been described in laboratory mice (Tillmann et al 1997, Le Calvez et al 2006) and rats (Elcock et al 2001). Recently, cases of fibrosarcoma and liposarcoma at microchip implantation sites have been reported in dogs (Vascellari et al 2004, 2006). The majority of microchip-associated tumors have been mesenchymal in origin and the mechanism of carcinogenicity is suspected to be foreign body-induced tumorigenesis (Elcock et al 2001). To our knowledge, this is the first report of a tumor adjacent to the site of a microchip in a cat.

Additional historical information was obtained from the referring veterinarian of the cat in this report. The cat was implanted with a HomeAgain microchip (Schering Plough, Kenilworth, NJ) in the interscapular area nearly 3 years prior to the diagnosis of fibrosarcoma. Although the microchip was attached to the tumor, one cannot definitively conclude that it induced the fibrosarcoma because the cat received annual vaccinations in the interscapular area including feline parvovirus, feline herpesvirus-1, feline calicivirus, and rabies. Vaccines were last administered 1 year prior to detection of the mass. In addition, feline leukemia virus vaccinations had been administered in this area until they were discontinued >4 years before development of the fibrosarcoma. Vaccine-associated sarcomas have been reported most commonly at the site of feline leukemia and rabies vaccinations (Vaccine-Associated Feline Sarcoma Task Force 2005) and have been reported from 2 months to 10 years after vaccination (McEntee and Page 2001). It is important to note that it is possible that this cat's tumor was induced by vaccinations and the location of the microchip was coincidental or that receiving vaccinations at the site of the microchip increased the risk of a foreign body-induced tumor.

It is not the intent of this report to discourage the use of microchips for pet identification. Development of tumors at microchip implantation sites appears to be a rare event. More than 4,900,000 companion animals have been implanted with a HomeAgain microchip (HomeAgain Pet Recovery Website 2007) and there are several other companies that manufacture similar identification chips worldwide. Despite the implantation of millions of microchips, this is the first reported tumor in the vicinity of a microchip in a cat. Veterinarians should be aware that tumors at microchip sites are possible and educate owners to monitor these sites. This will promote early detection as well as better definition of the incidence of tumors. In addition, clinicians are encouraged to follow current vaccine recommendations as described in the 2006 American Association of Feline Practitioners Feline Vaccine Advisory Panel Report (AAFP Advisory Panel 2006). This will prevent administration of vaccines at the site of microchip implantation, avoid consequent alterations in the microenvironment at the microchip site that could increase the risk of sarcoma development and allow veterinarians to more easily determine the etiology of tumors.