developing adjacent to the site of microchip implantation has been reported in a cat (Daly et al. 2008), and dog (Vascellari et al. 2006).
Most often younger cats are affected (6–7 years), with a second peak at 10–11 years. FISAS usually occur in the subcutis. In contrast, non‐injection site‐associated FSAs are often dermal in origin (Kass et al. 1993). One case has been reported suffering from hypertrophic osteopathy with concurrent FISAS (Salgüero et al. 2015). The reported prevalence in Poland differs between 0.1 and 0.85% in general and referral clinics, respectively (Kliczkowska et al. 2015). The incidence risk of FISAS per year in the UK has been estimated to be 1/16 000–50 000 cats registered by practices, 1/10 000–20 000 cat consultations, and 1/5000–12 500 vaccination visits (Dean et al. 2013). Early in the new millennium, the risk of developing a sarcoma in North America was evaluated at 0.63 sarcomas/10 000 cats and 0.32 sarcomas/10 000 doses of all vaccines (Gobar and Kass 2002). The number of vaccines administered increases the risk of developing FISAS. The risk for a cat to develop a sarcoma after administration of a single vaccine is 50% higher than the risk of a cat not receiving any vaccine. The risk for a cat given two vaccines is approximately 127% higher, and the risk for a cat given three to four vaccines is 175% higher (Kass et al. 1993).
FISAS are best diagnosed by biopsy as cytology is not reliable. A wedge or punch biopsy must be taken from a part of the mass that later will be included in the surgical resection.
MRI and/or CT of the tumor and thorax is indicated to look for pulmonary and skip metastases and evaluate size and extent of the tumor as these masses often have tendrils extending to or into underlying muscles and other tissues like bony structures (spinous process, scapula). In 10–25% of cases, pulmonary metastases are found. The lungs are the most common site for FISAS metastases, although they can also occur in the subcutaneous tissue, liver, and lymph nodes. Therefore, draining lymph nodes should be palpated and assessed by cytology and diagnostic imaging (Kuntz et al. 1997; Romanelli et al. 2008; Rousset et al. 2013). Skip metastases are highly correlated with tumor recurrence (P = 0.001) (Zardo et al. 2016).
Dual‐phase CT angiogram and MRI identify a similar number of peritumoral lesions. The extensive overlap between imaging features of neoplastic and nonneoplastic lesions precludes definitive identification of neoplastic peritumoral FISAS lesions using CTA or MRI (Nemanic et al. 2016).
STS grading system used in canines, depth of infiltration, surgical margins, and Ki‐67 index did not relate to recurrence. Instead, the size of the tumor, measured after formalin fixation, with an optimal cutoff of 3.75 cm, and the mitotic count, with an optimal cutoff of 20 mitoses/10 HPF were prognostic for recurrence and survival time (Porcellato et al. 2019). In another study, grade using the canine criteria was prognostic for metastasis but no local recurrence (Romanelli et al. 2008). In a study of cats with soft tissue sarcomas that included FISAS and non‐FISAS, modification of the criteria used to grade soft tissue sarcomas in canines was applied and was prognostic for survival time: median survival time for cats with low‐grade tumors was 900 days, with intermediate grade 514 days, and high grade 283 days (Dobromylskyj et al. 2021). In this modified system, mitotic score and tumor necrosis score were the same as in canine tumors, but inflammation score was used instead of tumor cell differentiation score (Dobromylskyj et al. 2021).
Aggressive first surgical excision with wide (Figure 4.8) to radical (3–5 cm) margins results in the best tumor‐free interval and survival time in cats with soft tissue FSA. Cats with complete excisions have significantly longer median tumor‐free interval (>16 vs. 4 months) and survival time (>16 vs. 9 months) than those with incomplete excisions (Davidson et al. 1997). Marginal excision is the major reason that high recurrence rates (up to 70%) have been reported (Davidson et al. 1997; Hershey et al. 2000). Advanced imaging of the tumor is recommended (McEntee and Samii 2000; Morrison and Star 2001) for appropriate treatment planning. Median time to recurrence was significantly longer when a cat was operated by a specialist surgeon (274 days) compared to a referring veterinarian (66 days) (Hershey et al. 2000). Other prognostic factors for survival time that are significant include local recurrence, presence of distant metastasis, and the number of surgeries (Cohen et al. 2001; Eckstein et al. 2009; Romanelli et al. 2008). Overall reported median survival time after surgical treatment has been 11.5–20.3 months (Davidson et al. 1997; Dillon et al. 2005), and median survival time after complete excisions (>16 months) has been significantly longer compared to incomplete excisions (9 months) (Davidson et al. 1997).
Figure 4.8 (a) Wide excision of a feline injection‐site‐associated sarcoma. The skin incision has been performed around the subcutaneous tumor. (b) En bloc resection of tumor mass and surrounding tissue barrier. (c) Visible dorsal spinous processes (arrows) of cervical vertebrae after tumor removal. (d) Closure in layers with simple interrupted suture patterns. Blue nylon skin sutures are visible.
The most important prognostic factor for local recurrence, and subsequent survival time, is the achievement of clean surgical margins (Banerji and Kanjilal 2006; Cronin et al. 1998; Hershey et al. 2000; Kobayashi et al. 2002). Cats undergoing limb amputation for FISAS did better than local excision anywhere else on the body (Hershey et al. 2000). Size of the tumor has been reported to influence survival time after surgery (Cohen et al. 2001; Dillon et al. 2005; Spugnini et al. 2007b). To achieve wide tumor resection, resection of the dorsal portion of interscapular vertebral spinous processes, partial scapulectomy (Trout et al. 1995), lateral body wall resection (Lidbetter et al. 2002), and hemipelvectomy (Barbur et al. 2015; Straw et al. 1992) may be necessary (Davidson et al. 1997; Davis et al. 2007; Hershey et al. 2000; Romanelli et al. 2008). Some surgeons promote using wider surgical margins than the commonly recommended 2–3 cm lateral margins with one tissue plane in depth, because of the high recurrence rate of FISAS. Fifty‐seven cats with FISASs were treated by wide resection using 4–5 cm lateral margins and one fascial plane deep to the tumor, including partial scapulectomy and removal of dorsal spinal processes if indicated. Histologically complete resections were reported for 95% of the tumors; 5% had tumor cells in the margins. Local tumor recurrence developed in 39%, with distant metastasis in 21%. About 51% of the cats were alive at an overall median follow‐up period of 366 days (median follow‐up period for the alive cats was 600 days) (Romanelli et al. 2008).
Phelps et al. (2011) reported in a case series on 91 cats treated by radical excision with five‐centimeter margins including 2 muscle planes or bone deep to the tumor (Figure 4.9). Any anatomic structures that fell within the determined margins were excised, including thoracic wall, abdominal wall, dorsal spinous processes, ilial wing, and scapula. If the tumor was subcutaneous and could be elevated away from underlying structures with 5‐cm margins, the underlying structures were not excised. Although excision of FISAS resulted in a metastasis rate similar to rates reported previously, the local recurrence rate appeared to be substantially less than rates reported after less aggressive surgeries. Overall median survival time was 901 days. Out of 91, 13 (14%) cats had local tumor recurrence, 18 (20%) cats had evidence of metastasis after surgery. The median survival time of cats with and without recurrence was 499 and 1461 days, respectively. The MST of cats with and without metastasis was 388 and 1528 days, respectively. Tumor recurrence and metastasis were significantly associated with survival time, whereas other examined variables were not. Major complications occurred in 10 cats, including 7 with incisional dehiscence. The best predictor for the development of wound healing complications after wide excision of FISAS is an increased duration of surgery (Cantatore et al. 2014).
