Feline injection site sarcomas (ISS) were first described nearly 2 decades ago. Recognition of this tumour coincided with the development of effective vaccines and increased administration of multiple vaccines in household cats. Epidemiological evidence has implicated rabies and feline leukemia (FeLV) vaccines as major causative factors, enhancing the risk of ISS 2–5 fold, with an intense inflammatory reaction as the inciting source for tumor development. The emergence of ISS also corresponded with the development of killed vaccines for common use. There is an increased risk with increasing number of vaccinations, and with repeated vaccination at the same site. Tumours usually arise 2–10 months after vaccination, and a risk of 0.32 sarcomas/10,000 doses given is reported. An inflammatory reactive granuloma is seen 11.8 times/10,000 doses of vaccine in cats, but it is not necessary to remove these masses unless malignant behaviour is apparent or they persist over 4 months In addition, adjuvants, such as aluminum, have been suggested as a causative factor for sarcoma development; other materials implicated include various long-acting injectable medications and suture material.

Most ISS are histologically classified as fibrosarcomas, with osteosarcoma, rhabdomyosarcoma, chondrosarcoma and malignant fibrous histiocytoma also described. Rapid growth and local invasion are considered more characteristic of ISS than non-ISS in cats with vaccination site tumours more likely to be larger in size at the time of diagnosis and recur post-surgery. There are often centrally located micro- or macro­abscesses giving rise to a cystic centre, yielding fluid on fine needle aspiration. Feline ISS historically arise in popular injection sites such as the dorsal interscapular area. Recent work has shown that following publication of vaccine guidelines in the mid-1990s recommending vaccination in the extremities (to allow for amputation should a tumour arise) tumours are now arising with increasing frequency in the hindlimbs and caudal flank (the latter likely due to misplaced injections designed for the pelvic limbs in sitting cats).

ISS are graded from I (least aggressive)–III (most malignant) depending on mitotic index, differentiation and necrosis. Multinucleated giant cells are seen in the more malignant tumours, and most have peritumoural lymphocyte inflammation and increased vascular density. Recent evidence has suggested a higher grade (i.e., grade III) is associated with an increased risk of metastasis, and the presence of metastasis has been shown to significantly decrease survival time in cats with ISS.

Investigation of a Suspected ISS

Diagnostic investigation for ISS has 3 aims.

• To accurately define the tumour ahead of treatment.
• To define the anatomical relations of the primary tumour for planning (surgery and/or radiation).
• To identify the presence or absence of metastatic disease.

Radiographs may yield some information regarding local behaviour, but may only confirm the mass is of soft tissue density. Ultrasound (esp. Doppler) can be useful but cross sectional imaging (CT or MRI) is the imaging of choice and is typically supportive of the diagnosis. Whereas MRI is traditionally regarded as superior for soft tissue detail, CT (esp. contrast CT) offers a fast, simple, and accurate alternative for all but the most complex STS.

In terms of biopsy, fine needle aspirates have an important role in ruling out other subcutaneous differentials for example mast cell tumours, lipomas or inflammatory lesions, all of which exfoliate cells well. If lucky enough mesenchymal cells will be aspirated from an ISS to make a diagnosis. If an aspirate of a SQ mass fails to yield many cells on the slide, your index of suspicion for an ISS or other sarcoma should be raised, and prompt a core biopsy. Percutaneous core biopsies (e.g., Tru-Cut) are the best technique for achieving a safe and accurate diagnosis and can easily be performed with local anaesthetic alone or with sedation in nervous patients. Tru-Cut biopsies will reliably differentiate benign from malignant disease and in most cases will also give a good indication of grade. The simplicity and accuracy of core biopsy for ISS means incisional biopsies are infrequently indicated and come with the added concerns of location and direction of scar, and tumour dissemination from post-incisional biopsy haematoma.

Surgical Resection is the Most Effective Treatment for ISS

Principles of surgical oncology and tumour resection are perhaps more applicable to ISS than any other tumour type. The Enneking system of margin classification we use in veterinary medicine evolved from human sarcomas and fits most comfortably with canine and feline sarcomas. He classified them as intralesional (intracapsular), marginal, wide or radical.

• An intracapsular margin is achieved by piecemeal removal (‘debulking’) of a lesion from within the capsule. This is also used if the capsule is accidentally entered during dissection as the surgical field is now contaminated. Gross and/or microscopic disease remains.
• A marginal margin is achieved by an extracapsular dissection through the reactive zone around the mass. Classically these are termed ‘shell-outs’ and involve peeling the mass out from its tissue bed and off local attachments. Both benign and malignant lesions may have extracapsular microextensions of disease, microsatellites (in the reactive zone, e.g., mast cell disease), and ‘skip’ metastases of high-grade lesions (in normal tissue of the same compartment, e.g., soft tissue sarcomas). These both have implications for marginal excisions in terms of potential for local recurrence.
• A wide margin is achieved by en bloc removal of the lesion, its capsule and the surrounding reactive zone but always working in normal uncontaminated tissue within the compartment of the lesion. Non-neoplastic, non­reactive intracompartmental normal tissue is left at the margins and there is the possibility of ‘skip’ metastases arising in the remaining portion of the compartment.
• A radical margin removes the lesion, reactive zone, and all the tissue of the associated compartment. There is no potential for residual neoplasm locally. The typical example is amputation, along with variants such as hemi­pelvectomy.

Treatment options broadly exist as

1.  Curative-intent excision with reconstruction, plus radiation if incomplete margins.

2.  Pre-operative radiation (48Gy) with sterilisation of tissue around tumour and planned marginal excision of mass.

3.  Planned marginal excision of mass then post-operative radiation (57 Gy) to sterilise tumour bed.

Treatment, including surgery and radiation therapy, has focused on reducing local recurrence and increasing survival time associated with ISS. Reported rates of local recurrence post-treatment range from 26–59%. Recurrence rates and survival times have been documented following various combinations of surgery (marginal vs. wide local vs. radical), radiation therapy (pre- or post-operative) and chemotherapy, with no reliable difference in outcome noted between various aggressive treatment protocols. In two studies, completeness of surgical excision was shown to affect rate of local recurrence, reporting 58–69% recurrence rates with incomplete versus 19–22% with complete excision. In addition, complete excision has been associated with a longer time to first recurrence and tumor free interval (time to first recurrence or metastasis).

Historically, an excision of ISS in cats with 2–3 cm margins and one fascial plane deep to the tumour bed has been adopted (from canine sarcomas), but it may not be adequate when using surgery alone considering the high historical rates of local recurrence. In response to this, several authors have suggested more radical methods, including surgical margins of greater than 3 cm and 1 to 2 fascial planes deep to the tumor, along with partial scapulectomy, osteotomy of spinous processes or hemipelvectomy when indicated. Currently, the Vaccine-­Associated Sarcoma Task Force recommends multi­modal treatment, including surgical resection with at least 2 cm margins in all planes, although the use of 3 to 5 cm margins is considered. A study on feline vaccine-associated sarcomas in cats following excision with 3 cm wide margins and one facial plane deep showed that 1 cm away from palpable tumor, the tissue was free of neoplasia in 13% cases, by 2 cm from the mass, 32% were free of cancer cells, and by 3 cm, 94% were free of neoplasia. The single fascial plane achieved a tumor-free deep margin in 94% cases.

Palpation may suggest a discrete mass, but post-contrast CT often shows the true extent of infiltrative disease, which cannot be appreciated by feel alone. Radical first excision of ISS yielded significantly longer median time to first recurrence (325 days) than did marginal first excision (79 days). Median survival time is significantly longer in cats with tumours <2 cm in diameter compared to those with tumours 2 cm in diameter. Work recently completed (to be presented) in 91 cats with ISS treated by an excision of 5 cm and 2 fascial planes (without radiation therapy or chemotherapy), shows the rate of recurrence to be 14% with rate of metastasis of 20%. Median survival time of cats with recurrence was 499 days and without recurrence was 1461 days. Median survival time of cats with metastasis was 388 days and without metastasis was 1528 days. Survival was significantly affected by both recurrence and metastasis. Major complications occurred in 11 cats; 7 had incisional dehiscence.

Radiation Therapy for ISS

Surgical resection of the ISS can facilitate adjunctive therapies by reducing the gross tumour burden; identifying tumour margins; decrease the risk of satellite and skip metastases, eliminate microscopic tumour extension into normal tissue and removing drug- and radiation-resistant cells, circulating immune complexes, and tumour-associated immunosuppressants.⁴⁴ Furthermore, radiopaque ligation clips can be placed at surgery to mark the margins of excision and facilitate radiation planning.

Although ISS response to external beam radiation can be unpredictable, it plays an important role in large fixed tumours where CT shows achieving tumour-free margins by surgery alone will be difficult. Pre-operative radiation will sterilise the reactive zone and surrounding ‘clean’ tissues and so allow for a potentially smaller resection to be performed and radiation therapy is theoretically more effective in the neoadjuvant setting because an unimpaired vascular supply to the tumour results in tumour cells being better oxygenated, less hypoxic, and more radiosensitive. Post-operative radiation can be used when a curative-intent surgery was unsuccessful and residual tumour disease remains. The orientation of the scar is an important factor in limiting radiation side-effects; a scar in the midline parallel to the spine will result in minimal dose to the spinal cord as the beams can be targeted from laterally to ‘skyline’ the scar. If the scar runs perpendicular to the spine and extends down over the thoracic wall on both sides, then there is a much greater risk of including the spinal cord within the planned treatment volume. The post-operative treatment is started 10–14 days after surgery when the surgical site has already healed. If radiation is given pre-operatively, the dose is often lower to avoid overly damaging the skin and so reduce the risk of creating a non-healing wound following surgery.

Using pre-operative cobalt 60 in combination with surgery, median time to recurrence was 2.7 years following complete excision and 0.8 years if incompletely excised. Surgery followed by megavoltage radiation gave median survivals of 23–24 months, but time to recurrence and survival were decreased when time to surgery and starting radiation was increased.

Chemotherapy for ISS

Although there are reports using drugs such as doxorubicin, cyclophosphamide, mitoxantrone, and carboplatin to treat cats with ISS, there is little evidence in larger numbers of cats with ISS that adjuvant chemotherapy improves overall survival. Neo-adjuvant doxorubicin in cats with macroscopic ISS has been shown to result in a 40% overall response rate which may allow for increased surgical success. Chemotherapy may delay local recurrence in cats also receiving curative-intent radiation therapy.

Reported rates of local recurrence range from 25–60% after various combinations of surgery, radiation (pre- or post-operative) and chemotherapy. Rates of metastasis range from 5.6% to 22.5%. Pulmonary metastasis is most commonly reported, with other sites including regional lymph nodes, skin, intestine, spleen, epidural and ocular infiltration, or multi-organ involvement.