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N. Bacon1
1Fitzpatrick Referrals Oncology and Soft Tissue, University of Surrey School of Veterinary Medicine, 70 Priestley Road, Surrey Research Park, Guildford, GU23 7HT, UK
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 macro- abscesses 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), and;
• 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 corebiopsy. Percutaneous core biopsies (e.g. Trucut) 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. Trucut 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
An Urban Experience

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