Page 163 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

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Injection site sarcoma
VetBooks.ir Malignant tumour of different lineage that may develop at sites of previous injections.

Clinical features
• It occurs in a range from 1:1000 to 1:10,000 tumours per vaccinated cat. It is very rare in dogs.
• Age in cats: as young as 3 years of age with an average of 8–10 years.
• Described after the injection of vaccines, long-acting antibiotics, steroids, non-ab-
sorbable material and microchip implant. Chronic inflammatory stimulation can
stimulate proliferation of fibroblasts and myofibroblasts and subsequent malignant
transformation through multiple cytokines and growth factors.
• Injection-site sarcoma can have different histotype origins. The most common forms
are fibrosarcoma, anaplastic sarcoma with giant cells, osteosarcoma, chondrosarcoma,
rhabdomyosarcoma and undifferentiated sarcoma.
• It presents as variably sized, firm, poorly demarcated, often multilobulated, cutane-
ous or subcutaneous nodules at sites commonly used for injections, such as neck,
interscapular space, thorax, lumbar region, flanks and limbs. Lesions often contain a
cystic centre containing a watery or mucinous fluid.
• Local recurrence after surgery is common due to the infiltrative growth pattern.
Metastatic risk is low to moderate and in cats ranges from 10% to 28%. Metastases
tend to occur late in the course of the disease and primarily affect regional lymph
nodes, mediastinum and lungs.

Cytological features
• The cellularity is variable, from low to high.
• Background: clear to lightly basophilic and variably haemodiluted. Depending on the histotype
of the tumour, variable amounts of matrix (e.g. osteoid, chondroid, collagen, etc.) can be present.
• The cell morphology depends on the histotype of the neoplastic cells (refer to fibrosarcoma,
anaplastic sarcoma with giant cells, etc.).
• Cytological features of atypia are variable, often marked, especially in poorly differentiated
forms. These include anisokaryosis, anisocytosis, multiple prominent nucleoli (round, oval,
angular or irregular), anisonucleoliosis, multinucleation and atypical mitotic figures.
• Neutrophils, macrophages and/or small lymphocytes are often present due to a concurrent in-
flammation. Macrophages may contain grey/brown intracytoplasmic material (compatible with
injected material). Necrosis can be observed.

Differential diagnoses
• Sarcoma of other origin
• Reactive fibroplasia