I find it interesting that a vaccine may induce a fibrosarcoma!
FIBROMA / FIBROSARCOMA
Fibromas exfoliate poorly and few cells will be on the slide. Fibrosarcomas produce cellular preparations consisting of numerous spindle cells with moderate to marked cellular and nuclear variability, including multinucleation.
On careful inspection, eosinophilic to purple granules may be seen in a few cells. There may also be a small amount of extracellular amorphous pink to eosinophilic material that resembles osteoid; interpreted to be collagen.
Fibrosarcomas are more common in cats than dogs. They can be associated with vaccine sites. The virally induced multicentric variety of FSA in young cats is rare. Vaccination related FSA can be associated with inflammation.
VACCINE INDUCED SARCOMA / VACCINE INDUCED “LESION”
These are recognized with increased frequency in cats. They are most commonly located at the base of the dorsum of the neck, between the shoulder blades or in a flank or rear limb or chest wall (vaccine sites). Lesions range from inflammatory to inflammation with “preneoplastic” changes; to suspect neoplasia; to full blown sarcoma or fibrosarcomas, with or without accompanying inflammation. When spindle cells with inflammatory cells are seen especially with extracellular, acellular amorphous pink material (globs? vaccine adjuvant? whatever) suspect a lesion of vaccine origin, with or without the historical information of location. Combining the presence of anaplastic spindle shaped cells, globs of extracellular amorphous material and location at the dorsum of the neck make this diagnosis easy.
“Early” lesions may only produce inflammatory cells of which neutrophils predominate but they will also be rich in lymphocytes, plasma cells, macrophages and occasionally eosinophils; possibly with a few multinucleated giant cells and reactive fibroblasts. The lesions seem to then proceed through stages in which the inflammatory components decrease and the neoplastic changes increase, culminating in a clearly recognizable sarcoma. At the latter stage the lesions exfoliate numerous cells with a wide variety of cellular and nuclear abnormalities.
Sometimes there are multinucleated giant cells with so many nuclei that they defy the pathologists ability to count them all. Even at this latter state some inflammatory cells, especially lymphocytes and macrophages remain. The extracellular amorphous pink to purple, acellular material is interesting and highly suggestive of these vaccine induced lesions. The composition of the extracellular material is unknown but it may be of adjuvant origin. It is nonbirefringent and it resembles ultrasound jelly.
Predicting biologic behavior is best done by recognizing what the lesion is, and then relying on published case series that report they are infiltrative, difficult to entirely excise and are notorious to recur but seldom metastasize to distant sites.
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