[3] The histology, association with the ETV6-NRTK3 fusion gene as well as certain chromosome trisomies, and the distribution of markers for cell type (i.e. cyclin D1 and Beta-catenin) within this tumor are similar to those found in the cellular form of mesoblastic nephroma.

Depending on this differentiation, tumour cells may resemble mature fibroblasts (spindle-shaped), secreting collagen, with rare mitoses.

These cells are arranged in short fascicles which split and merge, giving the appearance of "fish bone" known as a herringbone pattern.

[citation needed] Ancillary testing for fibrosarcoma includes IHC, where vimentin is positive, cytokeratin and S100 are negative, and actin is variable.

[10] Bostock DE, et al. performed a study of cats that had fibrosarcomas excised and were followed for a minimum of three years, or until death.