<unk> <unk>  is a rare type of<unk><unk><unk>  first described by H. L. Evans in 1987.<unk>  are soft tissue tumors of the mesenchyme-derived connective tissues; on microscopic examination, they are found to be composed of spindle-shaped cells that resemble fibroblasts. These fibroblastic, spindle-shaped cells are neoplastic cells that in most cases of<unk>  express fusion genes, i.e. genes composed of parts of two different genes that form as a result of mutations. The World Health Organization (2020) classified<unk>  as a specific type of tumor in the category of malignant fibroblastic and myofibroblastic tumors.<unk>  tumors occur in individuals of almost any age but up to 20% are less than 18 years/old. The tumors typically involve the proximal extremities but can occur virtually anywhere in the body. The progression of these tumors commonly takes an indolent, prolonged course involving many years or decades. Over this time, however, the tumors often recur at the site of their surgical removal and/or metastasize usually to the lung or pleural tissues surrounding the lungs. These metastasis can develop decades after the tumor's initial presentation and diagnosis.<unk>  can be difficult to distinguish from other mesenchymal tumors, particularly from sclerosing epithelioid fibrosarcoma (SEF).<unk>  tumors present with many clinical and pathological features that are similar to those in SEF. Indeed, current studies suggest that<unk>  may be an early form of SEF. For example, a tumor may present with features typical of<unk>  but over time progress to features typical of sclerosing epithelioid fibrosarcomas. This progression may be particularly evident in recurrent or metastatic<unk> " tumors. Since the World Health Organization has classified<unk>  as one of the malignant fibroblastic and myofibroblastic tumors that is distinctly different than SEF, SEF and<unk>  are here regarded as separate tumor forms.
Low-grade fibromyxoid sarcoma
Solitary fibrous tumor
Undifferentiated pleomorphic sarcoma
Ossifying fibromyxoid tumour
Sclerosing epithelioid fibrosarcoma
Collagenous fibroma
Mammary-type myofibroblastoma
Aggressive fibromatosis