Spindle cell and fibrohistiocytic soft tissue tumours of infancy and childhood

Spindle cell and fibrohistiocytic soft tissue tumours are a diverse group of neoplasms that are commonly seen in childhood. This paper provides an overview of the histopathological features of the more common entities that are likely to be encountered by general pathologists and paediatric pathologists in children and young adults. Fibroblastic and myofibroblastic tumours are characterized by the proliferation of fibroblasts or myofibroblasts, and they exhibit a wide range of histological appearances, from benign to malignant, and can occur in various anatomical locations. Nerve sheath tumours are derived from the Schwann cells of the peripheral nerves and are often associated with neurofibromatosis. Synovial sarcoma, a high-grade malignant tumour, is characterized by specific chromosomal translocations resulting in the SS18-SSX fusion gene. NTRK-rearranged tumours, a recently recognized entity, are characterized by gene fusions involving the neurotrophic tyrosine receptor kinase (NTRK) genes, leading to overexpression of the TRK protein. Fibrohistiocytic tumours of childhood, including fibrous histiocytoma and plexiform fibrohistiocytic tumour, show varying degrees of fibroblastic and histiocytic differentiation. They exhibit a wide range of clinical behaviour, from benign to locally aggressive. Understanding the histopathological features of these tumours is important to guide appropriate management. This review provides an overview of these features, along with the latest advances in molecular pathology, which can aid the pathologist in making a diagnosis.