Giant malignant fibrous histiocytoma of the pleura arising from solitary fibrous tumour

Undifferentiated and dedifferentiated soft tissue neoplasms: Immunohistochemical surrogates for differential diagnosis

Undifferentiated soft tissue sarcomas (USTS) are described in the current World Health Organization Classification of Soft Tissue and Bone Tumours as those showing no identifiable line of differentiation when analyzed by presently available technologies. This is a markedly heterogeneous group, and the diagnosis of USTS remains one of exclusion. USTS can be divided into four morphologic subgroups: pleomorphic, spindle cell, round cell and epithelioid undifferentiated sarcomas, with this combined group accounting for up to 20% of all soft tissue sarcomas. As molecular advances enable the stratification of emerging genetic subsets within USTS, particularly within undifferentiated round cell sarcomas, other groups, particularly the category of undifferentiated pleomorphic sarcomas (UPS), still remain difficult to substratify and represent heterogeneous collections of neoplasms often representing the common morphologic endpoints of a variety of malignant tumors of various (mesenchymal and non-mesenchymal) lineages. However, recent molecular developments have also enabled the identification and correct classification of many tumors from various lines of differentiation that would previously have been bracketed under 'UPS'. This includes pleomorphic neoplasms and dedifferentiated neoplasms (the latter typically manifesting with an undifferentiated pleomorphic morphology) of mesenchymal (e.g. solitary fibrous tumor and gastrointestinal stromal tumor) and non-mesenchymal (e.g. melanoma and carcinoma) origin. The precise categorization of 'pleomorphic' or 'undifferentiated' neoplasms is critical for prognostication, as, for example, dedifferentiated liposarcoma typically behaves less aggressively than other pleomorphic sarcomas, and for management, including the potential for targeted therapies based on underlying recurrent molecular features. In this review we focus on undifferentiated and dedifferentiated pleomorphic and spindle cell neoplasms, summarizing their key genetic, morphologic and immunophenotypic features in the routine diagnostic setting, and the use of immunohistochemistry in their principal differential diagnosis, and highlight new developments and entities in the group of undifferentiated and dedifferentiated soft tissue sarcomas.

The Current Status of Solitary Fibrous Tumor: Diagnostic Features, Variants, and Genetics

Solitary fibrous tumor (SFT) is a fibroblastic mesenchymal tumor originally described in the pleura but now shown at almost every anatomic site. Histopathologically, SFT is characteristically a circumscribed neoplasm composed of variably cellular and patternless distributions of bland spindle and ovoid cells within prominent collagenous stroma and shows diffuse expression of CD34, but it has a broad spectrum of both morphology and of biologic behavior. Many different names (particularly hemangiopericytoma) were previously used in the course of our understanding of this neoplasm but are now subsumed under the term "SFT," and the putative cell of origin was debated. However, it is now recognized that SFT is a translocation-associated neoplasm, consistently associated with NAB2-STAT6 gene fusions arising from recurrent intrachromosomal rearrangements on chromosome 12q, and this translocation is a likely major contributor to its pathogenesis. While most SFT with classical morphologic features behave in an indolent manner and those with overtly malignant histologic features tend to be aggressive neoplasms that behave as high-grade sarcomas, the behavior of SFT is unpredictable, and it is important to be aware of the propensity for aggressive behavior in a minority of histologically classical SFT and to ensure adequate clinical follow-up. Surgical excision remains the treatment gold standard; while radiotherapy and conventional chemotherapeutic agents have only shown limited efficacy, further understanding of the molecular events underlying tumorigenesis may allow the development of novel targeted treatments. We review SFT, discussing the morphologic spectrum and variants, including malignant and dedifferentiated subtypes, clinicopathological aspects, recent molecular genetic findings, and the differential diagnosis.