or adjacent structures [67]. The tumour affects young patients with a median of 25 years and a male predominance. A specific chromosomal translocation t(X;18)(p11;q11) that leads to the formation of the SS18-SSX fusion gene has been described in a subset of patients [68–70]. Histologically, synovial sarcoma may present as a pure spindle cell variant (monophasic) or biphasic when both spindle and epithelial components are present. The epithelioid component is typically formed of cuboidal or columnar epithelial cells forming cords, nests, and pseudoglandular spaces intermingled with a spindle cell proliferation. Although morphology is the cornerstone for diagnosis, ancillary markers, including keratin and epithelial membrane markers, can be used. In situ or PCR-based hybridisation for the t(X;18)(p11.2; q11;2) translocations may be of help, if positive [71, 72].
Differential Diagnosis
Spindle cell tumours including spindle cell carcinoma, fibrosarcoma, melanoma, and metastatic carcinoma to the base of skull are entities that may cause an initial diagnostic challenge, especially on small biopsy samples. The combined clinicopathologic and immunohistochemical marker features should be integrated in the diagnosis of this entity.
Angiosarcoma
Angiosarcoma of the skull base is exceedingly rare, and diagnosis is based on the identification of abnormal vascular proliferation and the invasive nature of tumour. Immunohistochemical positivity for ERG (nuclear expression) and CD31 confirms the vascular origin of the lesion. These markers will also be expressed by the vascular component of angiofibroma and haemangiopericytoma, although the associated stromal components will not stain [73–77].
Chondrosarcoma
Chondrosarcoma commonly develops from the nasal septum or nasoethmoidal complex, but may present as an extension from a maxillary primary. The tumour presents in different age groups, with the mesenchymal phenotype mainly affecting patients in the 2nd and 3rd decades of life [78]. The most common presenting symptoms are craniofacial bone expansion and pain, as well as nasal obstruction and orbital content displacement. Grossly, these tumours manifest translucent and cartilaginous features with scattered calcifications. Myxomatous areas with lobulation are commonly seen. The histologic spectrum seen in these tumours ranges from benign-appearing hyaline cartilaginous lesions to highly cellular malignant spindle cell sarcoma [79–81]. Malignant chondrocytic cells, a defining feature of these tumours, must be identified. The lesion can manifest as myxoid, clear cell, dedifferentiated, and mesenchymal phenotypes. Mesenchymal chondrosarcoma is rare and may cause differential diagnostic difficulties. These tumours are composed of highly cellular spindle cell proliferations in interlacing short fascicles with focal cartilaginous formations.
Differential Diagnosis
The main differential diagnoses include enchondroma, osteochondromas, chondroblastic osteosarcoma, and chordomas. Mesenchymal chondrosarcoma should be differentiated from other malignant spindle cell tumours. CD99 and SOX9 markers are typically positive in mesenchymal chondrosarcoma and can be used in the diagnosis of this entity.
Osteosarcoma
Osteosarcoma is a rare tumour in the skull base region, which most commonly represents an extension of maxillary origin. Grossly, osteosarcoma manifests as an ill-defined, irregular, and tan-yellow tissue mass with gritty (bone) sensation. Histologically, these tumours exhibit malignant cellular proliferation with osteoid bone formation. The degree of cellularity and anaplastic cellular features reflect the grade of the lesions. The most common type of osteosarcoma is the osteogenic phenotype [81, 82]. Chrondroblastic and fibroblastic variants are also encountered.
Differential Diagnosis
The differentiation of this entity from other bone-forming lesions, including osteoblastoma requires combined radiologic and histopathologic evaluation.
Biphenotypic Sinonasal Sarcoma
This is a recently described low-grade spindle cell sarcoma, which occurs in the nasal cavity and ethmoid [83, 84]. Histologically, the tumour displays spindle cell proliferation with intersecting fascicles and a herringbone pattern. Similar to synovial sarcoma, markers that may be used to assist diagnosis are S-100 and SMA. Biphenotypic spindle cell sarcoma is characterised by distinctive phenotypic features and the presence of translocation t(2;4)(q35;q31.1) PAX3-MAML3 gene fusion. The cardinal morphologic features include neural and myogenic differentiation. This is a low-grade malignancy prone to local recurrence, but no reports of metastasis or death due to disease have been published [85–88].
Chordoma
Chordoma is a low- to intermediate-grade malignant neoplasm originating from the notochord. Base of skull involvement occurs through the sphenoid occipital region, which is the site for approximately 20% of all chordomas. Chordomas of the head and neck region frequently affect patients in the 5th and 6th decades of life [89–93]. Patients typically present with neurological symptoms, headache, and progressive pain. Grossly, chordoma characteristically presents as a lobulated myxoid, expansive mass with mucous, and a slippery appearance. Histologically, chordoma is divided into classic, chondroid, and dedifferentiated phenotypes. The classic type is characterised by a lobulated growth pattern with cords and islands of polygonal and vacuolated cells in a myxomucoid background. The characteristic vacuolated and eosinophilic cell is called a physaliphorous cell. The chondroid type manifests the same features along with areas of hyaline cartilaginous tissue. Dedifferentiation signifying transformation to a high-grade sarcoma shows marked cytologic atypia and high cellularity.
Differential Diagnosis
Chordoma should be differentiated from mucinous adenocarcinoma, myxoma, and cartilaginous neoplasms. The use of immunohistochemical stains will typically aid in the diagnosis. In particular, the reactivity to keratin and EMA are helpful in the exclusion of chondrosarcoma. Chordoma is characteristically immunoreactive to cytokeratin and S-100.
Soft Tissue Tumours: Benign and Borderline Tumours
Fibromatosis
Fibromatosis is a histologically ill-defined benign proliferation of bland spindle cells and collagenous stroma. This entity is slowly
