General
- Mesenchymal chondrosarcoma is an exceptionally rare, malignant biphasic tumour.
- It is composed of islands of well-differentiated hyaline cartilage and undifferentiated small round, oval, or spindle-shaped cells.
- The tumour is genetically defined by the presence of a HEY1::NCOA2 gene fusion.
Definition
- Essential criteria include a poorly differentiated neoplasm consisting of small blue round cells with high N:C ratios and variable amounts of hyaline cartilage. AND
- In instances where cartilage is not present in the sample, the demonstration of the pathognomonic HEY1::NCOA2 fusion transcript is essential for diagnosis.
Numbers
- These tumours most commonly arise in the second and third decades of life.
- Cases have been documented in individuals at both ends of the age spectrum, including infants and older adults.
Grading
- Malignant no WHO grading
Histopathology
Macroscopic
- The tumours are typically lobular, firm, grey-brown masses.
- They frequently contain varying amounts of calcification.
Microscopic
- The hallmark feature is a biphasic pattern consisting of sheets and nests of malignant round cells with scant cytoplasm alongside islands of well-differentiated hyaline cartilage.
- Primary central nervous system examples often demonstrate more spindling and less necrosis than their musculoskeletal counterparts.
- A branching, staghorn-like vascular pattern reminiscent of solitary fibrous tumours is often present.
- Cartilaginous islands may show varying degrees of central calcification or enchondral ossification with an eosinophilic osteoid-like matrix.
Mesenchymal chondrosarcoma typically shows a biphasic pattern of small malignant cells with scant cytoplasm and islands of hyaline cartilage, both seen here.
Immunophenotype
- The primitive small round cells frequently express CD99, and may also show positivity for EMA, desmin, myogenin, MYOD1, and NKX3-1.
- The neoplastic cells are typically negative for GFAP, SMA, keratins, and ER.
- S100 and SOX10 expression can be identified in both components of the tumour.
- SMARCB1 (INI1) expression is preserved, while the Ki-67 labelling index is generally increased.
- CD99 expression in the poorly differentiated cells is typically diffuse and cytoplasmic.
Pathogenesis
- Nearly all cases of mesenchymal chondrosarcoma are driven by a highly specific HEY1::NCOA2 gene fusion.
Localisation
- Intracranial localisation is more frequent than intraspinal localisation.
- Favoured sites include the frontoparietal region and the thoracic spinal cord.
- Approximately 60 per cent of these tumours exhibit a dural attachment.
Clinical features
- Presenting symptoms are related to mass effect in intracranial cases.
- Intraspinal tumours typically cause symptoms of spinal cord compression.
Radiological features
- General
- Dural-based tumours can mimic the radiological appearance of meningiomas.
- CT
- Partially calcified mass.
- MRI
- MRI T2: the mass is hyperintense relative to adjacent muscle and may feature a very hyperintense necrotic centre.
- MRI + C: A ring of enhancement around a non-enhancing necrotic core.
- Ref: Lin 2012
Prognosis
- Intracranial mesenchymal chondrosarcomas are associated with a high rate of local recurrence.
- Rare instances of intracranial cases have shown leptomeningeal dissemination or metastasis outside the cranial vault.
- Spinal tumours generally show lower recurrence rates, with most patients surviving for more than 2 years after diagnosis.
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