Clinicopathological features and differential diagnosis of chondrogenic tumours

Chondrogenic or cartilaginous tumours represent the most common bone tumours and can be diagnostically challenging especially on scarce biopsy material. As morphologically they may overlap, correlation between radiological and pathological findings and discussion in a multidisciplinary tumour board is mandatory for a final diagnosis. The aim of this review is to discuss clinical and pathological features that are important in the differential diagnosis of benign (chondromesenchymal hamartoma of chest wall, osteochondroma, enchondroma, periosteal chondroma) and intermediate (synovial chondromatosis, central or secondary peripheral atypical cartilaginous tumour) or malignant cartilaginous tumours (central or secondary peripheral chondrosarcoma grades 1–3, periosteal chondrosarcoma, dedifferentiated chondrosarcoma, clear cell chondrosarcoma and mesenchymal chondrosarcoma). The 5^th edition WHO classification proposed a new concept for cartilage tumours in which the term atypical cartilaginous tumour is used for low grade tumours located in the long and short tubular bones of the appendicular skeleton, as they behave as locally aggressive lesions and therefore should not be considered full blown malignant or treated aggressively. Histologically similar tumours in the axial skeleton (including pelvis, scapula and skull base) are now diagnosed as chondrosarcoma grade 1. Immunohistochemistry is of limited value in the distinction between the different cartilaginous tumours. Molecular diagnostics can be helpful when used on small biopsies to distinguish mesenchymal chondrosarcoma (with HEY1-NCOA2 fusion) from other undifferentiated round cell sarcomas, or dedifferentiated chondrosarcoma (often with IDH mutation) from other high grade sarcomas.