Differential Diagnosis of Cartilage-Forming Tumours

Classification Overview

BENIGN TUMOURS

1. Enchondroma

• Definition: Benign hyaline cartilage within the medullary cavity; the most common intramedullary cartilage tumour.
• Age/Sex: Any age; found incidentally.
• Location: Metaphysis of long bones (proximal femur, humerus, distal femur); very common in the small bones of the hand.
• Imaging: Intramedullary lytic lesion, often with stippled/mottled calcifications; no cortical destruction. Minimal or absent endosteal scalloping.
• Histology: Hypocellular; bland binucleate chondrocytes in lacunar spaces; no pleomorphism, anaplasia, or hyperchromasia. Nodule of hyaline cartilage encased by thin reactive bone.
• Key DDx point: Distinguished from low-grade chondrosarcoma by serial plain films - cortical erosion >50%, cortical thickening or destruction, or lysis of mineralised cartilage all suggest malignancy. Hand enchondromas may be hypercellular and pathological fracture is common - must not be over-called as chondrosarcoma.
• Syndromes: Multiple enchondromatosis = Ollier disease (malignant transformation ~25-30%); Ollier + soft tissue angiomas = Maffucci syndrome (malignant transformation up to 100%).

2. Osteochondroma (Exostosis)

• Definition: Benign surface lesion with a cartilage cap overlying a bony stalk; cortex and medullary canal of the lesion are continuous with the underlying bone.
• Age/Sex: Adolescents and young adults; slight male predominance.
• Location: Around the knee, proximal femur, proximal humerus; sites of tendon insertion.
• Imaging: Pedunculated or sessile surface lesion; stalk confluent with the intramedullary canal; cartilage cap normally 2-3 mm (up to 1-2 cm in a growing child).
• Histology: Chondrocytes arranged in linear clusters resembling the normal physis; mature bone stalk with benign cartilage cap undergoing endochondral ossification.
• Key DDx point: A cartilage cap >2 cm in an adult is suspicious for malignant transformation. Pain without mechanical cause warns of secondary chondrosarcoma. Destruction of subchondral bone, mineralisation of a soft tissue mass, and inhomogeneous appearance are radiographic red flags.
• DDx: Parosteal osteosarcoma, heterotopic ossification.
• Syndrome: Multiple Hereditary Exostoses (MHE) - autosomal dominant, EXT1/EXT2 mutations; 5% lifetime risk of secondary chondrosarcoma; osteochondromas tend to be sessile and large.

3. Chondroblastoma

• Definition: Benign cartilage tumour arising in the epiphysis.
• Age/Sex: Children and adolescents with open physes (10-25 years); male predominance.
• Location: Epiphysis of long bones - proximal humerus, distal femur, proximal tibia; also proximal femur (may extend to greater trochanter).
• Imaging: Lytic epiphyseal lesion, sharply marginated, may have surrounding sclerosis; "chicken wire" calcifications on histology; can have an associated aneurysmal bone cyst (ABC).
• Histology: Sheets of polyhedral chondroblasts with distinct cell membranes ("chicken wire" calcifications around cells); scattered multinucleated giant cells; zones of chondroid matrix; mitotic figures may be present.
• Key DDx: Brodie's abscess (osteomyelitis), GCT of bone (usually post-skeletal maturity), ABC. Fewer than 5% metastasize to lungs.
• Treatment: Intralesional curettage and reconstruction.

4. Chondromyxoid Fibroma (CMF)

• Definition: Rare benign cartilage tumour containing variable amounts of chondroid, fibromatoid, and myxoid elements.
• Age/Sex: Any age; usually second to fourth decades.
• Location: Pathognomonic location is the proximal tibia; also pelvis, distal femur. Eccentric, metaphyseal.
• Imaging: Lytic, eccentric, sharply demarcated lesion with sclerotic margins; may scallop the cortex.
• Histology: Grows in lobules; loose myxoid tissue in the centre of lobules; more cellular fibrous tissue at the periphery; background chondroid but distinct hyaline cartilage is rare. Atypical pleomorphic hyperchromatic nuclei can be present - must not be misdiagnosed as chondrosarcoma if the overall picture is consistent.
• Key DDx: Chondrosarcoma, chondroblastoma, fibrous dysplasia, nonossifying fibroma, GCT, ABC, simple bone cyst.
• Treatment: Extended curettage with bone grafting; local recurrence ~20%.

5. Periosteal (Juxtacortical) Chondroma

• Definition: Benign hyaline cartilage proliferation on the bone surface beneath the periosteum.
• Age/Sex: Young adults.
• Location: Surface of long bones, especially humerus and femur; 1-5 cm in size.
• Imaging: Surface erosion of the underlying cortex with a soft tissue mass; may have peripheral calcifications.
• Key DDx: Periosteal chondrosarcoma, periosteal osteosarcoma.

INTERMEDIATE (LOCALLY AGGRESSIVE) TUMOUR

6. Atypical Cartilaginous Tumour / Chondrosarcoma Grade 1 (ACT/CS1)

• Definition (2020 WHO terminology): Low-grade hyaline cartilage-producing neoplasm. Lesions in the appendicular skeleton (long and short tubular bones) = ACT; lesions in the axial skeleton (pelvis, scapula, skull base) = CS1 due to higher local recurrence risk and difficulty of resection.
• Age: 40-60 years.
• Imaging: Endosteal scalloping >50% cortical width, cortical thickening; mineralized matrix (ring-and-arc / flocculent calcifications); may be indistinguishable from enchondroma.
• Histology: Low cellularity; plump vesicular nuclei; small nucleoli; occasional binucleate cells; cortical permeation and entrapment of trabecular bone.
• Key DDx: Enchondroma (see above), higher-grade chondrosarcoma.
• Note: Rarely metastasizes; treatment has shifted from wide resection to curettage or even watchful waiting with MRI for appendicular ACT.

MALIGNANT TUMOURS

7. Conventional Chondrosarcoma (Grade 1-3)

• Definition: Malignant tumour producing hyaline cartilage; second most common malignant matrix-producing bone tumour (after osteosarcoma). ~90% are of conventional type.
• Age/Sex: >40-50 years; men 2x more affected.
• Location: Axial skeleton preferentially - pelvis, shoulder (proximal humerus), ribs. Contrasts with osteosarcoma, which tends to affect the metaphyses of long bones.
• Origin: Primary (de novo) or secondary (arising from pre-existing enchondroma or osteochondroma, ~15%).
• Imaging: Bone destruction, cortical expansion, flocculent calcifications; may form soft tissue mass; periosteal reaction in high-grade tumours.
• Histology:
  • Grade 1: Low cellularity, plump vesicular nuclei, small nucleoli.
  • Grade 2: Moderate cellularity, more atypia.
  • Grade 3: High cellularity, extreme pleomorphism, bizarre tumour giant cells, frequent mitoses.
  • Cartilage infiltrates marrow space, entraps normal bony trabeculae.
• IHC/Molecular: IDH1/IDH2 mutations (sporadic and chondromatosis-related); EXT1/EXT2 loss in MHE-related tumours; CDKN2A silencing by methylation in sporadic tumours.
• Key DDx criteria for malignancy:
  • Many binucleate cells with plump nuclei
  • Large cartilage cells with large single or multiple nuclei with nuclear clumps
  • Infiltration of bony trabeculae
  • Cortical destruction, endosteal scalloping >50%
• Treatment: Wide surgical resection; chemotherapy does not improve survival for conventional type.
• Prognosis: Grade 1: excellent; Grade 2-3: worse, with metastatic potential.

8. Dedifferentiated Chondrosarcoma

• Definition: Most malignant cartilage tumour; a high-grade non-cartilaginous sarcoma (osteosarcoma, MFH/UPS, fibrosarcoma, rhabdomyosarcoma) juxtaposed to a low-grade chondrosarcoma.
• Age: Usually >50 years.
• Location: Proximal and distal femur, proximal humerus.
• Imaging: Characteristic "bimorphic" pattern - typical chondrosarcoma with a superimposed, highly destructive lytic area within or adjacent to it.
• Histology: Two distinct components side by side: low-grade cartilage and high-grade spindle cell sarcoma; no cartilage in the dedifferentiated component.
• Prognosis: Very poor; 5-year survival <10%.

9. Mesenchymal Chondrosarcoma

• Definition: Rare, high-grade bimorphic tumour with islands of well-differentiated cartilage within a background of undifferentiated small round cells.
• Age: Young adults (second and third decades, younger than conventional chondrosarcoma).
• Location: Jaw, ribs, spine; can also occur in soft tissue.
• Histology: Hemangiopericytoma-like vascular pattern; HEY1-NCOA2 gene fusion (diagnostic).
• Prognosis: Aggressive; late recurrences can occur decades later.

10. Clear Cell Chondrosarcoma

• Definition: Low-grade malignant variant with distinctive clear cytoplasm.
• Age: Adults, usually 30-60 years; male predominance.
• Location: Epiphysis of long bones - most commonly proximal femur and proximal humerus (mimics chondroblastoma by location).
• Histology: Large cells with clear or pale cytoplasm, central nuclei, scattered osteoclast-type giant cells; intermixed mature bone trabeculae.
• Key DDx: Chondroblastoma (younger patient, open epiphysis), conventional chondrosarcoma, GCT, metastatic clear cell carcinoma (e.g. renal cell carcinoma).
• Prognosis: Indolent but can metastasize late; wide resection required.

Summary Comparison Table

Key Points for Differentiating Benign from Malignant Cartilage Tumours

1. Cortical changes - endosteal scalloping >50% cortical width, cortical destruction, or periosteal reaction strongly favour malignancy.
2. Site - axial skeleton lesions behave more aggressively than appendicular; epiphyseal location suggests chondroblastoma or clear cell CS.
3. Age - chondroblastoma in children/adolescents; conventional CS in >40-year-olds.
4. Growth - new pain in a previously known cartilage lesion, especially osteochondroma, raises concern for malignant transformation.
5. Histology alone is insufficient - the combination of clinical, radiographic, and histological features is required; phalangeal cartilage lesions can appear histologically identical to pelvic chondrosarcoma but have vastly different prognosis.
6. Molecular markers - IDH1/IDH2 mutations in conventional and atypical cartilaginous tumours; HEY1-NCOA2 fusion in mesenchymal chondrosarcoma; EXT1/EXT2 mutations in MHE-related tumours.