Primary bone tumours, staging (Enneking), biopsy principles, limb salvage vs amputation, common tumours by age

Overview

• One of the most clinically useful diagnostic heuristics in musculoskeletal oncology is the patient's age at presentation.
• While no tumour is absolutely restricted to a given decade, the probability distribution of primary bone tumours is strongly age-dependent, and integrating age with anatomical location, radiological pattern, and histological findings is the foundation of accurate diagnosis.
• Pain is the most common presenting symptom but is non-specific.
• Progressive, unremitting, nocturnal, or rest pain is sinister.
• Some lesions remain entirely silent until discovered incidentally or after pathological fracture.
• Under age 20, benign bone tumours and cysts are the commonest cause of pathological fracture; over age 40, myeloma and metastatic disease dominate.

Age-Group Framework

Childhood (0-10 Years)

Simple (Unicameral) Bone Cyst

• Radiological features: Central, well-defined, unilocular lytic lesion within the metaphysis, most commonly the proximal humerus or proximal femur.
• The cortex is thinned but intact; the lesion does not cross the growth plate.
• The fallen fragment sign, a cortical fragment sinking to the dependent portion of the cyst after pathological fracture, is pathognomonic.
• Histological features: The wall is lined by a thin fibrous membrane; fluid content is straw-coloured or blood-stained.
• No epithelial lining is present.
• Histologically a reactive rather than truly neoplastic lesion: fibrous tissue, scattered giant cells, and haemosiderin deposits.

Eosinophilic Granuloma (Langerhans Cell Histiocytosis, Bone)

• Radiological features: A well-demarcated oval area of radiolucency within marrow-containing bone, flat bones, vertebral bodies, and metaphyses of long bones.
• Multiple skull lesions produce a characteristic punched-out appearance.
• Vertebral body involvement causes vertebra plana (near-complete collapse with preservation of disc height), a highly specific finding.
• Periosteal reaction may mimic Ewing sarcoma.
• Histological features: Sheets of Langerhans cells (large cells with reniform/coffee-bean nuclei and abundant eosinophilic cytoplasm) admixed with eosinophils, lymphocytes, plasma cells, and multinucleate giant cells.
• Immunohistochemistry: CD1a+ and S-100+.

Adolescence and Young Adulthood (10-25 Years)

Osteosarcoma (Conventional Intramedullary)

• The commonest primary malignant bone tumour in this age group; second commonest primary malignant bone tumour overall.
• Predilection for the metaphysis of long bones, distal femur, proximal tibia, and proximal humerus.
• Epidemiological data suggests the peak age of presentation may be somewhat older than the classic teaching of early adolescence, but it remains predominantly a tumour of the first two to three decades.

Radiological features:

• Pulmonary metastases are present at diagnosis in approximately 15-20% of cases; staging CT chest is mandatory.
• Skip lesions within the same bone must be sought on MRI.
• Histological features: The defining criterion is osteoid produced directly by malignant stromal cells.
• The background is highly pleomorphic.
• Multiple samples may be required, as pathologists require osteoid formation to confirm the diagnosis.

Three principal subtypes:

High mitotic activity, nuclear pleomorphism, and atypical mitoses are universal.

• Treatment context: Multi-agent neoadjuvant chemotherapy (methotrexate, doxorubicin, cisplatin, cyclophosphamide, vincristine) is given for 8-12 weeks preoperatively.
• Tumour necrosis >90% in the resected specimen indicates a good response.
• Long-term disease-free survival approaches 60% in optimal series.

Parosteal Osteosarcoma

• A low-grade variant arising on the outer cortical surface, most commonly the posterior distal femur.
• Presents in slightly older patients (20s-30s).
• Radiologically: a densely sclerotic, lobulated mass attached to the posterior cortex; CT demonstrates a cleavage plane between tumour and cortex.
• Histologically: well-differentiated bony trabeculae within a spindle-cell stroma with infrequent mitoses, frequently misinterpreted as benign.
• Classified as low-grade (Enneking Stage IA/IB); wide excision without neoadjuvant chemotherapy is the standard.

Ewing Sarcoma

• The second commonest primary malignant bone tumour in children and adolescents.
• Occurs predominantly in the first two decades; rare after age 30.
• Unlike osteosarcoma, Ewing sarcoma preferentially involves the diaphysis of long bones and flat/irregular bones (pelvis, ribs, scapula).

Radiological features:

Periosteal reaction and systemic features may closely mimic osteomyelitis, a critical differential in this age group.

• Histological features: Sheets of small, round, blue cells with scant cytoplasm and monotonous round nuclei, densely packed.
• PAS-positive, diastase-sensitive glycogen granules within the cytoplasm.
• Characteristic molecular finding: $t(11;22)(q24;q12)$ producing the EWS-FLI1 fusion gene, identified by FISH or RT-PCR.
• Immunohistochemistry: strongly CD99 (MIC2)+ and vimentin+.
• Treatment context: Multi-agent chemotherapy combined with surgery (or radiotherapy for unresectable/pelvic lesions).
• Long-term disease-free survival approximately 60% in optimal series.

Osteoid Osteoma

• Predominantly age 10-25; 2:1 male predominance.
• Classically presents with nocturnal pain dramatically relieved by aspirin/NSAIDs.
• Radiological features: A small (<2 cm) radiolucent nidus surrounded by a zone of dense reactive cortical sclerosis.
• In cortical bone (proximal femoral neck, tibial diaphysis), sclerosis may be so exuberant that the nidus is invisible on plain radiograph, CT is the investigation of choice.
• In cancellous bone, surrounding sclerosis is less prominent.
• Histological features: The nidus consists of a vascular fibrous stroma with disorganised woven bone trabeculae lined by osteoblasts and osteoclasts.
• Surrounding reactive bone is lamellar and hypervascular.
• Prostaglandin E2 production by the nidus explains both the pain and NSAID responsiveness.
• Classified as Enneking benign Stage 1 (latent).

Osteochondroma (Osteocartilaginous Exostosis)

• The commonest benign bone tumour overall.
• Arises from the metaphysis, pointing away from the adjacent joint; cortical and medullary continuity between lesion and host bone is pathognomonic.
• Radiological features: Pedunculated or sessile bony projection.
• The cartilage cap is visible on MRI.
• Cap thickness >2 cm in a skeletally mature patient raises concern for malignant transformation to secondary chondrosarcoma (a low-grade, slow-growing malignancy).
• Histological features: The cap is hyaline cartilage with a growth-plate-like architecture; beneath lies endochondral ossification, cancellous bone, and a fatty marrow core.
• Malignant transformation features: increased cellularity, nuclear atypia, myxoid change, and cap thickness >2 cm.

Aneurysmal Bone Cyst (ABC)

• Predominantly first two decades.
• Located eccentrically in the metaphysis; may extend to the epiphysis.
• Radiological features: Expansile, multiloculated lytic lesion with a thin cortical shell ("blown-out" appearance).
• Fluid-fluid levels on MRI are characteristic (layering of blood products of differing ages).
• May arise secondary to another lesion, giant cell tumour, chondroblastoma, or telangiectatic osteosarcoma (the last being a critical and dangerous differential).
• Classified as Enneking benign Stage 2 (active).
• Histological features: Blood-filled spaces lined by fibrous septa containing spindle cells, osteoclast-type giant cells, reactive woven bone, and haemosiderin deposits.
• No endothelial lining.
• A subset carries a USP6 gene rearrangement (supports primary ABC).
• Absence of endothelial lining distinguishes ABC from a vascular malformation.

Young to Middle Adult (25-50 Years)

Giant Cell Tumour (GCT) of Bone

• Rare before physeal closure and after age 55; presents in the third and fourth decades.
• Characteristically located in the epiphysis of long bones, distal femur, proximal tibia, distal radius, and sacrum.
• Enneking benign Stage 3 (aggressive).

Radiological features:

Approximately 1-2% of GCTs produce benign pulmonary implants (not true metastases in the conventional sense); ~1% undergo malignant transformation to giant cell sarcoma, which carries a high metastatic risk.

• Histological features: A highly cellular stroma of mononuclear cells (oval to spindle-shaped) with uniformly distributed osteoclast-type multinucleate giant cells.
• The key diagnostic feature is that the nuclei of the giant cells are morphologically identical to the mononuclear stromal cell nuclei.
• Mitoses may be numerous but are not atypical.
• Haemosiderin and areas of necrosis are common.
• Treatment context: Intralesional curettage with adjuvant (phenol, hydrogen peroxide, liquid nitrogen) and bone grafting/cementation for most lesions; wide excision for recurrent or aggressive (Stage 3) tumours.
• Denosumab (anti-RANKL) is used for unresectable or recurrent GCT.

Fibrosarcoma of Bone

• Rare; more likely to arise in previously abnormal tissue (bone infarct, fibrous dysplasia, post-irradiation).
• Usually an adult presentation (may extend into the >50 group).
• Radiologically: non-specific lytic bone destruction without distinctive matrix.
• Histologically: masses of fibroblastic tissue with atypical and mitotic cells; grade varies from well-differentiated to highly undifferentiated.
• Low-grade (Stage IA) lesions may be treated by wide excision; high-grade lesions (IIA/IIB) require radical resection or amputation.

Chondrosarcoma (Primary, Low Grade, Onset ~35-50 yrs)

(See also Older Adult section for high-grade disease.)

• Chondrosarcoma has a predilection for the axial skeleton (pelvis, ribs, sternum) and proximal long bones (proximal femur, proximal humerus).
• Most chondrosarcomas are slow-growing and metastasise late, the ideal case for wide excision with prosthetic replacement when achievable.
• Radiological features: Endosteal scalloping, cortical thickening, and intralesional mineralisation in the form of rings and arcs (stippled/lobular calcification, hallmark of cartilage matrix).
• Endosteal scalloping exceeding two-thirds of cortical thickness is a threshold for malignant concern.
• CT and MRI should be performed before biopsy.
• Histological features: Lobules of hyaline cartilage with variable cellularity.
• Low-grade tumours may be virtually indistinguishable from enchondroma, distinction requires clinical and radiological correlation.
• Higher-grade tumours show increased cellularity, binucleate cells, nuclear pleomorphism, hyperchromatism, and mitotic figures.
• Chondrosarcoma does not respond to conventional chemotherapy or radiotherapy; surgery is the only effective modality.
• Follow-up should extend to 10 years or longer due to propensity for late recurrence.

Histological variants:

• Dedifferentiated chondrosarcoma: A high-grade sarcoma (osteosarcoma or UPS) arising abruptly adjacent to a low-grade chondrosarcoma; very aggressive.
• Mesenchymal chondrosarcoma: Occurs in younger individuals; ~50% arise in soft tissues. Histology shows a bimorphic pattern, small round mesenchymal cells and islands of chondroid tissue. Clinical behaviour is more aggressive than conventional chondrosarcoma.
• Clear cell chondrosarcoma: Low-grade; epiphyseal location; may mimic chondroblastoma.

Older Adult (>50 Years)

Multiple Myeloma / Plasmacytoma

• The commonest primary malignant bone tumour in adults overall.
• Seldom seen before the sixth decade.
• Radiological features: Multiple punched-out lytic lesions throughout the axial skeleton and proximal long bones, skull (raindrop skull), spine, pelvis, ribs.
• Purely lytic with no sclerotic margin and no periosteal reaction.
• Bone scintigraphy is frequently falsely negative (absent osteoblastic activity), skeletal survey or whole-body MRI is preferred for staging.
• Histological features: Dense sheets of plasma cells with eccentric nuclei, clock-face (cartwheel) chromatin, and perinuclear hof.
• Well-differentiated plasmacytoma: mature plasma cells.
• Poorly differentiated myeloma: pleomorphic plasmablasts.
• Immunohistochemistry: CD138+, MUM1+, monoclonal immunoglobulin light chain restriction (kappa or lambda).

Undifferentiated Pleomorphic Sarcoma (UPS) / Malignant Fibrous Histiocytoma (MFH) of Bone

Note: The term MFH has been superseded by UPS in current WHO Classification of Soft Tissue and Bone Tumours (5th edition, 2020); both terms remain in clinical use.

• Predominantly middle-aged to elderly adults.
• Associated with pre-existing bone abnormality, medullary infarct, Paget's disease, prior irradiation, or fibrous dysplasia.
• Radiological features: Aggressive, poorly marginated lytic destruction; no specific matrix mineralisation.
• Often presents with or adjacent to an area of medullary infarction.
• Pathological fracture is common.
• Soft-tissue extension frequent.
• Histological features: Pleomorphic spindle cells arranged in a storiform (cartwheel) pattern with abundant giant cells, atypical mitoses, and inflammatory infiltrate.
• Diagnosis of exclusion, no osteoid, chondroid, or specific line of differentiation identified.
• Requires wide/radical resection; chemotherapy role is uncertain.

Summary Table: Characteristic Features by Tumour

Radiological Approach to Primary Bone Tumours

Systematic analysis should address:

1. Patient age and tumour location (epiphysis, metaphysis, diaphysis)
2. Zone of transition: narrow (benign) vs. wide (aggressive/malignant)
3. Bone destruction pattern: geographic, moth-eaten, permeative
4. Matrix mineralisation: osteoid (cloud-like density), chondroid (rings and arcs), no matrix
5. Periosteal reaction: Codman triangle, onion-skin, sunburst, solid/undulating
6. Cortical integrity and soft-tissue extension
7. MRI: intramedullary extent, skip lesions, neurovascular and articular relationships, compartmental status (essential for Enneking staging); perform before biopsy
8. CT chest: pulmonary metastasis staging for all suspected malignant lesions

Critical principle: Complete imaging (MRI and CT) must be performed before biopsy, as biopsy alters MRI/CT appearances and may compromise staging accuracy.

Paediatric Considerations

• In children, the periosteum is thicker and more biologically active, producing more exuberant periosteal reactions.
• Osteomyelitis is a critical differential for Ewing sarcoma and must always be excluded.
• Growth plate involvement has implications for limb length discrepancy and growth arrest following surgery or radiotherapy.
• Pathological fracture through a lesion in a child under 20 most commonly indicates a benign lesion (simple bone cyst, non-ossifying fibroma, aneurysmal bone cyst), though Ewing sarcoma and osteosarcoma must not be excluded without adequate investigation.

Physiological variants, normal distal femoral cortical irregularity, nutrient foramina, Caffey disease, must not be mistaken for tumours.

Key Examination Points

• Age is the single most important clinical filter; integrate with anatomical location and radiological pattern before histological diagnosis
• The triad of small round blue cells + CD99 positivity + EWS-FLI1 translocation $t(11;22)(q24;q12)$ confirms Ewing sarcoma
• Osteoid produced by malignant cells is mandatory for diagnosis of osteosarcoma, cartilaginous or fibroblastic areas alone are insufficient; multiple samples may be needed
• GCT diagnostic hallmark: giant-cell nuclei morphologically identical to the mononuclear stromal cell nuclei; mitoses present but not atypical
• Chondrosarcoma does not respond to chemotherapy or radiotherapy; surgery (wide excision) is the only effective modality; follow-up for ≥10 years
• Myeloma bone scan is frequently negative; skeletal survey or whole-body MRI are preferred staging investigations
• Fluid-fluid levels on MRI are characteristic of ABC but also occur in telangiectatic osteosarcoma, a dangerous diagnostic pitfall
• Enneking Stage 3 benign (aggressive) applies to GCT: growth not limited by natural barriers, tendency to recur, risk of soft-tissue extension
• Fibrosarcoma and MFH/UPS arising in bone are associated with pre-existing bone pathology (infarct, Paget's disease, irradiation, fibrous dysplasia), always seek an underlying cause
• Myeloma is the commonest primary malignant bone tumour overall; osteosarcoma is the commonest primary malignant bone tumour of childhood/adolescence