Bone tumors; classification, osteo- genic sarcoma pathology 2nd year
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Excellent. Let me now read the most relevant sources in parallel - Robbins Pathology for osteosarcoma pathology, Sabiston for bone tumor classification, and Miller's for a concise orthopaedic overview.
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osteosarcoma bone tumor histology Codman triangle radiograph
This is a plain radiograph (X-ray) of the femur demonstrating malignant transformation of Paget disease into osteosarcoma in the proximal femur. The image exhibits an aggressive, poorly demarcated lytic-destructive lesion with a permeative pattern, extensive cortical destruction, and a bulky soft tissue mass. Classic Paget disease changes are overlaid by malignant osteoid-producing tumor features, including sunburst-like periosteal reaction and Codman triangle, with a wide zone of transition. The lesion may be associated with a pathologic fracture and cortical erosion. The radiographic pattern aligns with high-grade osteosarcoma in the setting of pagetic bone; imaging may reveal mineralization within the tumor and strong aggression into surrounding soft tissues. Bone scans typically show reduced isotope uptake in lytic Pagetic regions, whereas MRI/CT could better delineate medullary involvement and soft-tissue extent. This case, attributed to The Radswiki, Radiopaedia.org (case rID: 11727), highlights the need for biopsy for definitive histology and staging. Clinically, older patients with longstanding Paget disease presenting with new, persistent pain or fracture should raise suspicion for malignant transformation to osteosarcoma; management involves oncologic assessment, surgical planning, and systemic therapy as indicated. The image is valuable for radiology education and differential diagnosis training in musculoskeletal oncology, Paget disease, and bone tumors. Utility.
Anterior-Posterior (AP) radiograph of the right humerus demonstrating an aggressive, primary bone tumor, consistent with osteosarcoma. The lesion is primarily centered in the proximal metaphyseal region with significant extension into the diaphysis. Key diagnostic features include an aggressive periosteal reaction characterized by a Codman triangle (indicated by a red arrow), where the periosteum is being lifted away from the bone by the rapidly growing tumor. Additionally, there is evidence of osteoid production resulting in a 'cloud-like' or fluffy ivory-white appearance of tumor bone formation within the soft tissue. The zone of transition between the malignant process and normal bone is wide and ill-defined, signifying high biological activity and infiltrative growth. The image serves as a classic educational example of malignant orthopedic oncology findings in a pediatric patient.
This composite diagnostic image illustrates a primary osteosarcoma in the left proximal fibula of a 10-year-old patient across four imaging modalities. (A) Plain radiograph demonstrates 'cloud-like' osteogenic destruction of the medullary and cortical bone, a suspicious soft tissue mass, and a Codman triangle, indicating aggressive periosteal reaction. (B) Transverse and sagittal CT scans provide high-resolution detail of the mineralized matrix and cortical destruction, including a visible lamellate periosteal reaction. (C) T1-weighted MRI (axial and coronal) shows the tumor with low signal intensity in the mineralized medullary compartment and intermediate signal in the non-mineralized soft-tissue component. (D) T2-weighted MRI (axial and coronal) highlights the heterogeneity of the lesion; the mineralized areas remain low signal, while non-mineralized soft tissue shows medium-high signal intensity, accompanied by high signal intensity peritumoral edema. These modalities collectively facilitate the assessment of tumor mineralization, soft tissue extension, and involvement of surrounding neurovascular structures for surgical planning.
Plain Radiography of a long bone diaphysis demonstrates an aggressive, ill-defined osteolytic lesion with cortical destruction and periosteal reaction. The radiograph shows a moth-eaten destruction pattern with diffuse bone loss extending along a substantial portion of the shaft. Periosteal elevation produces Codman triangle in the swept cortex, while layered deposition of reactive new bone beneath the periosteum can create an onion-skin appearance; in some cases perpendicular radiating spicules or sunburst-like projections may be seen. The lesion often breaches the cortex or expands to form a soft tissue mass that extends beyond the bone. Although ultrasound or CT can delineate the soft tissue component, CT and MRI best characterize the extent, relation to surrounding structures, and marrow involvement. The radiographic pattern is highly suggestive of an aggressive small round blue cell tumor, most notably Ewing sarcoma, particularly in children and adolescents. Differential considerations include osteosarcoma, osteomyelitis, lymphoma, metastasis, or eosinophilic granuloma, but the combination of an diaphyseal, permeative lesion with onion-skin periosteal reaction strongly favors Ewing. Clinical correlation with pain, fever, raised inflammatory markers, and prompt biopsy is essential to confirm diagnosis and guide treatment, which typically combines chemotherapy, limb-sparing surgery, and possibly radiotherapy.
Now let me get the bone tumor classification table from Robbins and the osteosarcoma histology image.
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Bone Tumors: Classification & Osteogenic Sarcoma Pathology
(Based on Robbins & Kumar Basic Pathology, Rheumatology 2-Vol Set, Miller's Review of Orthopaedics, and Campbell's Operative Orthopaedics)
1. Classification of Primary Bone Tumors
Bone tumors are classified by the tissue of origin (histogenetic type) and behavior (benign vs. malignant). The WHO recognizes >60 subtypes; the table below covers the major categories tested at 2nd year.
| Category | Behavior | Tumor Type | Common Location | Age (yr) | Morphology |
|---|---|---|---|---|---|
| Cartilage-forming | Benign | Osteochondroma | Metaphysis of long bones | 10-30 | Bony excrescence with cartilage cap |
| Benign | Chondroma (Enchondroma) | Small bones of hands/feet | 30-50 | Intramedullary hyaline cartilage nodule | |
| Malignant | Chondrosarcoma | Pelvis, shoulder | 40-60 | Extends from medullary canal through cortex; chondrocytes with atypia | |
| Bone-forming | Benign | Osteoid osteoma | Metaphysis of long bones | 10-20 | Cortical; interlacing microtrabeculae of woven bone |
| Benign | Osteoblastoma | Vertebral column | 10-20 | Posterior vertebral elements; histology similar to osteoid osteoma | |
| Malignant | Osteosarcoma | Metaphysis of distal femur, proximal tibia | 10-20 | Extends from medullary canal to lift periosteum; malignant cells producing woven bone | |
| Unknown origin | Benign | Giant cell tumor (GCT) | Epiphysis of long bones | 20-40 | Destroys medullary canal and cortex; sheets of osteoclast-like giant cells |
| Benign | Aneurysmal bone cyst | Proximal tibia, distal femur, vertebra | 10-20 | Hemorrhagic spaces separated by cellular fibrous septa | |
| Malignant | Ewing sarcoma | Diaphysis of long bones | 10-20 | Sheets of primitive small round cells; t(11;22) EWS-FLI1 fusion |
(Adapted from Robbins & Kumar Basic Pathology, Table 19.1)
Additional Clinical Notes on Classification
- Metastatic carcinoma is far more common in bone than primary tumors; the "BLT with a kosher pickle" mnemonic covers the 5 most common primary sources - Breast, Lung, Thyroid, Kidney (renal), Prostate.
- In patients >50 years with a destructive bone lesion and no known primary tumor, metastatic disease must be the first consideration.
- Primary malignant bone neoplasms are rare (<0.2% of all neoplasms) with a bimodal age distribution.
- Secondary osteosarcoma occurs in older adults in the setting of Paget disease, bone infarcts, or prior radiation.
2. Osteogenic Sarcoma (Osteosarcoma) - Detailed Pathology
Definition
Osteosarcoma is a malignant tumor that produces osteoid matrix or mineralized bone. It is the most common primary malignant bone tumor (excluding hematopoietic tumors).
Epidemiology
| Feature | Details |
|---|---|
| Incidence | 1-1.5 per million population |
| Age | Bimodal: ~75% before age 20 (adolescent growth spurt); second peak in older adults (secondary type) |
| Sex | Males > Females (1.6:1) |
| Most common site | Metaphysis of long bones; ~50% near the knee (distal femur or proximal tibia) |
Pathogenesis
The peak incidence coincides with the adolescent growth spurt - the tumor arises most often near the growth plate where rapid proliferation predisposes to oncogenic mutations. Key molecular abnormalities:
| Gene | Relevance |
|---|---|
| RB (retinoblastoma) | Mutated in up to 70% of sporadic cases; germline RB mutation increases risk 1000-fold |
| TP53 | Germline mutation in Li-Fraumeni syndrome greatly increases osteosarcoma incidence; common in sporadic tumors |
| MDM2 / CDK4 | Overexpressed in many low-grade osteosarcomas (inhibit p53 and RB function) |
| CDKN2A (INK4a/p16/p14) | Inactivated in many osteosarcomas |
| MYC | Amplified in up to 50% of cases; associated with poor prognosis |
No specific chromosomal translocation (unlike Ewing sarcoma which has t(11;22)). Instead, osteosarcoma shows complex karyotypes with numerous chromosomal aberrations.
Gross Pathology (Macroscopy)
Osteosarcomas are bulky, gritty, tan-white tumors with areas of hemorrhage. Key gross features:
- Destroys surrounding cortices and invades soft tissue
- Extensive intramedullary spread replaces the marrow
- Infrequently, the tumor penetrates the epiphyseal plate and enters the adjacent joint
- Confined to the metaphyseal side of the cartilaginous growth plate
Gross specimen: Osteosarcoma of distal femur with cortical disruption and hemorrhagic areas. Tumor is confined to the metaphyseal side of the growth plate. (Robbins & Kumar, Fig. 19.16)
Histology (Microscopy)
Diagnostic criterion: Presence of malignant tumor cells producing unmineralized osteoid or mineralized bone - this is the pathognomonic hallmark.
Microscopic features include:
- Cellular pleomorphism - large hyperchromatic nuclei
- Bizarre tumor giant cells
- Abundant mitoses, including abnormal (e.g., bipolar) forms
- Extensive necrosis
- Intravascular invasion
- Lacelike osteoid produced by pleomorphic malignant cells (fine and lacelike pattern, or broad sheets/primitive trabeculae)
- Bridges preexisting lamellar bone
H&E: Lacelike osteoid produced by pleomorphic malignant tumor cells bridges preexisting lamellar bone. Arrow = abnormal mitotic figure. (Robbins & Kumar, Fig. 19.17)
Radiological Features
On plain X-ray, osteosarcoma presents as a large, destructive, mixed lytic and sclerotic mass with infiltrative margins.
Key radiological signs:
| Sign | Description |
|---|---|
| Codman triangle | Tumor lifts the periosteum, resulting in a triangular wedge of reactive subperiosteal bone. Indicates aggressive tumor but is NOT pathognomonic of osteosarcoma |
| Sunburst pattern | Perpendicular periosteal spicules radiating outward |
| Mixed lytic/sclerotic | Combined bone destruction and tumor bone formation |
| Soft tissue mass | Extension beyond cortex into surrounding soft tissue |
X-ray: Distal femur osteosarcoma. Mixed lytic-sclerotic mass with Codman triangle (arrow) - triangular shell of reactive subperiosteal bone. (Robbins & Kumar, Fig. 19.15)
Here is a multi-modal imaging example from a real case:
Multi-modal imaging: Plain X-ray shows cloud-like osteogenic destruction with Codman triangle; CT delineates mineralized matrix; MRI shows medullary and soft-tissue extent.
Clinical Features
- Presents as painful, progressively enlarging mass
- Pathologic fracture may be the presenting complaint
- All osteosarcomas are assumed to have occult metastases at diagnosis
- Metastasizes hematogenously to lungs (most common), bones, brain, and other sites
Treatment and Prognosis
| Component | Details |
|---|---|
| Neoadjuvant chemotherapy | Given before surgery to reduce tumor bulk and treat occult metastases |
| Surgery | Wide surgical resection (limb-sparing preferred over amputation where possible) |
| Adjuvant chemotherapy | Post-operative |
| 5-year survival | ~70% in patients without overt metastases at initial diagnosis |
| With metastases | Prognosis is significantly worse |
3. Quick Comparison: Key Malignant Bone Tumors
| Feature | Osteosarcoma | Ewing Sarcoma | Chondrosarcoma |
|---|---|---|---|
| Age | 10-20 yr | 5-20 yr | 40-60 yr |
| Location | Metaphysis | Diaphysis | Pelvis, shoulder |
| Genetics | RB, TP53 mutations; complex karyotype | t(11;22) EWS-FLI1 | No specific translocation |
| X-ray | Mixed lytic/sclerotic, Codman triangle, sunburst | Onion-skin periosteal reaction | Stippled/ring calcifications |
| Histology | Malignant cells + osteoid production | Small round blue cells | Malignant chondrocytes in cartilage matrix |
| Chemo response | Good (70% survival) | Good - multiagent chemo curative | Resistant to chemo |
Sources:
- Robbins & Kumar Basic Pathology, Table 19.1, pp. 782-783
- Rheumatology 2-Volume Set (2022, Elsevier), Primary Bone Tumors, p. 3062-3087
- Miller's Review of Orthopaedics, 9th Ed., pp. 10111-10161
- Campbell's Operative Orthopaedics, 15th Ed. 2026
Bone tumours complete pathology
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Now reading all the major Robbins sections in parallel:
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Excellent content. Now let me read the chondrosarcoma and GCT sections from Robbins, plus get all the key images:
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bone tumors histology osteosarcoma chondrosarcoma Ewing sarcoma giant cell tumor
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I now have comprehensive content for all major bone tumors. Let me also quickly get the osteoid osteoma and aneurysmal bone cyst content from Robbins:
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Here is the complete bone tumors pathology reference, compiled from Robbins & Kumar Basic Pathology and supplementary textbooks.
Bone Tumours - Complete Pathology
(Source: Robbins & Kumar Basic Pathology; Rheumatology 2-Vol Set; Miller's Review of Orthopaedics; Campbell's Operative Orthopaedics)
SECTION 1: CLASSIFICATION OF PRIMARY BONE TUMOURS
Bone tumors are classified primarily by the tissue of origin (histogenetic type) and behavior (benign vs. malignant). The WHO recognizes >60 subtypes.
| Category | Behavior | Tumor Type | Common Location | Age (yr) | Key Morphology |
|---|---|---|---|---|---|
| Cartilage-forming | Benign | Osteochondroma | Metaphysis of long bones | 10-30 | Bony excrescence with cartilage cap |
| Benign | Chondroma (Enchondroma) | Small bones of hands & feet | 30-50 | Intramedullary hyaline cartilage nodule | |
| Malignant | Chondrosarcoma | Pelvis, shoulder, ribs | 40-60 | Infiltrates marrow; chondrocytes with atypia | |
| Bone-forming | Benign | Osteoid osteoma | Metaphysis of long bones | 10-20 | Small cortical nidus; interlacing woven bone trabeculae |
| Benign | Osteoblastoma | Vertebral column (posterior) | 10-20 | Like osteoid osteoma but >2 cm | |
| Malignant | Osteosarcoma | Metaphysis: distal femur, proximal tibia | 10-20 | Malignant cells producing woven bone/osteoid | |
| Unknown origin | Benign | Giant cell tumor (GCT) | Epiphysis of long bones | 20-40 | Osteoclast-type giant cells + mononuclear stromal cells |
| Benign | Aneurysmal bone cyst | Femur, tibia, vertebra | 10-20 | Multiloculated blood-filled spaces | |
| Malignant | Ewing sarcoma | Diaphysis of long bones | 10-20 | Sheets of small round blue cells; t(11;22) |
Memory tip for tumor locations: OsteoSarcoma = Metaphysis | Giant Cell Tumor = Epiphysis | Ewing = Diaphysis
SECTION 2: BONE-FORMING TUMOURS
2A. OSTEOID OSTEOMA
| Feature | Details |
|---|---|
| Incidence | Most common benign bone-forming tumor in young patients |
| Age/Sex | 10-20 yr; M > F |
| Location | Cortex of metaphysis of long bones (femur, tibia); posterior vertebral elements |
| Size | < 2 cm (key distinguishing feature from osteoblastoma) |
| Symptoms | Night pain, relieved by aspirin/NSAIDs (prostaglandin-mediated) |
Radiology: Round lucent nidus (<1.5 cm) surrounded by dense reactive sclerosis
Histology:
- Well-circumscribed round-oval mass
- Delicate interlacing trabeculae of woven bone rimmed by a single layer of osteoblasts
- Stroma: loose connective tissue with dilated, congested capillaries
Treatment: Radiofrequency ablation (RFA) or conservative NSAIDs
2B. OSTEOBLASTOMA
| Feature | Details |
|---|---|
| Size | > 2 cm (larger than osteoid osteoma) |
| Location | Posterior vertebral elements (laminae, pedicles) |
| Pain | Unresponsive to aspirin |
| Histology | Similar to osteoid osteoma; no marked reactive bony sclerosis around it |
Treatment: Curettage or en bloc excision
2C. OSTEOSARCOMA (Osteogenic Sarcoma)
Definition
Malignant tumor producing osteoid matrix or mineralized bone. The most common primary malignant bone tumor (excluding hematopoietic tumors).
Epidemiology
| Feature | Details |
|---|---|
| Age | Bimodal: ~75% <20 yr (adolescent); second peak in older adults (secondary type) |
| Sex | M > F (1.6:1) |
| Site | Metaphysis; ~50% near the knee (distal femur or proximal tibia) |
| Incidence | 1-1.5 per million |
Pathogenesis
Peak incidence at adolescent growth spurt - rapid proliferation near growth plate predisposes to mutations.
| Gene | Role |
|---|---|
| RB | Mutated in 70% of sporadic cases; germline mutation = 1000-fold increased risk |
| TP53 | Germline mutation in Li-Fraumeni syndrome; common in sporadic tumors |
| MDM2 / CDK4 | Overexpressed in low-grade osteosarcomas (inhibit p53 and RB) |
| CDKN2A | Inactivated (encodes p16 and p14 tumor suppressors) |
| MYC | Amplified in up to 50%; associated with poor prognosis |
No specific chromosomal translocation (unlike Ewing sarcoma). Instead: complex karyotypes with numerous chromosomal aberrations.
Secondary Osteosarcoma
Occurs in older adults in association with:
- Paget disease of bone
- Bone infarcts
- Prior radiation to bone
Gross Pathology
- Bulky, gritty, tan-white tumor with areas of hemorrhage
- Destroys cortex and invades soft tissue
- Extensive intramedullary spread
- Confined to metaphyseal side of growth plate (rarely crosses the epiphyseal plate)
Radiology
| Sign | Description |
|---|---|
| Codman triangle | Periosteum lifted by tumor → triangular wedge of reactive subperiosteal bone. Indicates aggressive tumor; not pathognomonic |
| Sunburst pattern | Perpendicular periosteal spicules radiating from bone surface |
| Mixed lytic/sclerotic | Combined bone destruction + new bone formation |
| Soft tissue mass | Extension beyond cortex |
Histology (Microscopy)
Diagnostic hallmark: Malignant tumor cells producing unmineralized osteoid or mineralized bone
Microscopic features:
- Cellular pleomorphism - large hyperchromatic nuclei
- Bizarre tumor giant cells
- Abundant mitoses including abnormal (bipolar) forms
- Fine lacelike osteoid produced by malignant cells
- Extensive necrosis
- Intravascular invasion
- Bridges preexisting lamellar bone
Clinical Features & Prognosis
| Feature | Details |
|---|---|
| Presentation | Painful, progressively enlarging mass; may present as pathologic fracture |
| Metastasis | All assumed to have occult metastases at diagnosis. Hematogenous spread to lungs (most common), then bone, brain |
| Treatment | Neoadjuvant chemo → Surgery (limb-sparing) → Adjuvant chemo |
| 5-year survival | ~70% without overt metastases at diagnosis |
| With metastases | Significantly worse prognosis |
SECTION 3: CARTILAGE-FORMING TUMOURS
3A. OSTEOCHONDROMA (Exostosis)
| Feature | Details |
|---|---|
| Definition | Benign cartilage-capped tumor arising from the bone surface |
| Type | Sessile or pedunculated (with bony stalk); 85% solitary |
| Age/Sex | Late adolescence/early adulthood; M:F = 3:1 |
| Location | Metaphysis near growth plate; especially around the knee |
| Origin | Bones of endochondral origin only |
| Genetics | Loss-of-function mutations in EXT1 or EXT2 genes (encode heparan sulfate enzymes) → disrupts hedgehog signaling → abnormal chondrocyte differentiation |
| Syndrome | Multiple hereditary exostoses (familial; bones bowed and shortened) |
Gross: Bony excrescence with hyaline cartilage cap covered by perichondrium
Histology:
- Cap = hyaline cartilage recapitulating growth plate
- Undergoes endochondral ossification inward
- Medullary cavity of osteochondroma is continuous with host bone
Clinical Features:
- Slow-growing; stops at growth plate closure
- Can cause pain if nerve impingement or stalk fracture
- Sarcomatous transformation rare in solitary lesions; more common in multiple hereditary exostoses
- Treatment: simple excision when symptomatic
3B. CHONDROMA (ENCHONDROMA)
| Feature | Details |
|---|---|
| Definition | Benign tumor of hyaline cartilage within the medullary cavity (enchondroma) or on bone surface (juxtacortical) |
| Age | 20-50 yr; equal sex distribution |
| Location | Most common site: small bones of hands and feet (proximal/middle phalanges); most common bone tumor of the hand |
| Genetics | IDH1 / IDH2 mutations → 2-hydroxyglutarate ("oncometabolite") → altered DNA methylation |
| Syndromes | Ollier disease (multiple enchondromas); Maffucci syndrome (enchondromas + hemangiomas, higher malignant potential) |
Radiology: Circumscribed lucency with central irregular calcifications, sclerotic rim, intact cortex
Histology:
- Benign-appearing chondrocytes in well-circumscribed nodules of hyaline cartilage
- Peripheral nodules may undergo endochondral ossification
- Center may calcify and become infarcted
- Syndromic enchondromas: more cellular + more atypia
Clinical: Usually asymptomatic; may cause pathologic fracture. Solitary lesions rarely undergo sarcomatous change.
3C. CHONDROSARCOMA
| Feature | Details |
|---|---|
| Definition | Malignant tumor producing cartilage |
| Incidence | 2nd most common malignant matrix-producing bone tumor (after osteosarcoma) |
| Age/Sex | 40s or older; M:F = 2:1 |
| Location | Axial skeleton - pelvis, shoulder, ribs (contrast with osteosarcoma which favors extremities) |
| Secondary type | 15% arise from preexisting enchondroma or osteochondroma |
| Genetics | EXT1/EXT2 mutations (in osteochondroma-related); IDH1/IDH2 mutations; CDKN2A silencing |
Subtypes: Conventional (90%), dedifferentiated, clear cell, mesenchymal
Gross:
- Bulky nodules of glistening gray-white, translucent cartilage with gelatinous myxoid areas
- Focal calcifications; central necrosis creating cystic spaces
- Extracortical extension common
Histology:
- Cartilage infiltrates marrow space and entraps normal bony trabeculae
- Graded 1-3 based on cellularity, atypia, mitoses (Grade 1 = low grade; Grade 3 = high grade with bizarre giant cells)
- Grade 1: low cellularity, plump vesicular nuclei, small nucleoli
- Grade 3: high cellularity, extreme pleomorphism, frequent mitoses
X-ray: "Flocculent" (popcorn/ring-and-arc) calcifications within cartilage matrix; cortical destruction; soft tissue mass
Clinical/Treatment:
- Painful, progressively enlarging mass
- Most are grade 1 (low grade, good prognosis)
- Treatment: Wide surgical resection only (chemo and radiotherapy largely ineffective - key exam point!)
- 10-year survival: Grade 1 = ~83%; Grade 2 = ~53%; Grade 3 = ~22%
SECTION 4: TUMOURS OF UNKNOWN ORIGIN
4A. GIANT CELL TUMOUR (GCT) OF BONE
| Feature | Details |
|---|---|
| Behavior | Benign but locally aggressive |
| Age | 3rd-5th decade (20-40 yr); more common in young women |
| Location | Epiphysis of long bones - most commonly distal femur and proximal tibia (around the knee); also distal radius |
| Key distinction | Epiphyseal location sets it apart from most other bone tumors |
Pathogenesis
- Neoplastic cells: primitive osteoblast precursors that express high levels of RANKL
- RANKL drives proliferation and differentiation of osteoclast precursors
- Multinucleate giant cells: are the non-neoplastic osteoclasts (express RANK receptor)
- Absence of normal osteoblast-osteoclast feedback → localized highly destructive bone resorption
Gross & Histology
Gross: Red-brown mass; destroys overlying cortex → bulging soft tissue mass bounded by thin shell of reactive bone; frequently undergoes cystic degeneration
Histology:
- Numerous multinucleate osteoclast-type giant cells (up to 100+ nuclei)
- Background of uniform oval mononuclear neoplastic stromal cells
- Giant cells are reactive (non-neoplastic); stromal cells are the true neoplastic component
Radiology
Predominantly lytic and expansile with cortical destruction; "soap-bubble" appearance; may show pathologic fracture
Clinical Features & Treatment
| Feature | Details |
|---|---|
| Symptoms | Joint pain (near epiphysis), arthritis-like symptoms, pathologic fracture |
| Treatment | Curettage (primary); 40-60% recur locally |
| Alternative | Denosumab (anti-RANKL antibody) - inhibits osteoclast maturation; used when surgery would be deforming |
| Metastasis | ~4% develop lung metastases; most cured by excision of metastases |
4B. EWING SARCOMA
| Feature | Details |
|---|---|
| Definition | Malignant tumor of small round cells |
| Incidence | ~10% of primary malignant bone tumors; 2nd most common bone sarcoma in children |
| Age/Sex | 80% <20 yr; slight male predominance |
| Location | Diaphysis of long bones (femur most common); 20% extraskeletal |
Pathogenesis
- >90% have balanced translocation t(11;22)(q24;q12) - EWSR1 + FLI1 genes
- Produces chimeric EWS/FLI1 fusion protein - binds chromatin, dysregulates transcription → uncontrolled growth
- Cell of origin: mesenchymal stem cells or primitive neuroectodermal cells (uncertain)
Gross & Histology
Gross:
- Arises in medullary cavity → invades cortex, periosteum, and soft tissue
- Periosteal reaction: layered reactive bone = "onion-skin" appearance on X-ray
- Soft, tan-white tumor; areas of hemorrhage and necrosis
Histology:
- Sheets of uniform small round cells - slightly larger and more cohesive than lymphocytes
- Scant cytoplasm, clear (rich in glycogen; PAS positive)
- Homer-Wright rosettes (circular cell groupings with central fibrillary core) may be present
- Tumor cells do NOT produce bone or cartilage
- One of the small round blue cell tumors (SRBCT) of childhood
Radiology
- Permeative, destructive diaphyseal lesion
- Onion-skin periosteal reaction (layered periosteal new bone)
- Soft tissue mass
Clinical Features & Treatment
| Feature | Details |
|---|---|
| Symptoms | Painful, enlarging mass; site is tender, warm, swollen (may mimic osteomyelitis/infection) |
| Fever/ESR | May be elevated - simulates infection |
| Spread | Hematogenous; lungs and other bones |
| Treatment | Multiagent chemotherapy + radiation ± surgery (chemo is highly effective; often curative) |
| 5-year survival | ~65-75% with localized disease |
4C. ANEURYSMAL BONE CYST (ABC)
| Feature | Details |
|---|---|
| Definition | Benign lesion characterized by multiloculated blood-filled spaces |
| Age | Adolescence (all ages affected) |
| Location | Femur, tibia, vertebral posterior elements |
| Behavior | Benign but locally aggressive |
| Genetics | Rearrangements of chromosome 17p13 → USP6 gene fusion (commonly with COL1A1) → USP6 overexpression → NF-κB activation → matrix metalloprotease upregulation → cystic bone resorption |
| Association | 20-40% are secondary to a preexisting lesion (most commonly GCT) |
Radiology:
- Eccentric, expansile, lytic metaphyseal lesion with well-defined margins
- MRI: fluid-fluid levels (pathognomonic - blood sediment by density)
Gross: Multiple blood-filled cystic spaces separated by thin tan-white septa
Histology:
- Cystic spaces lack endothelial lining (not true aneurysm)
- Septa contain: plump uniform fibroblasts + multinucleate osteoclast-like giant cells + reactive woven bone
Treatment: Curettage or excision; 10-50% recur
SECTION 5: NON-NEOPLASTIC LESIONS SIMULATING TUMOURS
Fibrous Cortical Defect / Nonossifying Fibroma (NOF)
| Feature | Details |
|---|---|
| Prevalence | Fibrous cortical defects in up to 50% of children >2 yr |
| Location | Eccentric metaphysis of distal femur / proximal tibia (often bilateral) |
| Size | <0.5 cm = fibrous cortical defect; 5-6 cm = nonossifying fibroma |
| Behavior | Sharply demarcated radiolucent mass with thin rim of sclerosis; rarely needs biopsy |
| Outcome | Most regress spontaneously |
SECTION 6: COMPARATIVE SUMMARY TABLE
| Feature | Osteosarcoma | Chondrosarcoma | Ewing Sarcoma | GCT | Osteochondroma |
|---|---|---|---|---|---|
| Age | 10-20 yr | 40-60 yr | 10-20 yr | 20-40 yr | 10-30 yr |
| Location | Metaphysis | Pelvis/axial | Diaphysis | Epiphysis | Metaphysis |
| Genetics | RB, TP53 mutations; complex karyotype | IDH1/2, EXT1/2 | t(11;22) EWS-FLI1 | RANKL overexpression | EXT1/EXT2 |
| X-ray | Mixed lytic/sclerotic; Codman triangle; sunburst | Flocculent calcifications; soft tissue mass | Onion-skin periosteal reaction | Lytic/expansile; soap-bubble | Bony excrescence with cap |
| Histology | Malignant cells + osteoid production | Malignant chondrocytes in cartilage matrix | Small round blue cells; no bone/cartilage | Giant cells + mononuclear stromal cells | Cartilage cap + endochondral ossification |
| Chemo response | Good (70% survival) | Poor/resistant | Excellent | N/A | N/A |
| Treatment | Neoadjuvant chemo + surgery + adjuvant chemo | Wide resection (no chemo) | Chemo + radiation ± surgery | Curettage ± denosumab | Excision |
SECTION 7: APPROACH TO A BONE TUMOUR (CLINICAL PEARL)
Patient >50 yr with destructive bone lesion → Think metastasis first!
Most common primary sites to metastasize to bone: Breast, Lung, Thyroid, Kidney (renal), Prostate
Mnemonic: "BLT with a Kosher Pickle"
Biopsy principles:
- Benign, asymptomatic lesions (fibrous dysplasia, simple bone cyst, enchondroma, osteochondroma) → observation, no biopsy needed
- Night pain + any suspicious lesion → mandatory biopsy + expert pathologic opinion
- All malignant lesions: biopsy before treatment planning
Source: Robbins & Kumar Basic Pathology (Table 19.1, pp. 781-787), Rheumatology 2-Vol Set 2022 (p. 3062), Miller's Review of Orthopaedics 9th Ed.
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