Osteosarcoma

Osteosarcoma is the most common primary malignant bone tumour of childhood and adolescence, defined histologically by malignant mesenchymal cells producing osteoid (immature bone). It is a perennial NEET PG favourite — the radiological signs, alkaline phosphatase, MAP chemotherapy protocol, and Enneking staging are repeatedly tested.

Definition & basic concept

High-yield: The single defining feature of osteosarcoma — the one that distinguishes it from every other sarcoma — is the production of osteoid directly by the malignant cells. If a tumour cell makes osteoid, it is an osteosarcoma, no matter how cartilaginous or fibrous it otherwise looks.

It is a spindle-cell sarcoma arising in bone. Roughly 80% are the conventional (high-grade intramedullary) type. The tumour typically arises in the metaphysis of long bones because this is the site of greatest osteoblastic activity during the adolescent growth spurt.

Epidemiology — the classic exam picture

• Bimodal age distribution: first and largest peak in the second decade (10–20 years, adolescents) during the growth spurt; second smaller peak in the elderly (>60 years), usually secondary to Paget's disease or prior irradiation.
• Slight male preponderance (~1.5:1).
• It is the most common primary malignant bone tumour overall if multiple myeloma is excluded (myeloma is technically the commonest primary malignant bone neoplasm in adults; osteosarcoma is the commonest non-haematological primary bone malignancy).

High-yield: Most common site = around the knee → distal femur metaphysis (most common single site) > proximal tibia > proximal humerus. "Around the knee, away from the elbow" — fast-growing physes are favoured.

Classification

Osteosarcoma is a family of tumours. Classify primarily by location relative to bone and by grade.

Type	Grade	Location	Age / Notes	Prognosis
Conventional (intramedullary)	High	Metaphysis, medullary	Adolescents; ~80% of all cases	Worst of the surface variants combined; needs chemo
Telangiectatic	High	Medullary, lytic/expansile	Mimics aneurysmal bone cyst (ABC)	Aggressive but chemo-responsive
Parosteal (juxtacortical)	Low	Surface, posterior distal femur	3rd–4th decade, female slight excess	Best prognosis
Periosteal	Intermediate	Surface (cortical), diaphysis of tibia/femur	Young adults; chondroblastic	Intermediate
High-grade surface	High	Surface	Rare	Like conventional
Secondary (Paget/radiation)	High	Pelvis, flat bones	Elderly	Worst prognosis
Multifocal / osteosarcomatosis	High	Multiple bones	Children	Dismal

Conventional osteosarcoma is further subtyped histologically by the predominant matrix: osteoblastic (most common, ~50%), chondroblastic, and fibroblastic. Subtype does not change conventional management.

High-yield: Parosteal = low grade, posterior distal femur, may show a cleavage/lucent line separating tumour from cortex, best prognosis, often treated by surgery alone (chemo not mandatory). Periosteal = intermediate grade, chondroblastic, diaphyseal.

Aetiology & risk factors

While most cases are sporadic, several associations are classic exam links:

• Hereditary retinoblastoma (germline RB1 mutation) — the strongest genetic association; these children have a dramatically increased lifelong risk, augmented further by radiotherapy.
• Li–Fraumeni syndrome (germline TP53 mutation).
• Rothmund–Thomson syndrome (RECQL4 helicase defect).
• Paget's disease of bone — the commonest cause of osteosarcoma in the elderly; suspect malignant transformation when a Pagetic bone develops sudden pain or a soft-tissue mass.
• Prior ionising radiation (radiation-induced sarcoma, latency often >10 years).
• Chronic osteomyelitis and bone infarcts (rare).

High-yield: RB1 and p53 are the two cardinal tumour-suppressor genes lost in osteosarcoma. Retinoblastoma survivors who receive radiotherapy are at extreme risk.

Pathophysiology

Genomic instability is the hallmark. Loss of RB and p53 removes cell-cycle checkpoints; complex karyotypes with chromothripsis are typical. The malignant osteoblasts secrete disordered, lace-like (woven) osteoid. As the tumour breaches the cortex, it elevates and irritates the periosteum, which lays down reactive new bone — producing the radiological signs (Codman's triangle, sunburst) described below. Spread is haematogenous, characteristically to the lungs (and then to other bones); lymphatic spread is rare. Skip metastases (discontinuous tumour foci within the same bone or across a joint) are an important Enneking concept.

Clinical features

• Deep, boring, progressive bone pain — often nocturnal, initially intermittent. Frequently attributed by patient/parents to "sports injury".
• Palpable, tender, firm swelling with overlying warmth and dilated veins as it enlarges.
• Restricted joint movement, limp, joint effusion.
• Pathological fracture in ~5–10% (more common in telangiectatic type).
• Systemic symptoms (weight loss, fever) are uncommon early and suggest advanced disease.

Examine for the metastatic and syndromic clues: chest signs, regional lymph nodes (usually negative), and signs of retinoblastoma history.

Investigations

Plain radiograph — the first and most exam-tested step

A NEET PG question almost always hinges on recognising these signs:

Radiological sign	Meaning
Sunburst / sunray appearance	Spiculated periosteal bone perpendicular to cortex from tumour osteoid
Codman's triangle	Triangular periosteal reaction at the margin where periosteum is lifted off cortex
Mixed lytic-sclerotic ("moth-eaten") lesion	Bone destruction + new bone formation
Soft-tissue mass with calcification	Tumour extension beyond cortex
Metaphyseal location, ill-defined margins	Aggressive, wide zone of transition

High-yield: Codman's triangle and the sunburst pattern are characteristic but not pathognomonic — Ewing sarcoma, infection, and other aggressive lesions can produce periosteal reaction too. The diagnosis is confirmed only by biopsy.

Subsequent imaging

• MRI of the whole involved bone — investigation of choice for local staging: defines intramedullary (marrow) extent, soft-tissue component, neurovascular involvement, skip lesions, and joint involvement. Essential for planning limb salvage.
• CT chest — investigation of choice for detecting pulmonary metastases (most common metastatic site).
• Radionuclide bone scan (Tc-99m) / PET-CT — to detect skip lesions and distant skeletal/whole-body metastases.

Laboratory

• Serum alkaline phosphatase (ALP) — raised in ~50%; reflects osteoblastic activity. High levels and failure to fall after treatment carry poor prognosis.
• Serum LDH raised — also an adverse prognostic marker.

High-yield: Both ALP and LDH are prognostic. Persistently high or rising ALP/LDH after surgery suggests residual/metastatic disease.

Biopsy — the confirmatory test

• Biopsy is mandatory for definitive diagnosis and grading.
• Golden rule: the biopsy should be performed at the centre that will do the definitive surgery, and the biopsy tract must be placed so it can be excised en bloc with the tumour at resection. A poorly placed biopsy can convert a limb-salvageable tumour into an amputation.
• Histology shows pleomorphic malignant spindle cells producing osteoid/lace-like bone.

Staging — Enneking (MSTS) system

The Enneking / Musculoskeletal Tumour Society (MSTS) surgical staging system is the most examined staging for bone sarcomas. It uses three variables: Grade (G), Local extent / compartment (T), and Metastasis (M).

Stage	Grade	Site (compartment)	Metastasis
IA	Low (G1)	Intracompartmental (T1)	None (M0)
IB	Low (G1)	Extracompartmental (T2)	None (M0)
IIA	High (G2)	Intracompartmental (T1)	None (M0)
IIB	High (G2)	Extracompartmental (T2)	None (M0)
III	Any grade	Any extent	Metastasis present (M1)

High-yield: Most conventional osteosarcomas present as Stage IIB (high-grade, extracompartmental, no mets). Any metastasis = Stage III, regardless of grade or local extent. Memorise: Grade → Site → Metastasis.

Management

Modern treatment is multidisciplinary and has transformed survival. The cornerstone is neoadjuvant chemotherapy + wide local excision (limb salvage) + adjuvant chemotherapy.

Stepwise approach (high-grade conventional osteosarcoma)

1. Biopsy & staging (MRI bone + CT chest) → 2. Neoadjuvant (pre-operative) chemotherapy ~8–10 weeks → 3. Restaging & surgical resection (limb salvage or amputation) with wide margins → 4. Assess histological tumour necrosis (Huvos grade) → 5. Adjuvant (post-operative) chemotherapy → 6. Surveillance (chest CT + clinical follow-up).

Chemotherapy — the MAP protocol (drugs of choice)

High-yield: Standard regimen = MAP → Methotrexate (high-dose, with leucovorin/folinic acid rescue), Adriamycin (doxorubicin), and Platinum (cisplatin). Ifosfamide ± etoposide may be added in some protocols.

• Neoadjuvant chemotherapy is given first to shrink the tumour, treat micrometastases early, facilitate limb salvage, and — crucially — to allow assessment of chemosensitivity via tumour necrosis.

Assessing response — Huvos grading

After resection, the percentage of tumour necrosis is the single most powerful prognostic factor:

• ≥90% (good responder) → excellent prognosis; continue same regimen.
• <90% (poor responder) → worse prognosis.

High-yield: ≥90% necrosis after neoadjuvant chemo = good prognostic response (Huvos grade III/IV). This is the most consistently tested treatment-response fact.

Surgery

• Limb-salvage surgery is now possible in the majority (>80%) when wide margins can be achieved without compromising the limb — reconstruction with endoprosthesis, allograft, or rotationplasty.
• Amputation reserved for: major neurovascular involvement, very large tumours where wide margins are unachievable, pathological fracture with contamination, or young children where limb-length discrepancy would be severe.
• The aim is always a wide or radical surgical margin (Enneking margin concept).

Role of radiotherapy

Osteosarcoma is relatively radioresistant; radiotherapy is not a primary modality. It is reserved for unresectable sites (e.g., axial/pelvic), positive margins, or palliation. (Contrast with Ewing sarcoma, which is radiosensitive.)

Pulmonary metastases

Isolated, resectable lung metastases should be treated with metastasectomy (thoracotomy) plus chemotherapy — patients can still be cured. This is a key conceptual point: lung mets in osteosarcoma do not automatically mean palliation only.

Prognosis

• 5-year survival with localised disease and modern chemo ≈ 60–70%; with metastatic disease at presentation it falls to ~20–30%.

Good prognostic factors	Poor prognostic factors
≥90% tumour necrosis (Huvos)	<90% necrosis
Distal/appendicular site	Axial / pelvic / proximal site
No metastases at diagnosis	Metastases (esp. multiple, bony)
Normal/falling ALP & LDH	High or rising ALP & LDH
Small tumour, complete resection	Large tumour, positive margins, pathological fracture

High-yield: The two strongest prognostic determinants are presence of metastases at diagnosis and degree of tumour necrosis after neoadjuvant chemotherapy.

Complications

• Pulmonary metastasis (most common and most feared) → respiratory failure.
• Pathological fracture (treatment-altering, may force amputation).
• Local recurrence after inadequate margins.
• Chemotherapy toxicities — important pharm overlap:
  • Doxorubicin → cardiomyopathy (dose-dependent; monitor ejection fraction; dexrazoxane is cardioprotective).
  • Cisplatin → nephrotoxicity and ototoxicity.
  • High-dose methotrexate → mucositis, myelosuppression, nephrotoxicity (needs hydration, urinary alkalinisation, and leucovorin rescue).
  • Ifosfamide → haemorrhagic cystitis (give MESNA).
• Secondary malignancies (leukaemia from alkylating agents).
• Limb-length discrepancy and prosthetic complications after salvage surgery.

Key differentials

Feature	Osteosarcoma	Ewing sarcoma	Chondrosarcoma	Osteomyelitis
Age	10–20 y	5–15 y (younger)	>40 y (older)	Any
Bone region	Metaphysis	Diaphysis	Axial/pelvis, metaphysis	Metaphysis
Matrix	Osteoid	Small round blue cells (no matrix)	Cartilage	None
Periosteal reaction	Sunburst, Codman's	Onion-skin (lamellated)	Endosteal scalloping	Sequestrum/involucrum
Systemic	Usually none	Fever, ↑ESR (mimics infection)	None	Fever, pus
Genetics	RB1, p53	t(11;22) EWS-FLI1	IDH1/2	—
Radiotherapy	Resistant	Sensitive	Resistant	N/A

High-yield: Onion-skin periosteal reaction + diaphysis + younger child + fever/↑ESR + t(11;22) = Ewing sarcoma, not osteosarcoma. This contrast is asked almost every year.

Mnemonics & memory hooks

• Drugs — "MAP your sarcoma": Methotrexate, Adriamycin (doxorubicin), Platinum (cisplatin).
• Enneking variables — "GSM": Grade → Site (compartment) → Metastasis.
• Radiology — "SCC": Sunburst, Codman's triangle, Cortical breach.
• Surface variants prognosis (best→worst): Parosteal > Periosteal > High-grade surface. ("Parosteal is par excellence.")
• Genes — "RB makes BAD bone" → RB1 + p53.

Recently asked / exam angle

• Most common site of osteosarcoma → distal femur (around the knee).
• Radiological sign that is "characteristic but not diagnostic" → Codman's triangle / sunburst — confirm by biopsy.
• Investigation of choice for local extent / marrow involvement → MRI of whole bone; for metastasis → CT chest.
• Most important prognostic factor after neoadjuvant chemo → percentage tumour necrosis (≥90% = good, Huvos grade).
• Chemotherapy protocol → MAP; the drug causing cardiomyopathy → doxorubicin.
• Lowest-grade, best-prognosis variant on posterior distal femur → parosteal osteosarcoma.
• Osteosarcoma in elderly → think Paget's disease or prior radiation.
• Genetic syndrome with highest risk → hereditary retinoblastoma (RB1); also Li–Fraumeni (p53).
• Staging system used → Enneking / MSTS; conventional tumour usually Stage IIB.
• Tumour producing osteoid by malignant cells → defining feature, single-best-answer style question.
• Telangiectatic osteosarcoma mimics → aneurysmal bone cyst on X-ray (lytic, expansile).

Rapid revision

1. Osteosarcoma = malignant tumour whose cells produce osteoid — the defining feature.
2. Commonest primary malignant bone tumour of adolescents; bimodal peaks (teens; elderly with Paget/radiation).
3. Classic site = distal femur metaphysis ("around the knee").
4. X-ray triad: sunburst pattern, Codman's triangle, mixed lytic-sclerotic lesion with soft-tissue mass.
5. MRI whole bone = local staging (skip lesions, marrow); CT chest = metastasis screen.
6. ALP and LDH raised and prognostic; biopsy confirms and must be tract-excisable.
7. Spread is haematogenous to lungs; lymphatic spread rare.
8. Enneking staging: Grade → Site → Metastasis; conventional tumour is usually Stage IIB.
9. Treatment = neoadjuvant MAP chemo → wide resection / limb salvage → adjuvant chemo.
10. ≥90% tumour necrosis (Huvos) post-chemo = good prognosis; the key response marker.
11. Doxorubicin → cardiotoxicity; cisplatin → nephro/ototoxicity; methotrexate → leucovorin rescue; ifosfamide → MESNA.
12. Parosteal = low grade, posterior distal femur, best prognosis; Ewing = diaphysis, onion-skin, t(11;22), radiosensitive.