Osteosarcoma

• Bimodal age distribution: a peak in adolescence during the growth spurt, and a second peak in older adults arising in Paget disease, prior radiation, or bone infarct.
• Predisposing conditions: hereditary retinoblastoma (RB1) and Li–Fraumeni syndrome (TP53).
• Site: the metaphysis of long bones, most often around the knee — distal femur and proximal tibia — where growth is most active.
• Presentation: progressive localised pain, often worse at night, with swelling and a palpable mass; pathological fracture in some.
• Radiographic hallmarks are highly testable: a "sunburst" periosteal reaction from aggressive spiculated new bone, and Codman triangle where the periosteum is lifted from the cortex. The lesion is typically mixed lytic and sclerotic with cortical destruction and a soft-tissue mass.
• Staging requires MRI of the whole bone plus chest CT — the lungs are the dominant site of metastasis.
• Treatment is neoadjuvant chemotherapy, limb-salvage resection with wide margins, then adjuvant chemotherapy. The degree of tumour necrosis on the resected specimen is a key prognostic marker.
• Contrast with Ewing sarcoma, which is diaphyseal, occurs in younger children, and shows an onion-skin periosteal reaction.

Non-modifiable — germline tumour suppressor loss

• **Hereditary retinoblastoma (RB1)**: biallelic loss of RB releases E2F and removes the G1/S checkpoint. Survivors of germline retinoblastoma carry a markedly increased lifetime risk of osteosarcoma, amplified further if they received orbital radiation.
• **Li–Fraumeni syndrome (TP53)**: loss of the guardian of the genome permits propagation of cells with unrepaired double-strand breaks. Osteosarcoma is one of the core Li–Fraumeni tumours alongside breast cancer, adrenocortical carcinoma, brain tumours and leukaemia.
• RecQ helicase disorders: Rothmund–Thomson (RECQL4), Bloom and Werner syndromes — defective DNA helicase leads to genomic instability and chromothripsis-type rearrangements.

Non-modifiable — host growth biology

• Adolescent growth spurt: peak incidence tracks the pubertal growth velocity curve, which is why patients are often tall for age and why boys peak slightly later than girls.

Acquired / potentially modifiable exposures

• Therapeutic ionising radiation: radiation-associated osteosarcoma arises within a prior treatment field after a latency of many years — a classic stem is a childhood-cancer survivor with a new painful mass in the irradiated bone.
• Paget disease of bone: disordered osteoclast–osteoblast coupling produces chaotic high-turnover mosaic bone; malignant transformation occurs in only a small minority of Paget patients but accounts for much of the older-adult peak. The signal is new or escalating pain, or a sudden further rise in alkaline phosphatase in stable Paget disease.
• Bone infarct, chronic osteomyelitis, and retained metallic implants: chronic reparative proliferation as the substrate for transformation.
• Alkylating-agent chemotherapy: contributes to secondary sarcoma risk in cancer survivors.

Why the metaphysis, why adolescence

• The metaphysis adjacent to the physis is the site of greatest osteoblast proliferation and highest mitotic rate during the pubertal growth spurt, giving the largest pool of dividing cells in which a second hit can be fixed. Hence the predilection for distal femur, proximal tibia, and proximal humerus — the fastest-growing physes.

Why the imaging looks the way it does

• Malignant osteoid production is the defining feature: the tumour cells lay down disorganised, unmineralised-to-partially-mineralised osteoid, which appears as cloud-like sclerosis intermixed with lytic destruction.
• Tumour outgrows and destroys cortex by stimulating osteoclastic resorption, then breaks through into soft tissue, elevating the periosteum. New bone is deposited along Sharpey fibres perpendicular to the cortex — the sunburst or sunray spiculation.
• At the advancing edge, periosteum is lifted faster than it can ossify, ossifying only at its margin — the Codman triangle. Both signs mean an aggressive lesion outpacing containment; neither is specific to osteosarcoma.

Classic symptoms

• Deep, progressive, localised bone pain: from periosteal elevation and stretching. Characteristically present at rest and worse at night, and increasingly unresponsive to NSAIDs — the opposite of osteoid osteoma, whose night pain is dramatically relieved by NSAIDs.
• Pain often attributed to sports injury, with weeks-to-months of delay before imaging. A "sprain" that fails to resolve and worsens is the intended red flag.
• Palpable, firm, fixed, tender mass with overlying warmth, skin tethering, and dilated superficial veins: the soft-tissue component after cortical breakthrough. Size and fixity distinguish it from a mobile benign lesion.
• Localised swelling and decreased range of motion of the adjacent joint; a sympathetic joint effusion may mimic monoarticular arthritis.
• Pathological fracture after trivial trauma, from cortical destruction — a presentation that historically worsened staging and complicates limb salvage.

Less common / late findings

• Limp or refusal to bear weight in a lower-limb lesion.
• Systemic B-type symptoms (fever, weight loss) are uncommon; prominent fever and leucocytosis should redirect toward osteomyelitis or Ewing sarcoma.
• Respiratory symptoms — cough, pleuritic pain, dyspnoea, or pneumothorax from a subpleural deposit — indicate pulmonary metastases and are usually absent at presentation despite occult micrometastatic disease.

Physical exam adjuncts

• Examine the whole limb, regional nodes and the contralateral side; nodal spread is rare, so palpable adenopathy argues for another diagnosis.
• In the older patient, look for the stigmata of Paget disease — bowed tibiae, enlarged skull, hearing loss — since sarcomatous transformation is the classic second-peak scenario.

• Mixed lytic and blastic metaphyseal lesion with a wide zone of transition and indistinct margins.
• ***Sunburst* spiculated periosteal reaction and *Codman triangle*** from lifted periosteum.
• Cortical destruction with an ossified soft-tissue mass ("cloud-like" tumour osteoid outside the bone).

Immediate issues

• Pathological fracture or impending fracture: immobilise, avoid intramedullary fixation (it seeds the canal and contaminates compartments), and refer urgently — definitive management is oncological resection, not orthopaedic fixation.
• Never perform an excisional biopsy or shell-out of a suspected bone sarcoma.

Neoadjuvant systemic therapy (first-line)

• **Cytotoxic chemotherapy — the MAP regimen: high-dose methotrexate with leucovorin rescue, an anthracycline (doxorubicin), and a platinum agent (cisplatin)**. Given for all patients with high-grade disease, including apparently localised tumours, because micrometastases are assumed present. Ifosfamide ± etoposide is added or substituted in some protocols.
• Neoadjuvant therapy shrinks the soft-tissue component, facilitates limb salvage, and — crucially — allows measurement of percent tumour necrosis (Huvos) in the specimen as a prognostic marker.

Definitive local control

• Wide en bloc resection with negative margins, most often limb-salvage surgery with endoprosthetic reconstruction, allograft, or rotationplasty in skeletally immature children. Amputation is reserved for tumours with unreconstructable neurovascular involvement, extensive contamination, or failed salvage.
• The biopsy tract must be excised with the specimen.

Relapsed/metastatic disease

• Pulmonary metastasectomy is potentially curative and is pursued aggressively, including for repeat or bilateral disease, when metastases are resectable.
• Second-line systemic options include ifosfamide/etoposide and multikinase inhibitors (for example regorafenib or sorafenib) in the relapsed setting.

What does not work / is contraindicated

• Osteosarcoma is relatively radioresistant; radiotherapy is not standard definitive local therapy and is reserved for unresectable sites or palliation.
• Denosumab treats giant cell tumour of bone, not osteosarcoma — a common distractor.

Disease-related

• Pulmonary metastases: haematogenous spread through venous sinusoids to the first capillary bed. Signalled by cough, pleuritic pain, or new nodules on surveillance chest CT; spontaneous pneumothorax from a rupturing subpleural deposit is an emergency.
• Pathological fracture: cortical destruction; sudden severe pain and deformity after trivial force. Complicates limb salvage by contaminating tissue planes with tumour-laden haematoma.
• Skip metastases within the same bone — missed if MRI does not cover the whole bone — cause local recurrence after otherwise "complete" resection.
• Local recurrence from inadequate margins or an unexcised biopsy tract; a new mass at the surgical bed mandates re-imaging and biopsy.

Chemotherapy-related

• Doxorubicin cardiotoxicity: free-radical and topoisomerase-IIβ–mediated myocyte injury, cumulative and dose-dependent, producing late dilated cardiomyopathy. Falling LVEF on serial echocardiography is the signal; dexrazoxane is used for cardioprotection. Acute decompensated heart failure is an emergency.
• Cisplatin toxicity: irreversible high-frequency sensorineural hearing loss (outer hair cell damage — monitor with audiometry), nephrotoxicity with magnesium wasting, and peripheral neuropathy.
• High-dose methotrexate: crystallisation in acidic tubules causing acute kidney injury, mucositis, myelosuppression and hepatotoxicity. Prevented by urinary alkalinisation, hydration, and leucovorin rescue; delayed methotrexate clearance with rising creatinine is a medical emergency treated with glucarpidase. Avoid NSAIDs, penicillins, proton-pump inhibitors and trimethoprim–sulfamethoxazole, which delay clearance.
• Ifosfamide: haemorrhagic cystitis from acrolein (prevented with mesna), Fanconi syndrome, and encephalopathy.
• Febrile neutropenia: an emergency — blood cultures and empiric broad-spectrum antipseudomonal beta-lactam within an hour, per IDSA neutropenic fever guidance.
• Secondary malignancy: anthracycline/alkylator-related therapy-related MDS/AML, presenting years later with cytopenias.

Surgical

• Endoprosthetic infection, aseptic loosening, periprosthetic fracture, and allograft nonunion; limb-length discrepancy in growing children; nerve injury and lymphoedema.

• ***Sunburst* periosteal reaction + Codman triangle + metaphysis around the knee in a teenager** = osteosarcoma until proven otherwise. Neither radiographic sign is specific — both simply mean an aggressive process, and both occur in Ewing sarcoma and osteomyelitis.
• Single best next step after a suspicious radiograph: MRI of the entire involved bone (whole-bone coverage detects skip lesions), then chest CT, then image-guided core biopsy at the centre that will resect the tumour. Biopsy before MRI, or biopsy at an outside hospital, is the classic wrong answer.
• The association examiners test most: **hereditary retinoblastoma (RB1) — a survivor of bilateral retinoblastoma who later develops a distal femoral mass. Second favourite: Li–Fraumeni (TP53), and Paget disease** in the older-adult peak.
• New or escalating bone pain with a further rise in alkaline phosphatase in a patient with stable Paget disease = suspect sarcomatous transformation. Elevated ALP and LDH are prognostic, never diagnostic.
• Ewing sarcoma is the distractor: diaphyseal, younger child, onion-skin periosteal reaction, **small round blue cells, CD99 positive, *t(11;22) EWSR1–FLI1*, and it may present with fever and leucocytosis mimicking osteomyelitis. Osteosarcoma instead shows malignant osteoid** and a complex aneuploid karyotype — no defining translocation.
• Osteoid osteoma is the benign distractor: small cortical nidus with reactive sclerosis, night pain dramatically relieved by NSAIDs.
• Treatment order matters: neoadjuvant MAP chemotherapy → wide limb-salvage resection with the biopsy tract excised → adjuvant chemotherapy (NCCN). Percent tumour necrosis (Huvos) in the specimen is the key prognostic readout.
• Osteosarcoma is relatively radioresistant — radiation is not definitive local therapy; and resection of pulmonary metastases can still be curative, so isolated lung nodules are not automatically palliative-only disease.