Differential Diagnosis: Proximal Humerus Mass in a 16-Year-Old Boy

High-Priority Differentials

1. Ewing Sarcoma (most important mimic)

• Key overlap with osteosarcoma: Both can show aggressive periosteal reaction (Codman triangle, hair-on-end), cortical destruction, and soft-tissue mass
• Distinguishing features: Ewing tends to be diaphyseal (vs metaphyseal in OS); laminated "onion-skin" periosteal reaction is more classic; no tumour osteoid matrix — sclerosis in Ewing is reactive, not tumour bone; systemic symptoms (fever, elevated ESR) may suggest infection
• Cytogenetics: t(11;22) EWS-FLI1 fusion gene — pathognomonic — Grainger & Allison's Diagnostic Radiology, pp. 1054–1055

2. Conventional Osteosarcoma Variants

• Telangiectatic osteosarcoma — predominantly lytic, blood-filled spaces; may mimic ABC radiologically; needs biopsy to distinguish
• Low-grade central osteosarcoma — less aggressive appearance, may resemble fibrous dysplasia
• Periosteal osteosarcoma — surface lesion, intermediate grade, chondroblastic; arises from deeper periosteum
• Parosteal osteosarcoma — posterior surface, dense ossification, low grade; typically older patients (20s–30s) — Grainger & Allison, p. 1048

3. Aneurysmal Bone Cyst (ABC)

• Expansile, blood-filled, multilocular cystic lesion; occurs in 10–20 year olds, metaphysis
• Fluid-fluid levels on MRI are characteristic
• Can be primary or secondary (arising within a telangiectatic osteosarcoma, chondroblastoma, or GCT — biopsy is mandatory)
• Usually no periosteal spiculation or soft-tissue mass

4. Chondroblastoma

• Classic: epiphyseal location in skeletally immature patients (15–25 years); proximal humerus is a favoured site
• Lytic lesion with well-defined sclerotic margins, may have chondroid calcifications
• Surrounding marrow/soft-tissue oedema on MRI — can simulate malignancy
• Distinguished from OS by epiphyseal location and benign radiographic margins — Grainger & Allison, p. 1217 (caption)

5. Unicameral (Simple) Bone Cyst

• Common in proximal humerus of children/adolescents
• Central metaphyseal lytic lesion, thin cortex, "fallen fragment" sign on fracture
• Non-aggressive; no periosteal reaction, no soft-tissue mass
• Presents often as a pathological fracture

6. Osteoid Osteoma / Osteoblastoma

• Osteoid osteoma: small (<1.5 cm nidus), cortical, intense night pain relieved by NSAIDs; dense sclerosis surrounds a lucent nidus
• Osteoblastoma: >2 cm, expansile; may be locally aggressive, mimicking low-grade OS; usually posterior elements of spine but can involve long bones — Robbins Basic Pathology, p. 782

7. Langerhans Cell Histiocytosis (LCH)

• Peak: 5–15 years; can be monostotic or polyostotic
• Lytic lesion with aggressive periosteal reaction, can simulate Ewing or osteosarcoma
• Key differentiator: rapid radiological change (bone disruption in 7–10 days vs 4–6 weeks for osteomyelitis); periosteal reaction may regress spontaneously
• Histology (CD1a, S100, Birbeck granules on EM) is diagnostic — Rockwood & Green, p. 3408–3412

8. Subacute Osteomyelitis / Chronic Osteomyelitis

• Can produce aggressive periosteal reaction, bone destruction, and soft-tissue swelling — strongly mimicking OS or Ewing
• Fever, elevated WBC, CRP; sequestrum/involucrum on imaging
• Brodie abscess: metaphyseal lytic lesion with surrounding sclerosis in adolescents
• This distinction is clinically critical: biopsy is often required to differentiate

9. Giant Cell Tumor (GCT)

• Typically epiphyseal, subarticular; more common in skeletally mature patients (20s–40s, after physeal fusion)
• Rare in a 16-year-old with open physis; if present, usually eccentrically placed, soap-bubble appearance
• More common in females

10. Chondrosarcoma

• Uncommon under 20 years; typically adults (30s–60s)
• Cartilaginous matrix ("rings and arcs") calcification
• Usually primary in adults; secondary to enchondroma/osteochondroma in skeletally immature
• Keep on the list if an underlying enchondroma or osteochondroma is identified

11. Non-Ossifying Fibroma (NOF) / Fibrous Cortical Defect

• Common incidental finding in adolescents; eccentric metaphyseal cortical lesion with scalloped sclerotic margins
• Rarely >5 cm; does not simulate osteosarcoma unless very large

Age-Based Triage Summary (Schwartz's Surgery)

Radiographic Red Flags Pointing to Osteosarcoma Specifically

• Metaphyseal location with mixed lytic/sclerotic pattern
• Codman triangle + sunburst/hair-on-end periosteal reaction with tumour osteoid in soft-tissue mass (vs reactive sclerosis in Ewing)
• Extra-osseous soft-tissue mass with amorphous mineralisation
• MRI: skip metastases in same bone (5% of cases)

Bone Tumors in the 2nd Decade of Life (Ages 10–19)

Master Age-Location Table

Malignant Tumors

1. Osteosarcoma (most common primary malignant bone tumor)

• Epidemiology: 75% occur before age 20; peak at adolescent growth spurt; M:F = 1.6:1
• Pathogenesis: Arises near the growth plate of rapidly growing bones; associated with mutations in RB (up to 70%), TP53 (Li-Fraumeni syndrome), CDKN2A, MDM2/CDK4 (low-grade), MYC amplification (poor prognosis)
• Location: Metaphysis of long bones; ~50% near the knee (distal femur, proximal tibia); also proximal humerus
• Radiology: Mixed lytic/sclerotic metaphyseal lesion; Codman triangle (lifted periosteum); sunburst/hair-on-end periosteal reaction; tumour osteoid in soft-tissue mass
• Morphology: Large gritty grey-white tumour with haemorrhage and cystic degeneration; anaplastic cells producing fine lacelike osteoid; pleomorphism, hyperchromatic nuclei, abnormal mitoses
• Subtypes: Conventional (75%), telangiectatic, low-grade central, small cell, periosteal, parosteal, high-grade surface
• Treatment: Neoadjuvant chemotherapy → limb-salvage surgery → adjuvant chemotherapy
• Prognosis: 5-year survival ~70% (non-metastatic); <20% with overt metastases at diagnosis

2. Ewing Sarcoma (second most common bone sarcoma in children)

• Epidemiology: ~80% under age 20; peak 5–15 years; M:F = 1.4:1; rare in non-Caucasians
• Pathogenesis: >90% have balanced translocation t(11;22) → EWSR1-FLI1 fusion gene, a chimeric transcription factor that dysregulates chromatin; minority involve other FET/ETS fusions. Cell of origin: possibly mesenchymal stem cell or primitive neuroectodermal cell
• Location: Medullary cavity of diaphysis (especially femur); also flat bones (pelvis, ribs, scapula); ~20% extraskeletal
• Radiology: Destructive lytic permeative lesion; classic "onion-skin" (laminated) periosteal reaction; large soft-tissue mass; sclerosis = reactive (not tumour osteoid — key distinction from osteosarcoma)
• Morphology: Sheets of uniform small round blue cells with scant clear (glycogen-containing) cytoplasm; Homer-Wright pseudorosettes when neuroectodermal differentiation present; PAS positive (glycogen)
• Clinical mimicry: Fever, elevated ESR, leukocytosis → can mimic osteomyelitis
• Treatment: Neoadjuvant chemotherapy + surgery ± radiotherapy
• Prognosis: Chemotherapy-induced necrosis is a key prognostic indicator; 5-year survival ~70% (localised)

Benign Tumors

3. Osteochondroma (Exostosis) (most common benign bone tumor overall)

• Cartilage-capped bony outgrowth from the bone surface; arises only in bones of endochondral origin
• Location: Metaphysis near growth plate, especially around the knee
• 85% solitary and sporadic; remainder = Multiple Hereditary Exostoses (MHE) — autosomal dominant, EXT1/EXT2 gene mutations; EXT1 carries higher malignancy risk
• Risk of malignant transformation to secondary chondrosarcoma: ~1% (solitary) vs ~10% (MHE); suggested by pain in a previously painless lesion or growth after skeletal maturity
• Grows parallel to the bone; stops growing when physis closes

4. Chondroblastoma

• Distinctive epiphyseal location in skeletally immature patients (<30 years; peak 2nd decade)
• Common sites: proximal humerus, distal femur, proximal tibia, proximal femur
• Radiology: Well-circumscribed lytic lesion with sclerotic rim; "stippled" chondroid calcifications; MRI shows massive surrounding marrow/soft tissue oedema disproportionate to lesion size
• Histology: Chondroblasts surrounded by "chicken-wire" calcification in a lace-like pattern; scattered osteoclast-type giant cells
• Lung metastases in <1%
• Treatment: Intralesional curettage and reconstruction
• Key differential from GCT: patient age, open physis, epiphyseal location (both epiphyseal but GCT = skeletally mature)

5. Osteoid Osteoma

• Small (<1.5 cm) intracortical benign tumour; peak: 2nd decade, strong male predominance
• Classic symptom: Night pain, dramatically relieved by aspirin or NSAIDs (prostaglandin-mediated)
• Radiology: Small lucent nidus surrounded by abundant reactive cortical sclerosis; CT is investigation of choice
• Histology: Anastomosing trabeculae of woven bone rimmed by osteoblasts in a vascular fibrous stroma
• Treatment: CT-guided radiofrequency ablation (RFA) is the current standard of care

6. Osteoblastoma

• Essentially a "giant osteoid osteoma" (>2 cm); similar histology but more expansile and locally aggressive
• Predilection for posterior elements of the spine but also long bones
• Pain is not reliably relieved by NSAIDs (distinguishes it from osteoid osteoma)
• Can rarely undergo malignant transformation

7. Aneurysmal Bone Cyst (ABC)

• Benign neoplasm with multiloculated blood-filled spaces; most cases present in adolescence
• Genetics: Rearrangements of chromosome 17p13 → USP6 gene fusion (most commonly with CDH11) → NF-kB activation → matrix metalloprotease upregulation → cystic bone resorption
• Location: Femur, tibia, posterior vertebral elements (lamina, pedicles)
• Radiology: Expansile, eccentric, lytic with thin cortical shell; fluid-fluid levels on MRI are hallmark
• Grossly: Hemorrhagic, sponge-like
• Can be secondary (arising within GCT, chondroblastoma, telangiectatic osteosarcoma) — biopsy is therefore mandatory
• Treatment: Curettage ± bone grafting; high recurrence rate

8. Non-Ossifying Fibroma (NOF) / Fibrous Cortical Defect

• Most common "tumour-like" lesion in children; found in up to 30% of children >2 years on radiographs
• Genetics: Activating mutations in KRAS and FGFR1; NF1 mutations in NF1/Jaffe-Campanacci syndrome — confirms these are true neoplasms
• Location: Eccentric, metaphyseal, cortical in distal femur, proximal/distal tibia
• Radiology: Sharply demarcated radiolucent lesion with sclerotic rim; never mineralised
• Histology: Bland fibroblasts in storiform (pinwheel) pattern; foamy macrophages, giant cells, haemosiderin
• Usually asymptomatic, incidental; resolves spontaneously
• Large lesions (>50% cortical width) risk pathological fracture — curettage ± bone graft

9. Fibrous Dysplasia

• Benign neoplasm with disorganised, inadequately mineralised bone and intervening fibrous tissue; arises during skeletal development
• Genetics: Somatic gain-of-function mutations in GNAS1 (fetal period) → constitutive Gs activation → elevated cAMP → altered osteoblast differentiation
• Forms: Monostotic (most common) | Polyostotic | McCune-Albright syndrome (polyostotic + café-au-lait spots + precocious puberty) | Mazabraud syndrome (fibrous dysplasia + soft-tissue myxomas)
• Radiology: "Ground-glass" matrix; "shepherd's crook" deformity of proximal femur
• Rarely undergoes sarcomatous transformation

10. Enchondroma

• Well-circumscribed medullary hyaline cartilage nodule; IDH1/IDH2 mutations → 2-hydroxyglutarate oncometabolite
• Usually small (<3 cm), gray-blue, translucent; benign chondrocytes with peripheral endochondral ossification
• Syndromic variants: Ollier disease (enchondromatosis, more cellular/atypical) and Maffucci syndrome (enchondromatosis + spindle cell haemangiomas + risk of brain glioma/other malignancies)
• Malignant transformation to chondrosarcoma is rare for solitary lesions but higher in syndromic cases

Mnemonic for Bone Tumour Location by Age (Schwartz's Surgery)

Radiographic Framework for the 2nd Decade