= OSTEOCLASTOMA = OSTEOBLASTOCLASTOMA = TUMOR OF MYELOPLEXUS

= nonmineralized eccentric lytic meta-epiphyseal lesion involving a long bone with extension to subarticular bone in the skeletally mature patient (= with closed physis)

Origin: probably arises from zone of intense osteoclastic activity (of endochondral ossification) in skeletally immature patients

Frequency: 5% of all primary bone tumors; 20% of benign skeletal tumors; unusually high prevalence in China + southern India

Path: friable vascular stroma of numerous thin-walled capillaries with necrosis + hemorrhage + cyst formation (DDx: aneurysmal bone cyst without solid areas)

Histo: large number of multinucleated osteoclastic giant cells in a diffuse distribution in a background of ovoid mononuclear cells intermixed throughout a spindle cell stroma (DDx: giant cells characteristic of all reactive bone disease as in pigmented villonodular synovitis, benign chondroblastoma, nonosteogenic fibroma, chondromyxoid fibroma, fibrous dysplasia)

Peak age: 3^rd (range, 2^nd–4^th) decade; <3% below age 14; 80% between 20 and 50 years; 13% >age 50; M÷F = 1÷1.1 to 1÷1.5 (in spine 1÷2.5)

May be associated with: Paget disease located in skull + facial bones (50–60%), pelvis, spine

Hormonal stimulation: occasionally dramatic increase in size during pregnancy

Staging:

• Stage 1 indolent radiographic + histologic appearance (10–15%)
• Stage 2 more aggressive radiographic appearance with expansile remodeling (70–80%):
  • wide zone of transition
  • cortical thinning
  • expansile remodeling
  • cortical bone destruction
• Stage 3 extension into adjacent soft tissues with histologically benign appearance (10–15%)

• pain at affected site (most common – in 10% pathologic fracture)
• local swelling + tenderness
• weakness + sensory deficits (if in spine)

Location:

• long bones (75–90%)
  • lower extremity:
    1. about knee (50%–65%):
       • distal end of femur (23–30%)
       • proximal end of tibia (20–25%)
    2. away from knee: proximal femur (4%) >distal tibia (2–5%) >proximal fibula (3–4%) >foot (1–2%)
       rare: patella (the largest sesamoid bone), greater trochanter (epiphyseal equivalent)
  • upper extremity (away from elbow): distal end of radius (10–12%) >proximal end of humerus (4–8%) >hand & wrist with predilection for metacarpal bones (1–5%)
• flat bones (15%)
  • pelvis: upper sacrum near SIJ (4%), iliac bone (3%) ← Paget disease
    • 2^nd most common primary sacral neoplasm after chordoma!
  • rib (anterior / posterior end)
  • skull (sphenoid bone) ← Paget disease
  • scapula (<1%)
• spine (3–7%): lumbar >thoracic >cervical spine (2^nd most frequent tumor after chordoma); vertebral body and pedicles (21%); frequent involvement of posterior arch

Site:

1. eccentric metaepiphyseal (42–93%) = in metaphysis of long bones adjacent to ossified epiphyseal line
2. extension to within 1 cm of subarticular bone (85–99%) after fusion of epiphyseal plate (MOST TYPICAL)
3. intraarticular extension possible / transarticular spread (rare)

◊The open epiphyseal plate acts as a barrier to tumor growth!

X-Ray:

• well-circumscribed expansile solitary lytic bone lesion:
  • nonsclerotic margin ← aggressive rapid growth
  • narrow (80–90%) / (less well-defined) wide zone of transition
  • large lesions more centrally located
• “soap bubble” appearance (47–60%) = expansile remodeling of multiloculated appearance:
  • NO internal mineralization of tumor matrix
  • prominent trabeculation (33–57%):
    1. reactive with appositional bone growth
    2. pseudotrabeculation of osseous ridges in endosteal scalloping
• periosteal reaction (10–30%)
• cortical penetration / disruption (33–50%):
  • cortical thinning
  • soft-tissue invasion (25%)
  • complete / incomplete pathologic fracture (11–12–37%)
• may cross joint space in long bones (exceedingly rare)
• Spine & sacrum (<3%)
  • destruction of vertebral body with secondary invasion of posterior elements (DDx: ABC, osteoblastoma):
    • purely osteolytic / with cortical expansion
    • absence of mineralization
    • lack of a sclerotic rim at tumor margins
  • frequently vertebral collapse
  • may involve adjacent vertebral disks + vertebrae
  • commonly involving both sides of the midline
  • joint transgression is unusual except for sacroiliac joint (38%) with sacral lesion
  • extraosseous involvement of soft tissues (79%)

NUC:

• diffusely increased uptake on delayed bone scintigraphy
• “doughnut” sign (57%) of central photopenia ← osteolysis / central necrosis
• increased uptake across an articulation + in adjacent joints (62%) ← increased blood flow + disuse osteoporosis and NOT secondary to tumor extension

Angio:

• hypervascular (60–65%) / hypovascular (20%) / avascular (10%) lesion

CT:

• tumor of soft-tissue attenuation similar to muscle with foci of low attenuation (hemorrhage / necrosis):
  • NO matrix mineralization
• well-defined margins ± thin rim of sclerosis (in up to 20%)
• soft-tissue extension (33–44%) usually at metaphyseal end of tumor
• secondary ABC (aneurysmal bone cyst) components of low density with fluid-fluid levels (in up to 14%)
• significant enhancement

MR: (nonspecific!)

• low to intermediate SI on T1WI ± areas of hyperintensity ← recent hemorrhage
• relatively well-defined heterogeneous lesion with low to intermediate SI on T2WI (63–96%) ← increased cellularity + high collagen content + hemosiderin
  • HELPFUL feature to distinguish from other subarticular lesions (solitary subchondral cyst, intraosseous ganglion, Brodie abscess, clear cell chondrosarcoma with hyperintense matrix on T2WI)
• focal aneurysmal bone cyst components with fluid-fluid level (in 14%) in tumor center with marked hyperintensity on T2WI
  • Direct biopsy toward peripheral solid-tissue component to prevent misdiagnosis!
• ± low-signal-intensity margin ← pseudocapsule
• significant enhancement of solid-tissue component

Cx: in <1% malignant transformation into high-grade sarcoma within first 9 years (M÷F = 3÷1) after radiation treatment / spontaneously after 19 years

Prognosis: ± locally aggressive; 10–20–90% recurrence rate within first 3 years after initial treatment

Dx: core needle / open bone biopsy

Rx: arterial embolization before surgery / for palliation; curettage + bone grafting (40–60% recurrence); curettage with filling of void with high-speed burr and polymethylmethacrylate (15–25% recurrence); wide resection (7% recurrence) + reconstruction with allografts / metal prosthesis; radiation therapy for inoperable GCT (39–63% recurrence); denosumab

DDx:

1. Aneurysmal bone cyst (contains only cystic regions WITHOUT enhancing soft-tissue component; in posterior elements of spine)
2. Brown tumor of HPT (lab values, multicentric)
3. Expansile lytic bone metastasis (primary renal / thyroid carcinoma)
4. Plasmacytoma / multiple myeloma
5. Chondroblastoma (extensive surrounding soft-tissue + marrow edema, sclerotic margin, central “rings-and-arcs” matrix)
6. Clear cell chondrosarcoma
7. Osteosarcoma subtypes: telangiectatic, giant cell-rich, fibroblastic
8. Osteoblastoma
9. Nonossifying fibroma
10. Bone abscess
11. Hemangioma
12. Fibrous dysplasia
13. Giant cell reparative granuloma