Giant-cell tumor of bone

Giant-cell tumor of bone (GCTB) is a locally aggressive, osteolytic neoplasm of intermediate malignancy with low metastatic potential, classified under ICD-O code 9250/1 by the 2020 WHO Classification of Bone Tumours.[1][2] The tumor is driven by somatic missense mutations in the H3F3A gene (most commonly p.G34W), which are restricted to the neoplastic mononuclear stromal cells and are considered the diagnostic hallmark.[3][4] GCTB characteristically arises in the meta-epiphyseal region of long bones in skeletally mature individuals and presents radiographically as an eccentric, expansile, purely lytic lesion extending to the subchondral bone plate.[5][6] The jaws are rarely involved; when a giant cell–rich lesion arises in the mandible or maxilla, central giant cell granuloma (CGCG) is far more common and is a molecularly distinct entity driven by RAS/MAPK pathway mutations rather than H3F3A alterations.[7][8] Surgical excision is the primary treatment, with denosumab (a RANKL inhibitor) reserved for unresectable or morbid-resection cases.[9][10]

• Current WHO designation (2020): Giant cell tumour of bone (GCTB).[1][2]
• Accepted synonyms: Giant-cell tumor of bone; osteoclastoma (historical, still occasionally encountered).
• Obsolete or discouraged terms: "Benign giant cell tumor" — discouraged because the WHO now classifies GCTB as intermediate (locally aggressive, rarely metastasizing) rather than benign.[1]
• Important distinction from jaw lesions: The 5th edition WHO Classification of Head and Neck Tumours separates giant cell–rich lesions distinctive to the jaws (central giant cell granuloma, brown tumor, cherubism) from the bone tumor classification; these are listed under odontogenic and maxillofacial bone tumours and are molecularly distinct from GCTB.[7]

• GCTB accounts for approximately 4–5% of all primary bone tumors.[1][11]
• Peak incidence occurs in the third and fourth decades of life; the tumor is rare before skeletal maturity and uncommon in patients with open physes.[12][13]
• A slight female predominance is reported (approximately 1.5:1 female-to-male ratio), though some series show near-equal distribution.[12][14]
• Approximately 50% of cases arise around the knee (distal femur and proximal tibia); other common sites include the distal radius, proximal humerus, and sacrum.[12][13]
• Craniofacial involvement is rare; when GCTB does arise in the skull, the sphenoid and temporal bones are the most frequent sites. True GCTB of the mandible or maxilla is exceedingly uncommon and must be distinguished from the far more prevalent CGCG.[6][15][16]
• Multicentric GCTB is rare and may be associated with Paget disease of bone.[6][17]

Symptoms and presentation

• Pain is the most common presenting symptom, typically localized, progressive, and activity-related.[12][18]
• Swelling develops as the tumor expands cortical bone, and a palpable mass may be evident in superficial locations.
• Decreased range of motion occurs when the tumor involves a peri-articular region.[12]
• Pathological fracture is present on initial radiographs in approximately 9% of cases and may be the presenting event.[13]

Tempo and complications

• Growth is typically progressive over weeks to months.
• Spinal and sacral lesions may produce neurological deficits from cord or nerve root compression.[12]
• Rare pulmonary metastases (histologically benign) occur in a small minority of patients; these may remain stable for years but occasionally progress.[5][14]
• Malignant transformation to high-grade sarcoma (osteosarcoma or undifferentiated pleomorphic sarcoma) is uncommon and is most often secondary to prior radiation therapy or multiple recurrences.[6][14]

Red flags

• Rapid enlargement, new neurological deficit, or a destructive lesion with aggressive periosteal reaction should raise concern for malignant transformation or an alternative diagnosis.

Location and distribution

• The characteristic site is the meta-epiphyseal region of a long bone, with extension to the subchondral bone plate; purely metaphyseal lesions without epiphyseal involvement are occasional.[5][13]
• In the jaws, true GCTB is exceedingly rare. When encountered, the mandible is more commonly affected than the maxilla. The vast majority of giant cell–rich jaw lesions are CGCGs, not GCTB.[16][19]
• Axial skeleton involvement (spine, sacrum, pelvis) is less common but well recognized; sacral GCTB may cross the sacroiliac joint or invade adjacent vertebrae.[20]

Periphery and shape

• The lesion is typically well-defined but nonsclerotic (geographic, Lodwick grade IB).[5][21]
• The margin is sharp without a sclerotic rim in most cases; a thin sclerotic border may occasionally be seen in less aggressive or treated lesions.[13][18]
• The shape is eccentric and expansile, often producing a "soap-bubble" or lobulated contour as the tumor remodels overlying cortex.[5][6]

Internal architecture

• The hallmark is a purely lytic (radiolucent) lesion with no intrinsic mineralized matrix.[5][6]
• Fine internal trabeculations or pseudoseptations may create a multiloculated appearance, but true bony septa are uncommon.[5]
• Secondary aneurysmal bone cyst (ABC) components, manifesting as cystic spaces with fluid-fluid levels, are present in approximately 14% of cases.[5][6]
• No periosteal reaction is expected in uncomplicated GCTB; its presence suggests pathological fracture, aggressive behavior, or an alternative diagnosis.[5][18]

Effects on teeth and adjacent structures

• In the rare jaw occurrence, GCTB may displace or resorb adjacent tooth roots and expand the cortical plates, similar to other aggressive central jaw lesions.[16][19]
• In long bones, the tumor characteristically extends to the articular surface without crossing the joint cartilage; trans-articular extension to an adjacent bone is reported in approximately 5% of cases.[13]

Growth pattern and cortical involvement

• Cortical thinning and expansion are common; cortical breakthrough with soft-tissue extension indicates aggressive behavior (Campanacci grade III).[13][22]
• The Campanacci radiographic grading system stratifies GCTB into three grades:[13]
  • Grade I: Well-marginated lesion with intact cortex (rare).
  • Grade II: Expanded but intact cortex with a well-defined margin (most common).
  • Grade III: Cortical destruction with soft-tissue extension (associated with higher recurrence).

Recognized variants and atypical presentations

• Multicentric GCTB: Rare; may be synchronous or metachronous and is associated with Paget disease.[6][17]
• GCTB without giant cells: A rare histologic variant that retains the H3F3A mutation but lacks the characteristic multinucleated giant cells, making radiologic and histologic diagnosis challenging.[1]
• Post-denosumab appearance: Treatment produces peripheral osteosclerosis, intralesional ossification, and cortical reconstitution, which may obscure the original tumor margin and complicate surgical planning.[11][23]

CT

• CT delineates cortical integrity, the presence and extent of cortical breakthrough, and any soft-tissue component more precisely than radiography.[6][11]
• The solid tumor shows soft-tissue attenuation without mineralized matrix; cystic areas appear as low-attenuation foci.[20]
• Quantitative CT density measurements are used to monitor denosumab response, with progressive re-ossification indicating a favorable treatment effect.[24]

MRI

• The solid component characteristically demonstrates low to intermediate signal intensity on T2-weighted sequences, a feature that helps distinguish GCTB from many other bone tumors.[6]
• On T1-weighted images, the tumor is isointense to slightly hypointense relative to muscle; heterogeneous enhancement is seen after gadolinium administration.[20][25]
• Fluid-fluid levels within secondary ABC components are well depicted on MRI.[5][18]
• Cystic changes, penetrating irregular margins, and adjacent soft-tissue invasion on preoperative MRI have been associated with a higher risk of local recurrence.[11]
• A well-defined "multiple shells" pattern on MRI has been described as characteristic.[26]

Nuclear medicine

• On bone scintigraphy, GCTB classically shows a "doughnut sign" — peripheral increased uptake with a relatively photopenic center.[25]
• ¹⁸F-FDG PET/CT demonstrates intense, heterogeneous uptake; reduction in FDG avidity is an early sensitive marker of response to denosumab.[23][25]

Giant cell–rich lytic lesions of the jaws and long bones share overlapping radiographic and histologic features. The critical first step is to determine whether the lesion arises in the jaw (where CGCG predominates) or in a long bone (where GCTB is the leading consideration). Serum calcium and parathyroid hormone levels should be obtained for any giant cell–containing bone lesion to exclude brown tumor.[9][27]

The highest-risk diagnostic pitfall is misidentifying a brown tumor as GCTB (or vice versa), as management differs fundamentally. Serum calcium and PTH should be checked before biopsy in every case, and H3.3 G34W immunohistochemistry should be performed on all giant cell–rich bone lesion biopsies to confirm or exclude GCTB.[4][9]

Microscopic findings

• GCTB is composed of two principal cell populations: neoplastic mononuclear stromal cells (ovoid to spindle-shaped) and numerous reactive multinucleated osteoclast-like giant cells evenly interspersed throughout the lesion.[12][33]
• The giant cells are non-neoplastic and are recruited from the monocyte-macrophage lineage via RANKL overexpression by the stromal cells.[34][35]
• The stromal cells are the true neoplastic component and are responsible for the proliferative capacity of the tumor.[33][36]

Key diagnostic markers

• H3F3A mutation (predominantly p.G34W, rarely p.G34L/M/R/V) is present in >95% of GCTB and is restricted to the mononuclear stromal cells.[3][4]
• H3.3 G34W immunohistochemistry is a highly sensitive and specific surrogate marker; variant mutations (G34V, G34R, G34L) require additional antibodies or sequencing for detection.[4][37]
• H3.3 G34 alterations are absent in all histologic mimics (brown tumor, CGCG, ABC, giant cell–rich osteosarcoma) except sarcomas arising from pre-existing GCTB.[4][14]

Radiologic-pathologic correlation

• The purely lytic radiographic appearance reflects the intense osteoclastic bone resorption driven by RANKL-RANK signaling.[34][38]
• Secondary ABC components correspond to cystic, blood-filled spaces within the tumor; biopsy must target the solid regions to obtain diagnostic tissue.[5][6]
• After denosumab therapy, giant cells are depleted and the stromal cells undergo osteogenic differentiation, producing woven bone and a sclerotic radiographic appearance — but the H3F3A-mutant neoplastic cells persist.[35][39]

Malignant transformation

• Malignant GCTB most commonly shows osteosarcomatous or chondrosarcomatous differentiation and retains the H3F3A mutation.[14]
• Complex chromosomal copy number alterations distinguish malignant GCTB from conventional, atypical, and metastasizing conventional tumors.[14]

Workup and referral

• NCCN guidelines recommend history and physical examination, imaging of the primary site (radiographs and MRI with and without contrast ± CT), chest imaging, optional bone scan, and biopsy to confirm diagnosis.[9]
• Serum calcium and PTH should be obtained to exclude brown tumor.[9]
• Referral to a musculoskeletal oncology center is appropriate for all suspected GCTB.

Definitive treatment

• Surgical excision is the primary treatment for resectable disease; intralesional excision (curettage) with an effective local adjuvant (high-speed burr, phenol, liquid nitrogen, or polymethylmethacrylate cement) is the preferred joint-sparing approach.[9][34]
• En bloc resection is indicated when joint salvage is not feasible (e.g., extensive articular destruction, intra-articular fracture with soft-tissue component).[13][34]
• Denosumab (120 mg subcutaneously) is the preferred systemic agent for unresectable disease, lesions where resection would cause unacceptable morbidity, and as neoadjuvant therapy to facilitate less morbid surgery.[9][10]
• Radiation therapy (40–55 Gy) is reserved for rare cases where surgery is impossible and RANKL inhibitors are contraindicated, given the risk of radiation-induced malignant transformation.[9][34]
• Serial embolization is an alternative or adjunct for axial and sacral lesions.[9]

Recurrence

• Local recurrence rates after curettage with adjuvants range from approximately 8–27%; recurrence after en bloc resection is substantially lower (0–5%).[5][13][34]
• A recent meta-analysis found that perioperative denosumab use was associated with higher