Chondroblastoma

Chondroblastoma is a rare, benign cartilage-producing bone neoplasm classified as benign in the 2020 WHO Classification of Soft Tissue and Bone Tumours.[1] It accounts for approximately 1%–2% of all primary bone tumors.[2][3] The tumor characteristically arises in the epiphyses and apophyses of long bones in skeletally immature patients, but craniofacial involvement—including the temporal bone, mandible, temporomandibular joint (TMJ), and maxilla—occurs in an estimated 1%–7% of cases.[2][4] Pathogenesis is linked to a recurrent somatic H3F3B p.K36M (H3.3K36M) point mutation identified in approximately 95% of chondroblastomas, which reprograms the H3K36 methylation landscape and alters chondrocyte differentiation.[5][6] In the craniofacial skeleton, the tumor may originate from embryonal cartilaginous rests entrapped in sutures of bones formed by endochondral ossification.[2] The signature radiographic pattern is a well-defined osteolytic lesion with a sclerotic rim and variable internal calcification. Treatment is surgical, with intralesional curettage as the standard approach, though craniofacial lesions carry a notably higher recurrence rate than appendicular tumors.[4]

• Current WHO designation: Chondroblastoma, classified as a benign chondrogenic tumor in the 5th edition WHO Classification of Soft Tissue and Bone Tumours (2020).[1]
• Accepted synonyms: Codman tumor (historical eponym, after Ernest Codman's early descriptions of epiphyseal chondromatous giant cell tumors).
• Obsolete terms: "Epiphyseal chondromatous giant cell tumor," "calcifying giant cell tumor"—these older designations may appear in archival literature but are no longer recommended.
• The 2020 WHO reclassified chondroblastoma from "intermediate (locally aggressive)" to benign, reflecting its very low metastatic rate.[1][7]

• Frequency: 1%–2% of all primary bone tumors.[2][3]
• Age: In long bones, peak incidence occurs in the second decade (mean ~16–18 years). Craniofacial chondroblastomas present at a significantly older mean age (~43.5 years in the largest skull/facial series of 30 cases). Adult-onset tumors (≥20 years) in general show a shift toward flat bones and short tubular bones.[4][8][9][10]
• Sex: Male predominance with a male-to-female ratio of approximately 2–3:1 across most series.[9][10][11]
• Anatomic distribution:
  • Long bones (proximal tibia, proximal and distal femur, proximal humerus) account for ~80% of all cases.[8]
  • Craniofacial skeleton: the temporal bone (squamous portion) is the most common craniofacial site; the mandible is the second most common, followed by the TMJ region and maxilla. Maxillary sinus involvement is exceedingly rare.[2][4][12]

Presentation and symptoms

• Pain is the most common presenting complaint in appendicular lesions (~73%–84% of patients), with a mean symptom duration of approximately 8–9 months before diagnosis.[9][13]
• Craniofacial chondroblastomas are often characterized by slow, initially asymptomatic growth until the mass reaches sufficient size to produce symptoms dictated by location:[14]
  • Temporal bone lesions: hearing loss, preauricular or temporal swelling, and occasionally middle ear symptoms.[2]
  • Mandibular and TMJ lesions: facial swelling, pain, limited jaw opening, and TMJ dysfunction.
  • Maxillary lesions: nasal obstruction, facial asymmetry.[12]

Examination findings

• Palpable, firm swelling over the affected region.
• Temporal bone tumors may extend into the temporal fossa, infratemporal fossa, TMJ, and tympanic cavity.[2]

Tempo and complications

• Growth is typically slow, but the tumor can behave in a locally aggressive fashion, invading surrounding structures.[2][14]
• Secondary aneurysmal bone cyst (ABC) formation occurs in a substantial minority of cases (reported in up to ~64% histologically in one large series), which may accelerate clinical presentation.[8][15]
• Rare pulmonary metastasis has been documented.[7][16]

Location and distribution in the jaws

• In the craniofacial skeleton, the squamous portion of the temporal bone is the most frequent site; the adjacent sphenoid bone is often co-involved, placing the tumor in the middle cranial fossa. The mandible (condylar process and ramus) is the next most common jaw site, followed by the maxilla.[2][4]
• In long bones, the tumor is characteristically epiphyseal or apophyseal in origin, always related to a growth plate; approximately half remain confined to the epiphysis, while many extend across the physis into the adjacent metaphysis.[8][10]

Periphery and shape

• The hallmark is a well-defined, geographic osteolytic lesion with a narrow zone of transition and a peripheral rim of sclerosis.[8][10]
• Margins are typically sharply demarcated and may appear lobulated.[17]
• Most craniofacial and flat-bone lesions maintain well-defined borders but show a greater tendency toward osseous expansion compared with long-bone counterparts.[8]
• Lodwick classification: the majority grade as 1A or 1B (geographic destruction with sclerotic or well-defined margins), consistent with benign biologic behavior.[18]

Internal architecture

• The lesion is predominantly radiolucent (osteolytic).
• Internal calcification is present in approximately one-third of long-bone cases on radiographs and in up to 50% of temporal bone cases on CT. Calcification patterns include:[2][8]
  • Fine, stippled or flocculent densities (reflecting chondroid matrix mineralization)[2][19]
  • Curvilinear calcification
  • Ground-glass opacity pattern (occasionally in temporal bone lesions)[2]
• When a secondary ABC component is present, the lesion may show multicystic spaces with fluid–fluid levels and internal septations, sometimes producing a soap-bubble appearance.[20][21]

Effects on teeth and adjacent structures

• Mandibular lesions may displace or resorb adjacent teeth, though tooth involvement is infrequently described given the rarity of jaw cases.
• Condylar process involvement can produce TMJ dysfunction and may extend into the glenoid fossa.[2]
• Temporal bone tumors commonly invade the temporal fossa, infratemporal fossa, TMJ, and tympanic cavity.[2]

Growth pattern and cortical involvement

• Slow, expansile growth with cortical thinning and remodeling is typical.[8]
• Periosteal reaction is common (seen in ~83% of long-bone cases on imaging) and does not necessarily indicate cortical breakthrough.[18][22]
• Despite benign classification, cortical erosion and extension into adjacent soft tissues can occur, particularly in craniofacial lesions.[2][23]
• Temporal bone tumors characteristically produce smooth compression of the adjacent temporal lobe rather than irregular invasion, reflecting the slow growth rate.[2]

Variants

• Secondary ABC-dominant variant: Extensive cystic change with fluid–fluid levels may obscure the underlying chondroblastoma, producing a predominantly multicystic radiolucent lesion with minimal or absent matrix mineralization. This pattern is a well-recognized diagnostic pitfall.[20]
• Aggressive variant: Rare cases show permeative growth, cortical destruction, and soft-tissue extension; these may correspond to the recently characterized "malignant chondroblastoma," which occurs in older patients and favors flat bones.[23][24]

CT (medical CT)

• Noncalcified portions of temporal bone chondroblastomas demonstrate high attenuation on CT, a feature attributed to dense cellularity, hemosiderin, and calcification.[2]
• CT delineates the extent of bony destruction, involvement of the skull base foramina, and relationship to the middle cranial fossa more precisely than radiographs.[14][22]
• Variable calcification patterns (flocculent, curvilinear, ground-glass) are better characterized on CT than on plain films.[2]

MRI

• Signal intensity is characteristically heterogeneous:
  • T1-weighted: isointense to low signal.[2]
  • T2-weighted: predominantly low to intermediate signal intensity, a relatively distinctive feature among bone tumors; areas of high T2 signal correspond to secondary ABC components or hyaline cartilage islands.[2][17][18]
• Perilesional bone marrow edema is prominent in the majority of cases (~77%–92%) and is more conspicuous on MRI than on other modalities.[10][17][18]
• Enhancement is variable (mild to strong) after gadolinium administration.[2][18]
• Lobular internal architecture and fine lobular margins are well delineated on high-resolution T2-weighted sequences.[17]
• The combination of low-to-intermediate T2 signal, heterogeneity, and prominent surrounding edema can mimic infection or aggressive neoplasm if the diagnosis is not considered.[17]

FDG-PET

• Temporal bone chondroblastomas show hypermetabolism on FDG-PET (SUV values reported up to 16.4), which can be misleading and suggest malignancy.[2]

Chondroblastoma of the jaws and craniofacial skeleton is rare, and its radiographic features overlap with several other osteolytic lesions. The differential is guided by location (epiphyseal/apophyseal predilection, temporal bone, condylar region), internal calcification, and patient age.

The highest-risk diagnostic pitfall is misidentifying a chondroblastoma with extensive secondary ABC change as a primary ABC or giant cell tumor, particularly on limited biopsy material. H3.3 K36M immunohistochemistry resolves this distinction with high sensitivity (96%–100%) and specificity.[20][26]

Diagnostic microscopic features

• Proliferation of uniform, round to polygonal mononuclear chondroblasts with well-defined cytoplasmic borders, eosinophilic to clear cytoplasm, and ovoid nuclei with characteristic longitudinal nuclear grooves.[7][10]
• "Chicken-wire" (pericellular) calcification surrounding individual chondroblasts is a classic finding, present in ~26% of cases.[15][29]
• Scattered osteoclast-like multinucleated giant cells are interspersed among the mononuclear population.[7]
• Variable amounts of chondroid matrix and areas of mature cartilage.[7]
• Secondary ABC-like changes (blood-filled cystic spaces with fibrous septa) are common, reported in up to ~64% of cases, and may dominate the histologic picture.[15][20]

Atypical features

• Prominent hemosiderin deposition, numerous giant cells, mitotic activity, and focal necrosis may be encountered but do not reliably predict recurrence.[7][30][31]

Immunohistochemistry and molecular findings

• H3.3 K36M immunohistochemistry is positive in ~95%–100% of chondroblastomas and is the most sensitive and specific diagnostic marker, particularly valuable in cases with prominent ABC-like changes or limited biopsy material.[5][15][20]
• The underlying mutation is a heterozygous H3F3B p.K36M (rarely H3F3A p.K36M) point mutation, which inhibits H3K36 methyltransferases SETD2 and NSD2, reprogramming the epigenome.[5][6]
• DOG1 and SOX9 immunoreactivity have been described but are less sensitive than H3.3 K36M.[7][26]
• S-100 protein is variably positive.[29][31]

Radiologic-pathologic correlation

• Areas of low T2 signal on MRI correspond to dense cellularity, hemosiderin, and calcification; high T2 signal areas correlate with secondary ABC components or hyaline cartilage.[2][18]

Malignant chondroblastoma

• A recently characterized, rare variant occurring in older patients (median ~39–52 years), favoring flat bones, with cytologic atypia and permeative growth. It retains the H3.3 K36M mutation but shows a distinct DNA methylation profile and carries risk of metastasis and death from disease.[23][24]

Workup and referral

• Radiographic evaluation with plain films and CT (or CBCT for jaw lesions) for initial characterization; MRI for soft-tissue extent and relationship to neurovascular structures.[22][32]
• Referral to an oral and maxillofacial surgeon or head-and-neck surgical oncologist for craniofacial lesions; orthopedic oncology for appendicular lesions.
• Biopsy with H3.3 K36M immunohistochemistry is essential for definitive diagnosis, especially when imaging is nonspecific or ABC-like changes predominate.[20][26]

Definitive treatment

• Intralesional curettage (with or without adjuvants) is the standard of care.[3][9][11]
  • Adjuvants include high-speed burr, phenol, cryotherapy, hydrogen peroxide, and radiofrequency ablation; their benefit in reducing recurrence remains unproven.[3]
  • The resultant cavity is filled with autograft, allograft, bone substitute, or bone cement.[3][11]
• En bloc resection is reserved for expendable bones, recurrent or aggressive lesions, and rare malignant variants.[16][33]
• For craniofacial lesions, the surgical approach is dictated by tumor location and extent (e.g., preauricular approach for temporal bone/infratemporal fossa tumors).[14]

Recurrence

• Overall recurrence rates vary widely: 3%–32% across series, depending on location, surgical technique, and follow-up duration.[3][9][13][31]
  • Large multicenter series report rates of ~5%–10% after thorough curettage with adjuvants.[9][11][16]
  • Craniofacial lesions carry a higher recurrence rate: approximately 50% after curettage in the largest skull/facial series.[4]
• Mean time to recurrence is approximately 14 months.[16]
• Recurrence is managed with repeat curettage or wider resection, with high ultimate cure rates.[4][31]

Prognosis

• Prognosis is excellent for conventional chondroblastoma; functional outcomes after curettage are generally very good (mean MSTS score ~29/30).[11][16]
• Pulmonary metastasis is exceedingly rare.[7][16]
• Malignant chondroblastoma carries a substantially worse prognosis, with high rates of local recurrence (~50%), metastasis (~31%), and disease-related death.[23][24]
• Growth plate disturbance and angular deformity are concerns in skeletally immature patients with epiphyseal lesions.[3]

Pediatric and skeletal maturity considerations

• In skeletally immature patients, the proximity of chondroblastoma to the growth plate poses a risk of physeal damage during curettage, potentially resulting in angular deformity or limb-length discrepancy. Proximal tibial lesions appear particularly susceptible (incidence ~17% in one series).[3][13]
• Strictly epiphyseal lesions in children carry a higher recurrence risk compared with metaphyseal or apophyseal tumors.[13]

Distinction from malignant chondroblastoma

• Malignant chondroblastoma is a recently defined, epigenetically distinct variant with a predilection for flat bones in skeletally mature individuals (median age ~39 years). It retains the H3.3 K36M mutation but shows a unique DNA methylation profile, frequent copy number alterations, and occasional SETD2 co-mutations. A subset transforms into high-grade pleomorphic sarcoma. Recognition of this entity is important because it requires more aggressive surgical management and closer surveillance than conventional chondroblastoma.[23][24]