Giant cell tumor of bone is a primary bone tumor that is generally classified as benign but behaves more aggressively than most benign tumors, destroying bone locally and, in rare cases, spreading to the lungs. It accounts for roughly 4 to 10 percent of all bone tumors and overwhelmingly strikes young adults, typically appearing near the end of a long bone close to a joint.1PubMed Central. Giant cell tumor of bone revisited The tumor gets its name from the large multinucleated cells visible under the microscope, which resemble the body’s normal bone-resorbing cells. Because it sits in a gray zone between benign and malignant, managing it involves balancing the risk of recurrence against the goal of preserving a functioning joint.
Where It Grows and Who It Affects
Giant cell tumor of bone (often shortened to GCTB) has a strong preference for the ends of long bones, specifically the region that in adulthood is called the epiphysis and its neighboring metaphysis. The distal femur, the wide end of the thighbone just above the knee, is the single most common location. After that come the distal radius (near the wrist), the sacrum at the base of the spine, and the proximal humerus (the shoulder end of the upper arm bone).2PubMed Central. Giant cell tumor of bone revisited Most patients are between 20 and 40 years old, and the tumor almost always shows up after skeletal growth is complete, meaning the growth plates have closed. It does occur in children and teenagers, but that is rare enough to prompt extra scrutiny when a biopsy shows giant cell tumor in a skeletally immature patient.3PubMed Central. Giant Cell Tumor of Bone in Skeletally Immature Patients – A Clinical Perspective
In pediatric cases, the imaging picture can differ from adults. In children, the tumor tends to sit in the metaphysis alone rather than crossing into the epiphysis, and in smaller bones like the radius or fibula, it tends to fill the entire width of the bone and expand the cortex outward rather than staying off to one side.4PubMed Central. Characteristics of Giant Cell Tumor of the Bone in Pediatric Patients: Our 18-Year, Single-Center Experience These differences can make the diagnosis harder to suspect on initial imaging.
What Drives the Tumor at a Molecular Level
The neoplastic cells in GCTB are not the giant cells themselves but rather the smaller stromal cells that surround them. These stromal cells carry a highly recurrent mutation in a gene called H3F3A, which encodes a histone protein involved in packaging DNA. The mutation changes one amino acid at a specific position (G34), leading to abnormal gene regulation and increased proliferation of the stromal cells.5PubMed. The two faces of giant cell tumor of bone This mutation is considered the essential oncogenic driver of the tumor, and it is found consistently in both primary and recurrent disease.6PubMed Central. Diagnostic value of H3F3A mutations in giant cell tumour of bone compared to osteoclast‐rich mimics
The giant cells themselves are osteoclast-like, meaning they behave like the body’s normal bone-destroying cells. The stromal cells recruit them by producing a signaling protein called RANKL, which is a key trigger for osteoclast formation and activation. Early research assumed the stromal cells were the dominant source of RANKL, but expression profiling revealed that the osteoclast-like giant cells themselves also express RANKL at high levels, complicating the picture somewhat.7The American Journal of Pathology. Molecular Profiling of Giant Cell Tumor of Bone and the Osteoclastic Localization of Ligand for Receptor Activator of Nuclear Factor κB The practical upshot is the same, though: RANKL signaling is central to the bone destruction these tumors cause, and blocking it became a therapeutic target.
How It Looks on Imaging
On a standard X-ray, GCTB typically appears as a lytic lesion, meaning a dark area where bone has been eaten away. The lesion sits at the end of the bone and extends to or very near the joint surface. It usually appears off-center within the bone and has well-defined but non-hardened borders, a pattern radiologists call geographic destruction. There is generally no periosteal reaction (the kind of new bone layering you see with aggressive infections or high-grade malignancies) unless the tumor has caused a fracture through the weakened bone.8PubMed Central. Imaging of giant cell tumor of bone As the tumor grows larger, it tends to become more centrally located and may balloon out the cortex rather than staying eccentric.9PubMed. Giant cell tumor of bone: review, mimics, and new developments in treatment
MRI adds soft-tissue detail that X-rays miss, including how far the tumor extends into surrounding structures and whether there are fluid-filled cystic areas. CT scanning is useful for surgical planning because it shows the remaining cortical shell and helps determine whether a joint-preserving approach is feasible. Distinguishing GCTB from look-alikes such as aneurysmal bone cysts can be tricky on imaging alone, and newer approaches using CT texture analysis are being explored as quantitative tools to help tell them apart.10Clinical Radiology. Differentiation of giant cell tumours of bone, primary aneurysmal bone cysts, and aneurysmal bone cysts secondary to giant cell tumour of bone
Confirming the Diagnosis
Imaging is suggestive but not conclusive. A tissue sample, obtained through core needle biopsy or at surgery, is needed. Under the microscope, the hallmark is a mixture of mononuclear stromal cells and large multinucleated osteoclast-like giant cells. The difficulty is that several other bone tumors also contain giant cells, including aneurysmal bone cysts, chondroblastomas, and brown tumors from hyperparathyroidism. To narrow the diagnosis, pathologists can use an immunohistochemical stain that detects the specific mutant histone protein (H3G34W) produced by the H3F3A mutation. This stain is highly specific for GCTB among giant-cell-rich bone tumors, meaning it can reliably confirm the diagnosis even when the biopsy sample is small.11PubMed Central. Immunohistochemistry for histone H3G34W and H3K36M is highly specific for giant cell tumor of bone and chondroblastoma, respectively, in FNA and core needle biopsy
Surgery and the Recurrence Problem
Because most GCTBs occur near joints in young adults, treatment philosophy generally favors preserving the joint whenever possible. The standard joint-preserving approach is intralesional curettage: the surgeon scrapes out the tumor cavity, often using adjuvants like liquid nitrogen (cryotherapy), phenol, or high-speed burring to kill residual tumor cells at the cavity walls, and then fills the defect with bone cement or bone graft. The alternative is wide (en bloc) resection, which removes the entire tumor-bearing segment of bone, usually requiring reconstruction with a prosthesis or allograft.
The trade-off is straightforward: curettage preserves far better function but carries a substantially higher recurrence rate. In one large multicenter study of tumors around the knee, the overall recurrence rate was about 23 percent, but it jumped to 53 percent in patients treated with intralesional curettage alone, compared to much lower rates with en bloc resection.12Scientific Reports. Recurrence Rates and Risk Factors for Primary Giant Cell Tumors around the Knee: A Multicentre Retrospective Study in China Similarly, another analysis found wide resection had a recurrence rate of about 5 percent versus 25 percent for intralesional surgery. That study also found that using bone cement (polymethylmethacrylate) to fill the cavity after curettage reduced recurrence compared with bone graft alone, while phenol by itself did not show a clear benefit.13PubMed Central. Giant Cell Tumor of Bone: Risk Factors for Recurrence
Beyond surgical technique, other factors influence recurrence. Soft-tissue invasion at the time of diagnosis and the presence of a pathological fracture are both associated with significantly higher rates of local recurrence.14Current Orthopaedic Practice. Risk factors for local recurrence of giant cell tumor of bone of the extremities: a retrospective study Tumor location matters too: tumors in the proximal fibula appear to recur at far higher rates than those in the distal femur.15Scientific Reports. Recurrence Rates and Risk Factors for Primary Giant Cell Tumors around the Knee: A Multicentre Retrospective Study in China Recurrences typically show up within the first two to three years after surgery, so close follow-up imaging during that window is standard practice.
Functional Outcomes After Treatment
Given that these tumors tend to occur near weight-bearing joints in people in their twenties and thirties, long-term function is a real concern. Studies comparing outcomes show a clear advantage for curettage over wide resection when it comes to day-to-day function. In one long-term review of lower-limb tumors, patients treated with intralesional curettage scored significantly higher on standardized musculoskeletal function scales than those who underwent wide resection.16PubMed. The clinical, functional and oncological outcomes of patients with giant cell tumour of bone. A long-term review This functional gap is one of the main reasons surgeons often accept the higher recurrence risk of curettage, particularly when the tumor is accessible and does not involve extensive soft-tissue extension. A recurrence after curettage can often be retreated with another curettage, whereas loss of a joint or limb segment from wide resection is permanent.
Denosumab and Its Role
The understanding that RANKL drives the bone destruction in GCTB led to trials of denosumab, a monoclonal antibody that blocks RANKL. In a phase II trial, denosumab produced an objective tumor response in about 88 percent of patients when assessed by any response criterion, and bone-resorption markers dropped sharply within weeks of starting treatment.17PubMed Central. Objective tumor response to denosumab in patients with giant cell tumor of bone: a multicenter phase II trial Patients also experienced meaningful pain relief: more than half reported clinically significant improvement within two months, with a median time to pain improvement of roughly two to four weeks depending on the cohort.18PubMed. Effects of denosumab on pain and analgesic use in giant cell tumor of bone: interim results from a phase II study
Denosumab is now used in two main scenarios. For patients with tumors that cannot be surgically removed, or where surgery would cause unacceptable loss of function, it can serve as the primary treatment. It is also used before surgery with the aim of shrinking the tumor and hardening its borders, potentially converting what would have been a resection into a less aggressive curettage.19PubMed Central. Preoperative Denosumab With Curettage and Cryotherapy in Giant Cell Tumor of Bone: Is There an Increased Risk of Local Recurrence?
Denosumab is not without downsides. Long-term use carries the risk of osteonecrosis of the jaw (reported in about 4 percent of patients across pooled studies) and atypical femoral fractures.20PubMed Central. Denosumab regimens in the treatment of giant cell tumor of bone: A systematic review with meta-analysis The optimal duration of treatment remains unclear, and there have been isolated reports of malignant transformation to osteosarcoma in tumors treated with denosumab, though it is difficult to know whether the drug played a causal role or whether those tumors would have transformed anyway.21PubMed Central. Challenges of denosumab in giant cell tumor of bone, and other giant cell-rich tumors of bone
When Radiation Therapy Enters the Picture
Radiation is not a first-line treatment for GCTB, but it has a role in tumors that cannot be fully removed surgically, such as those in the sacrum or spine. When radiation is used as the primary therapy, local control rates are generally considered acceptable.22PubMed. Radiotherapy in the management of giant cell tumor of bone The historic concern, however, is that radiation to GCTB carries a risk of inducing a secondary sarcoma years later. This risk is real but depends heavily on the type of radiation used. Older orthovoltage radiation was associated with a malignant transformation rate of about 14 percent, while modern megavoltage radiation drops that to roughly 2 percent, an eightfold reduction.23PubMed Central. Risk of Malignant Transformation of Giant Cell Tumors of Bone Is 8 Times Lower with Megavoltage vs. Orthovoltage Radiation Therapy Much of the fearsome reputation of radiation in GCTB comes from historical series that predate modern techniques.
Malignant Transformation
The word “benign” in GCTB comes with a significant asterisk. Across large patient series, the cumulative incidence of malignancy is about 4 percent. Most of these are secondary malignancies, meaning a previously treated GCTB later develops into a high-grade sarcoma, and about 2.4 percent of all GCTB cases fall into this category. Primary malignancy, where sarcomatous tissue is already present alongside conventional giant cell tumor at the initial diagnosis, accounts for roughly 1.6 percent.24PubMed Central. Malignancy in Giant Cell Tumor of Bone: A Review of the Literature
Secondary malignancies most often arise after prior radiation therapy, though spontaneous transformation can also occur years after surgery without radiation. In one series, the average time between the original GCTB diagnosis and the appearance of a post-radiation sarcoma was about 9 years, while spontaneous transformations took even longer, averaging 19 years. The sarcomas that develop are usually osteosarcomas, and outcomes are generally poor, especially for post-radiation cases.25PubMed. Malignancy in giant cell tumor of bone The takeaway is that long-term follow-up matters even for tumors that appear successfully treated. A patient who had GCTB in their twenties should remain alert to new pain or swelling at the original site even a decade or two later.
Pulmonary Metastases From a “Benign” Tumor
One of the most puzzling aspects of GCTB is that it can spread to the lungs even when the primary tumor looks entirely benign under the microscope. Most tumors that metastasize are by definition malignant, but GCTB is the exception: the lung nodules themselves are histologically benign, made up of the same stromal and giant cell mixture found in the bone. In one large institutional series, this happened in about 3 percent of patients, and the lung deposits tended to appear as rounded, solid nodules in the peripheral and lower regions of the lungs.26PubMed. Benign giant-cell tumor of bone with pulmonary metastases: clinical findings and radiologic appearance of metastases in 13 cases
Patients with local recurrence at the primary bone site were more likely to develop pulmonary metastases, and distal radius tumors seemed particularly prone to this behavior.27PubMed. Benign giant-cell tumor of bone with pulmonary metastases: clinical findings and radiologic appearance of metastases in 13 cases Overall, the prognosis for patients with benign pulmonary metastases is relatively favorable compared to true sarcoma metastases, but it is not entirely benign either: the overall mortality directly attributable to GCTB and its lung deposits was about 23 percent in that series. Some patients with limited lung disease can undergo surgical removal of the nodules, while others are managed with observation or denosumab. The possibility of lung spread is one reason why many clinicians recommend periodic chest imaging during follow-up, even when the bone tumor appears fully controlled.
Tenosynovial Giant Cell Tumor, a Different Disease Entirely
The name “giant cell tumor” also applies to a separate tumor arising from the joint lining, tendons, or bursae rather than from bone. Tenosynovial giant cell tumor (TGCT), historically known as pigmented villonodular synovitis (PVNS) when it affects the synovium of a large joint, shares the same name and some superficial microscopic resemblance but has a completely different molecular cause. Where GCTB is driven by the H3F3A histone mutation, TGCT is driven by overexpression of CSF1, a cytokine that attracts inflammatory cells. In about 60 percent of cases, this overexpression results from a chromosomal translocation involving the CSF1 gene at chromosome 1p13.28PubMed Central. A landscape effect in tenosynovial giant-cell tumor from activation of CSF1 expression by a translocation in a minority of tumor cells In the remaining cases, CSF1 is overexpressed through some other mechanism that does not involve a translocation, suggesting that different upstream events can produce the same downstream biology.29PubMed. Translocation and expression of CSF1 in pigmented villonodular synovitis, tenosynovial giant cell tumor, rheumatoid arthritis and other reactive synovitides
TGCT can present as a localized nodule on a tendon sheath (often in the fingers) or as a diffuse process that engulfs an entire joint, most commonly the knee. The localized form is usually cured with surgical excision, but the diffuse form is notorious for recurring after surgery because it infiltrates widely through the synovium and is nearly impossible to remove completely.
Pexidartinib for Tenosynovial Giant Cell Tumor
The discovery that CSF1 signaling drives TGCT led to the development of pexidartinib, a small-molecule inhibitor that blocks the CSF1 receptor. In the phase III ENLIVEN trial, patients with advanced TGCT who were not candidates for improvement through surgery were randomized to pexidartinib or placebo. At 25 weeks, 39 percent of patients on the drug had an objective tumor response, compared with zero in the placebo group. With longer follow-up, the best overall response rate climbed to 53 percent.30PubMed Central. Pexidartinib for advanced tenosynovial giant cell tumor: results of the randomized phase 3 ENLIVEN study This led to FDA approval, making pexidartinib the first systemic therapy approved for TGCT.31PubMed. Pexidartinib: first approved systemic therapy for patients with tenosynovial giant cell tumor
The approval came with a significant caveat: liver toxicity. In the trial, three patients developed a pattern of liver injury that included elevated liver enzymes combined with elevated bilirubin, a combination that signals serious hepatic damage. One case lasted seven months and was confirmed by liver biopsy. Hair color changes, fatigue, and altered taste were also common side effects.32The Lancet. Pexidartinib versus placebo for advanced tenosynovial giant cell tumour (ENLIVEN): a randomised phase 3 trial Because of the liver risk, pexidartinib is prescribed under a restricted monitoring program, and patients undergo regular liver function tests throughout treatment. Other CSF1R inhibitors are under investigation for TGCT, and some may ultimately offer a better safety profile, but for now pexidartinib remains the only approved option for patients whose disease cannot be managed surgically.
Why These Two Tumors Share a Name
The naming overlap between giant cell tumor of bone and tenosynovial giant cell tumor causes genuine confusion among patients. Both tumors contain conspicuous multinucleated giant cells on biopsy, which is the historical reason for the shared label, but the resemblance largely ends there. They arise from different tissues, are driven by different mutations, affect different age groups (GCTB peaks in the twenties and thirties while TGCT’s diffuse form often presents in the third through fifth decades), and are treated with entirely different drugs. A patient diagnosed with “giant cell tumor” benefits from asking specifically which type they have, because the treatment pathways, recurrence patterns, and systemic therapy options are fundamentally different. The bone variant involves RANKL-targeted therapy with denosumab; the tenosynovial variant involves CSF1R-targeted therapy with pexidartinib. Mixing up the two in an online search can lead to deeply misleading information.