A giant cell tumor is a bone tumor that grows at the end of a long bone, most often near the knee. It is classified as benign but “locally aggressive,” meaning it does not typically spread to other organs yet can destroy surrounding bone if left untreated. These tumors account for roughly 1.7 cases per million people per year and are most common in adults around age 35, with women affected slightly more often than men.
What’s Happening Inside the Bone
Giant cell tumors get their name from the enormous cells visible under a microscope. The tumor contains two main cell types: smaller, oval-shaped stromal cells that are the actual tumor cells, and much larger “giant cells” that can contain dozens of nuclei in a single cell body. Those giant cells behave like osteoclasts, the cells your body normally uses to break down and remodel bone. When packed together in large numbers inside a tumor, they erode bone from the inside out, creating a cavity that weakens the bone’s structure over time.
Where Giant Cell Tumors Develop
About half of all giant cell tumors form around the knee, either in the lower end of the thighbone or the upper end of the shinbone. The upper arm bone near the shoulder and the forearm bone near the wrist are the next most common sites. In nearly every case, the tumor sits at the very end of the bone, right next to the joint surface. This location is one of the features that helps doctors distinguish it from other bone tumors, which tend to grow in the middle of the shaft.
Symptoms to Recognize
Pain is the earliest and most common symptom. It often starts as a mild, intermittent ache near a joint and gradually worsens as the tumor enlarges. Over weeks to months, you may notice swelling or a visible lump near the affected joint. Range of motion can decrease, especially if the tumor is pressing against the joint space. Because the tumor hollows out the bone from within, some people first discover it when the weakened bone fractures during a routine activity. Joint pain that steadily worsens without an obvious injury is the pattern that most often leads to imaging.
How It’s Diagnosed
An X-ray is usually the first step. Giant cell tumors appear as a dark, bubble-like area at the end of the bone, often off-center, that reaches all the way to the joint surface. The surrounding shell of bone may be thinned and expanded outward, sometimes with no visible rim of dense bone around it. Doctors grade the tumor’s aggressiveness based on what the X-ray shows:
- Grade 1 (latent): well-defined borders with the outer bone shell still intact.
- Grade 2 (active): relatively clear borders but the bone shell is thinned and ballooning outward.
- Grade 3 (aggressive): indistinct borders with the outer bone clearly broken through.
MRI adds detail by showing exactly how far the tumor extends into surrounding soft tissue and whether cystic (fluid-filled) areas have developed inside it. On MRI, giant cell tumors appear dark on one type of image and bright on another, with strong contrast enhancement, a pattern that helps confirm the diagnosis. A biopsy, where a small sample of tissue is examined under a microscope, provides the definitive answer.
Surgical Treatment
Surgery is the primary treatment for most giant cell tumors. The standard approach is called intralesional curettage: the surgeon opens a window in the bone, scoops out the tumor, and then treats the cavity walls to destroy any remaining tumor cells before filling the space to restore structural support.
What happens during that cavity treatment makes a significant difference in whether the tumor comes back. A large meta-analysis found that using a high-speed burr to grind down the cavity walls after scooping cut the recurrence rate dramatically, from about 48% down to 12%. Filling the cavity with bone cement (polymethyl methacrylate) also reduced recurrence, dropping the rate from roughly 33% to 20%. The heat generated by the cement as it hardens likely kills residual tumor cells at the margins. Chemical agents like phenol or hydrogen peroxide are sometimes added to the procedure, but the evidence suggests they don’t provide a meaningful additional benefit when used alone.
For tumors that have already broken through the bone and invaded surrounding tissue, or for tumors in locations where curettage would leave the bone too unstable, a wider resection may be necessary. This means removing the entire segment of affected bone and reconstructing it, often with a metal implant or bone graft. Wide resection carries a lower recurrence rate (around 12%) but involves a longer recovery and greater impact on joint function.
Medication for Tumors That Can’t Be Removed
Some giant cell tumors grow in locations where surgery would cause unacceptable damage, such as the spine or pelvis. For these cases, a targeted medication called denosumab can slow or stop bone destruction. Denosumab works by blocking a signaling molecule that activates the giant cells responsible for eating away bone. In a long-term clinical trial of patients with unresectable tumors, 99% responded to treatment. The drug is given as an injection under the skin every four weeks, with extra loading doses in the first few weeks. It is also sometimes used before surgery to shrink a tumor and make it easier to remove.
Recurrence and Long-Term Outlook
Giant cell tumors have a well-known tendency to come back after treatment. With curettage alone, recurrence rates historically reached nearly 50%. Modern techniques combining thorough burring with bone cement have brought that number closer to 12 to 20%, depending on the tumor’s size and grade. Recurrences typically appear within the first two to three years after surgery, so follow-up imaging during that window is important.
Most recurrences are local, meaning the tumor regrows in the same spot. They are generally treated with repeat surgery using the same approach, though a wider resection may be considered if curettage has already failed once.
Can Giant Cell Tumors Spread?
Despite being classified as benign, giant cell tumors can spread to the lungs in roughly 7 to 10% of cases, and up to 14% when the primary tumor is in the spine. These lung deposits typically appear about two years after treatment of the original tumor and often show up as multiple small nodules in both lungs on a chest CT.
The reassuring finding is that lung metastases from giant cell tumors behave very differently from lung metastases of a true cancer. They grow slowly, with a longer doubling time than most metastatic tumors, and they look identical to the original tumor under the microscope. Around 70% of patients with lung spread have a favorable outcome, and some of these nodules remain stable or even shrink without treatment. Monitoring with periodic chest imaging is the typical approach, with surgery or denosumab reserved for nodules that continue to grow.