Musculoskeletal
Giant cell tumor of bone
A locally aggressive but non-metastatic bone tumor where radiation controls disease in sites that cannot be safely operated on.
Definition
Giant cell tumor (GCT) of bone is a benign but locally aggressive neoplasm characterized by multinucleated osteoclast-like giant cells. It most often arises in the metaphysis–epiphysis of long bones around the knee (distal femur, proximal tibia), the distal radius, and the sacrum or spine. Although classified as benign, GCT is expansile and destructive, causes pain, swelling, pathologic fracture, and can rarely metastasize to the lung as benign implants.
How radiation treats it
Radiation kills the proliferating mononuclear stromal cells (the true neoplastic component of the tumor), which the multinucleated giant cells depend on for recruitment. Over weeks to months the tumor undergoes sclerosis and re-ossification, restoring structural bone. Denosumab can be used to stop progression, but radiation offers durable local control in sites where complete resection would be multilating.
Side effects
- Skin erythema and mild soft-tissue swelling during treatment.
- Temporary worsening of pain or nerve symptoms as the tumor undergoes necrosis and re-ossification.
- Permanent local changes include soft-tissue fibrosis and, in extremity sites treated over many years, a very small risk of radiation-induced malignancy, which modern conformal planning minimizes.
Simulation process
The patient is positioned to allow a conformal CT-planned approach, often supine with immobilization devices matched to the anatomic site (vac-lock for the sacrum/pelvis, custom cast or knee immobilizer for extremities). A CT simulation with contrast defines the gross tumor volume plus any medullary extension identified on MRI. Organs at risk (bowel, bladder, spinal cord, growth plates) are contoured. For sacral lesions, prone positioning with a belly board may be used to displace small bowel, and target volumes are drawn from diagnostic MRI fused to the planning CT.
Treatment process
Treatment is typically 40–50 Gy in 20–28 once-daily fractions using intensity-modulated radiation therapy (IMRT) or 3D conformal fields. Fractions are delivered five days per week over four to six weeks. Response is slow and assessed with serial imaging: bone healing and sclerosis continue for many months after the last fraction. Combined curettage with adjuvant RT is used for extremity lesions; definitive RT is reserved for the spine, sacrum, or other unresectable sites.
Key takeaways for students
- Definitive radiation for unresectable locations such as the sacrum and mobile spine.
- 40–50 Gy with IMRT; response evolves over months to years.
- The neoplastic cell is the stromal mononuclear component — the giant cells are reactive.