Are Tumors of the Knee Common?
No, tumors of the knee are rare—primary malignant bone tumors account for less than 0.2% of all malignancies, though benign bone tumors are more common and vastly outnumber malignant lesions. 1
Epidemiologic Context
Benign lesions far outnumber malignant tumors:
- Benign bone tumors clinically outnumber primary malignant bone tumors and may present similarly 1
- In the broader context of leg masses, benign lipomas are by far the most common soft tissue mass, vastly outnumbering malignant lesions 2
Primary malignant bone tumors are exceedingly rare:
- Overall incidence of primary bone tumors is less than 0.2% of all registered malignancies 1
- Osteosarcoma, the most frequent primary bone cancer, has an incidence of only 0.2-0.3 per 100,000 per year in the general population 3
Age-Specific Considerations
The rarity varies significantly by age group:
Adolescents and Young Adults
- Osteosarcoma incidence rises to 0.8-1.1 per 100,000 per year in ages 15-19, where it comprises 4% of solid cancers in children and 3% in teenagers and young adults 1, 3
- Ewing sarcoma has an incidence of 0.3 per 100,000 per year in white Caucasians under age 25 1
- Despite these relative increases, these remain numerically rare conditions 1
Older Adults
- Primary bone tumors are considerably outnumbered by bone metastases, which may mimic primary tumors 1
- Chondrosarcoma becomes more common, with peak incidence between ages 30-60 years at 0.1 per 100,000 per year 1
Anatomic Predilection for the Knee
When bone tumors do occur, the knee region is the most common site:
- Osteosarcoma usually arises in the metaphysis of long bones, most commonly around the knee 1, 3
- The distal femur and proximal tibia are the most frequent locations 3
- Approximately 50% of Ewing sarcoma cases involve extremity tumors 1
Critical Clinical Implications
Despite rarity, certain presentations demand immediate investigation:
- Non-mechanical pain or night pain around the knee in any age group should cause concern and lead to immediate further investigation 1
- Pain at rest or during the night is a critical "red flag" requiring urgent evaluation 3
- Swelling indicates tumor progression through the cortex and periosteal distension 1
Common diagnostic pitfall:
- Primary malignant bone tumors are frequently difficult to recognize as malignant by clinicians, radiologists, and pathologists, leading to major diagnostic difficulties in non-specialized centers 1
- All patients with suspected primary malignant bone tumors should be referred to a bone sarcoma reference center before biopsy 1, 3
- Inappropriate biopsy techniques can irrevocably compromise chances for limb salvage or cure 1, 3