Cancer in or around the knee typically shows up as a firm, visible swelling on one side of the joint, often with skin that feels warm to the touch and may appear discolored or reddish. The lump tends to grow steadily over weeks to months, unlike the symmetrical puffiness you’d see with a sprained knee or bursitis. What makes knee cancer tricky is that early on, it can look a lot like a sports injury or an inflamed joint, which is why many cases get dismissed before the right tests are done.
Types of Cancer That Affect the Knee
The knee is one of the most common sites for primary bone cancer. Osteosarcoma, the most frequent type, develops from bone-forming cells and typically strikes in the leg near the knee, especially in children, adolescents, and young adults. It tends to grow in the lower end of the thighbone or the upper end of the shinbone, both of which meet at the knee joint.
Giant cell tumor of bone is another type that gravitates toward the ends of long bones, often right next to the knee joint. Ewing sarcoma can also appear in the legs, though it more commonly forms in the pelvis or ribs. In rarer cases, soft tissue cancers like synovial sarcoma develop in the structures surrounding the knee, including tendons, bursae, and the joint lining. These soft tissue tumors can be especially deceptive because they closely mimic benign conditions on initial imaging.
What It Looks Like on the Surface
The most visible sign is a mass or lump you can feel near the bone. It’s usually firm, fixed in place, and located on one side of the knee rather than evenly distributed around the joint. The surrounding skin may feel noticeably warm compared to the other knee. Some people develop skin discoloration over the area, ranging from a subtle redness to a deeper change in tone. In more advanced cases, dilated veins may become visible on the skin’s surface as the tumor grows and demands more blood supply.
The swelling from a bone tumor looks different from the puffy, fluid-filled swelling of a twisted knee or arthritis flare. Instead of a soft, squishy fullness, a cancerous mass feels more solid and doesn’t shift when you press on it. It also doesn’t respond to ice, elevation, or anti-inflammatory medications the way an injury would.
How the Pain Differs From an Injury
Pain is the most common symptom of bone cancer near the knee, but it behaves differently than pain from a torn ligament or overuse injury. At first, it may come and go. A hallmark feature is that it often worsens at night, even when you’re resting and not putting any weight on the leg. With a typical injury, rest usually brings relief. With bone cancer, the pain tends to persist or intensify during rest.
Over time, the pain becomes more constant and gets worse with activity like walking or climbing stairs. Unlike muscle soreness that fades over a few days, cancer-related knee pain gradually escalates over weeks. It doesn’t follow the healing arc you’d expect from a strain or bruise, where things steadily improve. If knee pain has been lingering for several weeks without a clear cause and isn’t getting better, that pattern is worth investigating.
How It Affects Movement
As a tumor grows near the knee, it starts to interfere with the joint’s ability to bend and straighten. Range of motion decreases, and bending or extending the knee may become painful. People often develop a limp, and the muscles in the thigh on the affected side can begin to shrink from disuse. In some cases, patients are unable to bear weight on the leg for more than brief periods.
A particularly alarming scenario is a pathological fracture, where the bone weakened by the tumor breaks during a minor activity that wouldn’t normally cause injury. Someone might step off a curb or twist slightly and feel sudden, severe pain. This can be the first indication that something is wrong inside the bone. After successful treatment, many patients do regain full motion of the knee, but the degree of functional loss before diagnosis can be significant.
How Knee Cancer Looks on Imaging
On an X-ray, bone cancer near the knee has a distinctive and often dramatic appearance. High-grade osteosarcoma typically shows up as a destructive lesion that eats into the bone, with irregular, blurry margins instead of clean edges. Two classic patterns that radiologists look for are the “sunburst” pattern, where streaks of new bone radiate outward from the tumor like rays from a sun, and the “Codman triangle,” a small triangular area where the tumor has lifted the outer layer of bone away from the shaft.
Different subtypes create different pictures. Some tumors produce dense, white areas on X-ray where abnormal bone has formed. Others appear as dark holes where bone has been destroyed. Many show a mix of both. A rare subtype called telangiectatic osteosarcoma looks purely destructive on X-ray and can show fluid-filled cystic spaces on MRI. Tumors growing on the bone’s surface, like parosteal osteosarcoma, appear as a lumpy, cauliflower-shaped mass fused to the outside of the bone.
These imaging features are important because they help distinguish cancer from benign bone conditions. Some aggressive tumors can initially be mistaken for fibrous dysplasia or other non-cancerous bone abnormalities, especially when the X-ray shows a mix of dense and destroyed areas.
Why It’s Often Mistaken for Something Else
Slow-growing tumors near the knee are frequently misdiagnosed as bursitis, ganglion cysts, arthritis, or tendon problems. Synovial sarcoma, a soft tissue cancer that can develop around the knee, is a prime example. On a standard MRI without contrast dye, it can look almost identical to a fluid-filled bursa or a simple cyst sitting next to a tendon. The key difference often only becomes clear when contrast-enhanced imaging reveals solid tissue that lights up, something a benign cyst wouldn’t do.
This overlap is one reason a mass near the knee that doesn’t resolve on its own within a few weeks deserves further evaluation. A lump that’s growing, pain that’s worsening without a clear mechanical cause, or swelling that doesn’t respond to standard treatments for joint problems are all patterns that should prompt imaging beyond a basic X-ray.
How Diagnosis Is Confirmed
The diagnostic process typically starts with an X-ray, which can reveal bone destruction or abnormal bone growth. If the X-ray raises suspicion, an MRI provides detailed information about the tumor’s size, its relationship to surrounding muscles and blood vessels, and whether it has spread within the bone. CT scans and PET scans may follow to check whether cancer has traveled to other parts of the body.
A biopsy is required to confirm the diagnosis. This involves removing a small sample of tissue, either through a needle inserted through the skin or during a minor surgical procedure. The placement and technique of the biopsy matters more than most people realize. It needs to be done in a way that won’t complicate the surgery to eventually remove the tumor. For this reason, biopsies for suspected bone cancer are best performed at a center with experience treating these cancers, ideally by the same team that would handle the surgery if cancer is confirmed.