A lump behind the knee can be concerning. While unsettling, most lumps are benign, meaning non-cancerous. Many non-cancerous conditions can cause these lumps, often without serious health implications. Understanding common causes and when to seek medical evaluation can help alleviate anxiety and guide appropriate next steps.
Common Benign Causes
Most lumps behind the knee are non-cancerous. A frequent cause is a Baker’s cyst, also known as a popliteal cyst. This fluid-filled sac forms when excess knee joint fluid pushes through the back of the knee capsule, creating a noticeable bulge that may feel soft or firm. Lipomas are another common benign cause, representing slow-growing, fatty tumors located just beneath the skin. These lumps typically feel soft and movable when pressed.
Ganglion cysts can also appear behind the knee, though less commonly than Baker’s cysts. They form as fluid-filled sacs originating from a joint capsule or tendon sheath and are usually firm and non-tender. Enlarged lymph nodes, small, bean-shaped glands part of the immune system, can sometimes be felt behind the knee. These nodes may swell in response to an infection or inflammation in the leg, feeling tender and movable.
When to Seek Medical Advice
While many lumps behind the knee are harmless, certain characteristics or accompanying symptoms should prompt a medical evaluation. Rapid growth of a lump, noticeably increasing over weeks or months, is a significant indicator for concern. Persistent or increasing pain associated with the lump, especially if it interferes with daily activities or sleep, warrants professional assessment. Changes in the skin overlying the lump, such as redness, warmth, or ulceration, also suggest the need for immediate medical attention.
Systemic symptoms can also be important clues, even if the lump itself does not appear alarming. Unexplained weight loss, fever without an obvious cause, or night sweats should be discussed with a healthcare provider. A lump that feels unusually hard, fixed in place, or irregular in shape may also be a reason to seek medical advice.
Diagnostic Process
Evaluation of a lump behind the knee typically begins with a physical examination and a detailed review of the patient’s medical history. The doctor assesses the lump’s size, shape, texture, mobility, and tenderness, while inquiring about associated symptoms, their duration, and progression. This initial assessment helps understand the lump’s characteristics.
Imaging techniques are often employed to gain insight into the lump’s composition and relationship to surrounding structures. An X-ray can help identify any bone abnormalities or calcifications within the mass. An ultrasound is frequently the first imaging choice for soft tissue lumps, as it can differentiate between solid and fluid-filled masses and assess blood flow. Magnetic Resonance Imaging (MRI) provides highly detailed images of soft tissues, offering comprehensive views of the lump’s internal structure and its extent. Computed Tomography (CT) scans can also be used, particularly for evaluating bone involvement or complex masses.
Ultimately, a definitive diagnosis, especially if malignancy is suspected, often requires a biopsy. During a biopsy, a small tissue sample from the lump is removed and examined under a microscope by a pathologist. This can be done using a needle (needle biopsy) or by surgically removing a larger portion or the entire lump (excisional biopsy). Microscopic analysis of the tissue is the only way to confirm the presence of cancerous cells.
Types of Malignant Lumps
While less common, certain types of cancerous lumps can develop behind the knee, originating from either bone or soft tissues. Primary bone cancers, such as osteosarcoma or Ewing sarcoma, can arise in the bones around the knee joint. Osteosarcoma typically forms in the long bones, often near the knee, and is characterized by malignant bone-forming cells. Ewing sarcoma is another aggressive bone cancer that can affect the femur or tibia in this region.
Soft tissue sarcomas are a group of rare cancers that develop in the connective tissues, such as fat, muscle, nerves, or blood vessels. Examples that can occur behind the knee include liposarcoma, which originates from fatty tissue, and synovial sarcoma, which, despite its name, does not necessarily arise from the synovium of joints but can occur near them. Metastatic cancer, which is cancer that has spread from another part of the body to the knee area, is also a possibility, though less frequent than primary tumors.
Treatment Strategies
Treatment for a malignant lump behind the knee is highly individualized, depending on the specific type of cancer, its stage, location, and the patient’s overall health. Surgery is a primary treatment modality, often involving the complete removal of the tumor along with a margin of healthy tissue. For bone cancers, limb-sparing surgery aims to remove the cancerous bone while preserving the limb, often replacing the removed section with a prosthesis or bone graft.
Chemotherapy, which uses powerful drugs to kill cancer cells throughout the body, may be administered before surgery to shrink the tumor (neoadjuvant chemotherapy) or after surgery to eliminate any remaining cancer cells (adjuvant chemotherapy). This systemic treatment is particularly effective for cancers that have a high risk of spreading. Radiation therapy uses high-energy rays to target and destroy cancer cells in a specific area. It may be used alone or in combination with surgery and chemotherapy to reduce the risk of recurrence.