From the Guidelines
For a lump on the back of the knee, an ultrasound is the recommended initial imaging study. This is based on the most recent and highest quality evidence from the study published in 2025 1, which suggests that ultrasound provides an effective initial triage tool for soft tissue masses. Ultrasound can effectively visualize soft tissue structures and determine if the lump is solid or fluid-filled, such as a Baker's cyst, which is common in this location.
The key benefits of using ultrasound as the initial imaging study include:
- Effective visualization of soft tissue structures
- Ability to determine if the lump is solid or fluid-filled
- Highly user-dependent, but can demonstrate pathognomonic features in certain conditions
- Can be used to diagnose benign lipomas, which are the most common type of soft tissue tumor
If the ultrasound results are inconclusive or suggest a more complex condition, MRI would be the next appropriate imaging study. MRI provides detailed images of soft tissues, ligaments, tendons, and can better characterize the nature of the mass. The study published in 2025 1 also suggests that MRI is the most accurate imaging modality for diagnosis and surgical/radiotherapy planning for soft tissue tumors affecting the extremity, trunk, and pelvis.
It's worth noting that X-rays have limited value for soft tissue masses, but might be ordered if there's suspicion of bone involvement, as they can be used to identify bone involvement and risk of fracture, or to detect calcification, as suggested by the study published in 2018 1. However, the choice of imaging depends on several factors, including the lump's characteristics, how long it has been present, associated symptoms, and the patient's medical history. The patient should consult with their healthcare provider, who can perform a physical examination and determine the most appropriate imaging based on their specific clinical presentation.
From the Research
Imaging Options for a Lump on the Back of the Knee
The patient's lump on the back of the knee could be a cyst, such as a Baker's cyst, meniscal cyst, or ganglion cyst, as discussed in 2, 3, 4.
- To determine the cause of the lump, imaging studies such as magnetic resonance imaging (MRI) or ultrasound may be necessary, as mentioned in 5.
- MRI is particularly useful for visualizing soft tissue structures, such as cysts and tendons, and can help identify the presence of a Baker's cyst or other types of cysts, as noted in 4.
- Ultrasound can also be used to evaluate the lump and surrounding tissue, and can help guide needle aspiration or injection procedures if necessary, although this is not explicitly mentioned in the provided studies.
Considerations for Imaging
- The choice of imaging modality will depend on the patient's symptoms, medical history, and physical examination findings, as discussed in 5.
- It is essential to consider the possibility of malignant tumors, although rare, as mentioned in 6, and to evaluate the patient's overall condition before selecting an imaging study.
- The patient's symptoms, such as pain, swelling, or limited range of motion, should be taken into account when selecting an imaging study, as noted in 2, 3, 4.
Next Steps
- Based on the imaging findings, the patient may require further evaluation or treatment, such as arthroscopy or surgical excision, as discussed in 2, 3, 4.
- The patient's primary care physician or a specialist, such as an orthopedic surgeon, should be consulted to determine the best course of action, as implied in 5.