Soft Tissue Tumors Around The Knee Joint
hemangioma, lipoma, synovial chondromatosis, synovial sarcoma
P Chatra. Soft Tissue Tumors Around The Knee Joint. The Internet Journal of Radiology. 2012 Volume 14 Number 2.
Knee joint is a common site for many of the soft tissue tumors. Lipoma and hemangioma are the most common. Knowledge of individual tumors is of utmost importance in differentiating one from the other as each tumor demands unique management. MRI is the imaging modality of choice as it best characterizes the soft tissue. In this pictorial essay we describe morphology, imaging findings and clinical significance of each of the soft tissue tumors.
Soft tissue tumors around the knee joint can be classified as benign and malignant. Benign tumors include lipoma, synovial hemangioma, synovial chondromatosis and pigmented villonodular synovitis. Malignant tumors include synovial sarcoma and soft tissue sarcoma. Knee joint is the most common site for majority of these soft tissue tumors. These tumors being superficial are usually felt on clinical examination and imaging is necessary to evaluate its morphology, extent, differential diagnosis and post operative follow-up. MRI is the modality of choice as it can characterize the soft tissue better than all other modalities.
It is the most common soft tissue tumor seen in the extremity(1). Lipoma can be classified as cutaneous and deep seated, based on its location. Intra muscular lipoma is a rare deep seated lipoma(2). It can be classified as well circumscribed type and infiltrating type, on histology(3). Intra muscular lipomas may compress the adjacent neurovascular bundle causing symptoms. Lipoma and well-differentiated liposarcoma are difficult to distinguish on imaging. Radiological evaluation is mainly aimed at differentiating lipoma from well-differentiated liposarcoma and also to look for fat plane between it and surrounding structures in cases of compression.
On CT, lipoma appears as fat density lesion compressing and displacing adjacent structures (Figure 1A-C)(1). On MRI, lipoma displays hyperintense signal on T1 and T2W images and are suppressed on STIR images. Presence of thick septa and solid component which show enhancement on post contrast T1 W images goes on favor of well-differentiated liposarcoma rather than lipoma(4).
Hemangioma may be well-circumscribed or have poorly defined margins, with varying amounts of hyperintense T1 signal owing to either reactive fat overgrowth or haemorrhage(5). Hemangioma appears as a bunch of grapes on T2W images (Figure 2A, B). This appearance is due to cavernous vascular spaces containing stagnant blood. Some hemangiomas demonstrate fluid-fluid levels (Figure 2C, D)(8). Areas of signal void correspond to phleboliths. Post contrast T1 W images demonstrate extra-articular involvement(9). Open or arthroscopic surgical excision is the treatment of choice.
Radiological diagnosis is usually straight forward on plain radiographs with presence of multiple loose bodies in the affected joint showing ring and arc type of matrix mineralization (Figure 3A,B)(10). Cross sectional imaging is required in patients where there is no mineralization of loose bodies (Figure 3). In addition we can also demonstrate the bony erosions and extent of the disease process. MR is indicated to look for extent of lesion and also follow-up of patients with suspected recurrence(12). On MRI, multiple loose bodies are seen in the affected joint which display variable signal depending on the amount of mineralization (Figure 3D,E). Well mineralized lesions display hypointense signal on T1 W and T2W images. Surgical resection is the treatment of choice. Synovial chondromatosis is known to reccur after treatment and recurrence rate is between 3- 23%(10).
On MR imaging localized PVNS is seen as asymmetric nodular thickening of the synovium with lobulated contours. These lesions display characteristic low signal on all sequences (Figure 4A, B) and blooming on gradient images (Figure 4C) due to presence of hemosiderin pigmentation(13). On post contrast T1W images there is enhancement of the abnormal synovium (Figure 4D). Surgical excision is the treatment of choice. Localized disease could be excised on arthroscopy. PVNS has a high recurrence post surgery & recurrence rate is between 8- 56% (16).
On T1-weighted MR images, synovial sarcoma typically appear as a prominently heterogeneous multilobulated soft tissue mass (Figure 5A, B)(21). On T2-weighted MR images it appears heterogeneous with calcification, haemorrhage or necrosis and a solid component (Figure 5 C). This finding is characteristically called as triple sign(22). The solid component show homogenous enhancement on post contrast T1 W images. Synovial sarcomas are known to invade into the adjacent bone and joint(18). Synovial sarcoma shows high uptake of radio tracer on PET-CT (Figure 5D) (23). Wide local surgical excision with removal of normal cuff of surrounding tissue is the treatment of choice.
MRI is the imaging modality of choice. Most tumors have non-specific features in the form of hypointense signal on T1W images (Figure 6A) and hyperintense signal on T2W images. Hyperintense signal on T1W images points to fatty component; in this case diagnosis goes in favor of liposarcoma(4). In addition look for encasement of vessels (Figure 6B), inter compartmental extension (Figure 6), extension into joint & skin (Figure 6D), multifocal lesions and marrow infiltration. Wide surgical excision is the treatment of choice. Entire muscle could be excised if the functional loss is not too great(28). Encased vessels are bypassed and then resected along with the tumor tissue. If the fat plane between the lesion and the neurovascular structures is ill defined, then patient is subjected to pre and post operative radiotherapy(28). These tumors are known to have high recurrence rate depending on the histological type and grade.
Benign soft tissue tumors are much more common around the knee joint than malignant tumors. MRI is the imaging modality of choice in the evaluation of soft tissue tumors because of its superior soft tissue characterization. Many of the soft tissue tumors have risk of recurrence and they should be evaluated clearly looking for minute details to avoid recurrence. Imaging in malignant tumors is mainly aimed at evaluation of extent, staging and post treatment follow-up.