Soft-tissue tumors are very common. Although the exact incidence of benign tumors is unknown, and impossible to determine, it is estimated that benign soft-tissue tumors are at least 100 times more common than sarcomas. Although this is reassuring for orthopaedic surgeons and their patients, this commonality can lead to expectations of benignity which may result in unplanned resections of sarcomas. Unplanned resections, although still treatable, can cause a significant increase in morbidity for the patient. For this reason, adequate evaluation is essential.

History and physical are highly variable for soft-tissue masses. Size, consistency, depth, and absence or presence of growth are all important features to consider. Masses that are small, soft, superficial, and dormant are most likely benign and may be observed. Firm, deep, and enlarging masses require a higher level of suspicion. For masses that do not demonstrate a classic presentation, such as Dupuytren contracture or a dorsal wrist ganglion that transilluminates, imaging is indicated.

Quality imaging is critical in the evaluation of soft-tissue masses and criteria for the appropriateness of imaging exist.¹ Radiographs can be beneficial in certain cases such as identifying phleboliths in a hemangioma, a peripheral rim of calcification in myositis ossificans, or cortical involvement, but radiographs alone are rarely diagnostic. For small subcutaneous tumors, ultrasonography can be very useful. Ultrasonography can be an excellent modality for identifying superficial cystic and lipomatous masses, but its accuracy is dependent upon user experience and it may lack specificity.² MRI is the modality of choice for deep-seated masses and in certain situations, such as lipomas and vascular malformations, MRI can be diagnostic. Unfortunately, except for lipomas, most benign soft-tissue masses and sarcomas will demonstrate some degree of low signal on T1 images, high signal on T2 images, and contrast enhancement. So, although MRI is useful in raising suspicion for sarcoma or developing a differential diagnosis, it is not always diagnostic, and a systematic approach is encouraged.³ One common error is trusting the diagnosis of a cyst on noncontrast MRI because a myxoid neoplasm (solid) can mimic a true cyst (fluid). In both cases MRI can demonstrate homogenous low signal on T1 images and high signal on T2 images (Figure 1). The important message is that contrast is needed for differentiation to avoid an unplanned sarcoma resection. A cyst will demonstrate only peripheral or septal enhancement, whereas myxoid lesions will have more diffuse enhancement.

For any mass that has not been definitively determined as benign following a thorough evaluation, biopsy before resection is warranted. Depending on the location, small (<3 cm) subcutaneous masses may be considered for excisional biopsy. For all other indeterminate masses, needle biopsy is warranted. Needle biopsy is preferred over an open biopsy because there is less potential for tumor contamination with a needle biopsy and diagnostic accuracy is excellent. Although needle biopsy is not technically demanding, the overall process that ranges from proper biopsy placement to accurate pathologic assessment is complex. A
reasonable guideline for the nononcologic orthopaedic surgeon is that any deep mass that is 4 cm or greater in size should be considered malignant and referral to an orthopaedic oncologist for assessment and biopsy is appropriate.⁴

Lipoma is the most common soft-tissue tumor that will be encountered by an orthopaedic surgeon and is one of the few soft-tissue masses that can be definitively identified on imaging. On MRI, the mass simply needs to be compared to the subcutaneous fat. A lipoma will demonstrate the same imaging features as subcutaneous fat on all MRI sequences (Figure 2). Lipomas can be solitary or multiple and can occur in an unlimited number of subcutaneous, intramuscular, intermuscular, and even intraosseous locations. Dormant, small (<5 cm), asymptomatic lipomas can be safely observed. Larger (>5 cm), deep, enlarging, or symptomatic lipomas can be marginally excised and rarely recur.

When treating the larger variety of lipoma, it is important to be aware of a variant known as atypical lipoma/well-differentiated liposarcoma (ALT/WDL). This variant can be classified as a low-grade malignancy with higher recurrence rates than typical lipomas but, when located in the extremity, rarely dedifferentiate or metastasize. For this reason, marginal resection is appropriate with the knowledge that the recurrence rate is higher than garden variety lipomas. On imaging, atypical lipomas tend to be larger, more septated, and less homogenous on MRI when compared with standard lipomas⁵ (Figure 3). Following resection, atypical lipomas can be identified solely by histologic features, but when equivocal, the authors of a 2016 study have identified FISH for MDM2 as the ancillary study of choice.⁶

Several variants of lipoma exist: angiolipoma, fibrolipoma, spindle cell lipoma, hibernoma. Not only do these variants differ histologically from a lipoma, they also display unique chromosomal aberrations. The imaging features can also be variable when compared with a classic lipoma. For example, a hibernoma, which is composed of brown fat, demonstrates high signal on T1 and T2 images. It is not uncommon for lipoma variants to require biopsy before definitive diagnosis. Despite the unique differences when compared with lipoma, the treatment for lipoma variants is the same and marginal resection is typically curable.

Ganglion cysts arise from the joint capsule or tendon sheath and are the most common mass of the hand. They are typically firm, nonmobile, superficial mucin-filled masses with a classic predilection for the dorsum of the hand and foot although they can appear in any juxta-articular or peritendinous location. Physical examination can be diagnostic for most cases via transillumination or complete decompression at the time of aspiration. Ganglions are anechoic to hypoechoic on ultrasonography. MRI demonstrates low signal on T1 images and high signal on T2 images with peripheral or septal enhancement. For asymptomatic cysts, observation is appropriate whereas aspiration/injection or excision may be considered for symptomatic cysts.

Meniscal cysts are the result of joint fluid accumulation outside of the knee capsule and may present as a painful, firm mass along the joint line of the knee. This collection of fluid is due to a capsular defect that acts similar to a one-way valve. The meniscal cyst is formed secondarily and is merely a symptom of the true underlying problem. In most cases, a meniscal tear will be identified as the primary source of pathology. MRI is the imaging modality of choice because it can identify the cyst, meniscal tear, and communication of the cyst with the tear. Management of the symptomatic meniscal cyst typically involves management of the meniscal tear and aspiration or decompression of the cyst.

Popliteal cysts, also known as Baker cysts, are synovial lined cysts that form in the popliteal region of the knee. Patients may present with a pain or fullness in the posterior knee and will typically have symptoms of intra-articular pathology such as degenerative joint disease or meniscal tear. Like meniscal cysts, popliteal cysts develop secondarily via extravasation of joint fluid. These cysts typically can be easily identified on MRI because the classic cyst will demonstrate low T1 and high T2 signal with communication of joint fluid with the semimembranosus-gastrocnemius bursa. Popliteal cysts can become quite large and are not always homogenous on MRI because of debris, loose bodies, or hemorrhage that can accumulate in the cyst. The lining of the cyst will show enhancement on contrast MRI and can be quite thickened and septated because of inflammation. In such cases with atypical MRI features, clear communication with the joint should be verified before the assumption of a popliteal cyst. The primary treatment of the popliteal cysts requires management of the underlying intra-articular pathology. Following appropriate management, popliteal cysts may resolve without
excision. For larger cysts that are causing symptoms in the popliteal fossa, excision through a posterior or posteromedial approach may be performed.

Popliteal cysts in children are considered separately because typically they are not associated with intra-articular pathology and may not always communicate with the joint. The fluid may involve the semimembranosus-gastrocnemius or subgastrocnemius bursa and usually resolve spontaneously without treatment.

There is uncertainty regarding the classification of fibroma as a reactive or neoplastic process. However, this makes little difference as far as clinical management is concerned. Fibromas present as firm nodules, typically of the hands and feet, that overly tendinous structures. They are typically slow growing and may be painful. MRI will demonstrate a low signal mass on T1 and T2 images that is intimate with the tendon sheath. Nonpainful lesions that are not demonstrating growth can be observed. Enlarging or painful lesions can be marginally excised, but the local recurrence rate is not negligible. Therefore, management should be dictated by symptoms rather than the mere presence of the fibroma.

Fibromatosis exists along the intermediate behavior spectrum of benign fibrous lesions and demonstrate infiltrative growth patterns. The palmar (Dupuytren) variant is more common than plantar (Ledderhose), but some patients may be affected by both variants. Palmar fibromatosis is more common in the elderly, whereas plantar fibromatosis can be seen in children.

Palmar fibromatosis tends to affect the ulnar aspect of the hand. There is typically slow progression that eventually results in contracture of the little and ring finger. Although not necessarily painful, the contracture can ultimately lead to significant dysfunction. Given the classic presentation, imaging is not typically required to make the diagnosis. Historically, the treatment of choice has been surgical resection. However, according to a 2018 meta-analysis, collagenase injection is equally efficacious.⁷

Plantar fibromatosis typically presents in the arch of the foot, arising from the plantar fascia. They typically arise as a small, isolated nodule but can progress to large, diffuse masses. The appearance is not as classic as the palmar variety and imaging can be beneficial to rule out a more threatening process. MRI will demonstrate a low-signal lesion arising from the plantar fascia. This process can often be self-limited and may be appropriately treated with shoe modification. Given the significant recurrence rate following surgical resection, wide margins is the goal. However, given the infiltrative nature, this is not always possible without associated morbidity. For this reason, more conservative treatment measures are encouraged.