Neither Dr. Kafchinski nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter.
Benign soft-tissue tumors range from small, asymptomatic superficial masses without functional consequence to large, symptomatic, intramuscular, or intra-articular lesions. Many benign soft-tissue tumors can be successfully managed with marginal excision when nonsurgical measures have failed. Some lesions, such as desmoid tumors, have a high risk of local recurrence, even when wide surgical margins are achieved. Malignant transformation of benign soft-tissue tumors is exceedingly rare.
The soft-tissue tumors most frequently encountered benign by orthopaedic surgeons are lipomatous tumors, tenosynovial giant cell tumor, and cysts. These conditions can often be diagnosed with a combination of physical examination, radiographs, ultrasound, and MRI. If a mass does not follow the typical presentation, location, and imaging findings of a benign lesion, biopsy according to orthopaedic oncologic principles is warranted to rule out a malignant process.
Lipomatous tumors represent the most common benign soft-tissue masses, although the true prevalence is unknown because of underreporting of these often-asymptomatic lesions. Lipomatous tumors are generally classified as lipomas, atypical lipomatous tumors (ALTs), and other variants (angiolipomas, chondroid lipomas, hibernomas, lipoma arborescens, lipoblastomas, spindle cell lipomas, and atypical spindle cell lipomas). Table 1 summarizes the characteristics of these lipomatous tumor categories.
Appropriate preoperative evaluation according to the principles of orthopaedic oncology is of the utmost importance. In summary, if a small mass can be confidently identified as superficial on physical examination or ultrasonography, it is generally safe to perform a wide excision without MRI.1 Lesions that are deep, abutting or attached to fascia, or larger than 5 cm should be evaluated with MRI with and without contrast before surgery. Lesions with concerning imaging findings (ie, nodular or heterogeneous enhancement, large areas of necrosis) should be biopsied before surgery.1
Solitary lipomas present most commonly in male patients between 40 and 60 years of age, with equal distribution across races.2 They can occur in either superficial or deep locations. Superficial or subcutaneous lipomas are easier to detect and typically noticed when the mass is smaller in dimension. Deep lipomas may be further characterized as intramuscular or intermuscular. Lipomas are usually asymptomatic, although symptoms may arise from nerve compression, restricted range of motion, or a sense of fullness within the extremity. Surgical resection of lipomas is reserved for symptomatic lesions or for cosmetic purposes. Lipomas can be excised marginally with a low risk of local recurrence. Deep and particularly intramuscular lipomas have a higher risk of local recurrence, possibly related to incomplete excision.2,3,4
MRI of lipomas demonstrates homogeneous isointensity to subcutaneous fat on all sequences, and no abnormal enhancement on postcontrast imaging. There may be thin (<2 mm), nonenhancing septations within the lipoma and an occasional traversing blood vessel.4,5 Thus, lipomas are considered determinant lesions when imaging is concordant with subcutaneous adipose, and biopsy is not necessary to confirm the diagnosis.5 Lesions with MRI characteristics that are incongruous to fat warrant a biopsy before surgical intervention; myxoid, fibrous, nodularity, or heterogeneous areas of enhancement are more likely to indicate a sarcoma than a simple lipoma.2,4,5 Both grossly and microscopically, lipomas are by definition similar to the surrounding mature adipose tissue2 (Figure 1).
ALTs are histologically identical to well-differentiated liposarcomas. Differences in location, treatment, and clinical outcomes led to distinguishing these two entities with separate nomenclature. The World Health Organization (WHO) Classification of Tumours of Bone and Soft Tissue defines well-differentiated liposarcomas as tumors located in the mediastinum, retroperitoneum, or paratesticular regions.2,3 This entity has a higher risk of local recurrence because complete surgical resection may not be attainable, compared with ALTs that arise in the extremities.
ALTs are located deep to fascia in the extremities and have a low risk of either local recurrence or malignant transformation.2,3,4,6 ALTs affect both men and women equally, typically occurring in patients older than 60 years.3,6 MRI characteristics of ALTs, compared with those of lipomas, often demonstrate thicker septa (>2 mm), areas of nodular enhancement on postcontrast imaging, and large tumor diameters (>130 mm).7
Histologically, ALTs are composed of mature white adipose tissue, with areas of focal atypia, hyperchromatic stromal cells, and multinucleated stromal cells.2,3 ALTs express amplification of murine double minute 2 (MDM2) in close to 100% of cases via fluorescence in situ hybridization,8 with a 92% to 100% specificity and 97% to 100% sensitivity in differentiating ALT from a simple lipoma.9,10CDK4 is often co-amplified with MDM2 in ALT, as both genes are located within the 12q13-15 region.2
Because of the low risk of local recurrence or metastatic potential, marginal resection is recommended for ALTs.6,11,12,13,14 Reports of local recurrence range from 8.2% to 50.61%, with recurrence noted an average of 6 to 8 years after surgery.11,12,13,15,16 The largest case series examining 151 patients had a local recurrence rate of 10%.12 Reported rates of dedifferentiation and metastasis range from zero to 5% in the literature.11,12,13,15,16
Figure 1 Axial MRI sequences of a forearm intermuscular lipoma (arrow). A, T1-weighted magnetic resonance image. B, T2-weighted fat-suppressed magnetic resonance image. C, Gadolinium-enhanced T1-weighted fat-suppressed magnetic resonance image.
Variants—Benign Lipomatous Tumors
There are several variants of lipomas, all of which can be categorized as benign lipomatous tumors. The presentation, location, and imaging findings of each vary from those of a simple lipoma; however, once a diagnosis is confirmed, marginal excision remains the standard of care.4
Angiolipomas most commonly occur in the subcutaneous tissue of the upper extremities of patients in their second and third decades; conversely, they are uncommon in children or patients older than 50 years.2 Angiolipomas are tender to palpation, which helps distinguish them from an ordinary lipoma.
Chondroid lipomas are rare, benign masses that occur more frequently in women.2 They are often mistaken for sarcoma (myxoid liposarcoma or extraskeletal myxoid chondrosarcoma) because of both histologic and imaging similarities. As such, accurate diagnosis requires a skilled musculoskeletal pathologist.
Hibernomas are lipomatous tumors composed primarily of brown fat and have increased vascularity. These lesions tend to be subcutaneous and occur in the thigh region. They are most common in patients around the third decade of life, with a slight male predominance. Hibernomas do not tend to recur after surgical excision.2 Lipoma arborescens is considered a lipomalike lesion and is typically located in the suprapatellar pouch of the knee, and rarely in the shoulder, hip, or elbow.2 Lipoma arborescens is most likely a reactive process, strongly associated with degenerative intra-articular pathology, where irritated and hypertrophic synovial tissue becomes infiltrated with fat.2,17,18
Lipoblastomas occur in children younger than 3 years and are rarely seen in children older than 9 years. Histologically, this benign condition is very similar to fetal adipose tissue and is found in the extremities, trunk, and the head and neck.2 Lipoblastomas may be circumscribed or diffuse, with the latter having a higher rate of local recurrence after surgical resection, likely because of incomplete excision.2
Spindle cell lipomas, also called pleomorphic lipomas, classically arise (80%) in the posterior shoulder or neck of middle-aged males. Histologically, these tumors can be challenging to diagnose; they can include areas of spindle cells, myxoid changes, mast cells, and collagen fibers, hence the name pleomorphic lipoma.2 Notably, spindle cell lipomas/pleomorphic lipomas do not show MDM2 amplification.2 Surgical excision is typically curative.4
Atypical spindle cell or pleomorphic lipomatous tumor constitutes a distinct tumor, separate from spindle cell/pleomorphic lipomas or ALT as described previously. A recent addition in the 2020 WHO Classification of Tumours of Bone and Soft Tissue, this entity chiefly arises in the hands and feet of men (3:2 male:female ratio), in the fifth decade of life.19,20,21 Histologically, this tumor is more infiltrative and can vary greatly with regard to pleomorphism, lipoblasts, and level of nuclear atypia.21MDM2 amplification is negative in atypical spindle cell/pleomorphic lipomatous tumors.7