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Chapter: Pathology: Skeletal Muscle and Peripheral Nerve Pathology

Soft Tissue and Peripheral Nerve Tumors

Lipoma is a benign adipose tissue tumor that most often arises in subcutaneous tissue of trunk, neck, or proximal extremities.


Lipoma is a benign adipose tissue tumor that most often arises in subcutaneous tissue of trunk, neck, or proximal extremities. It is the most common benign soft tissue tumor. The tumor is usually more of a cosmetic problem than a medical one. Microscopically, it is composed of mature fat cells but can contain other mesen-chymal elements.

Liposarcoma is a malignant adipose tissue tumor that most often arises in the thigh or retroperitoneum. It is the most common adult sarcoma. It is distinguished from lipoma by the presence of lipoblasts. Grossly, it tends to be larger than lipoma, and the cut surface shows fibrous bands. Microscopically, well-differentiated liposarcoma consists of mature fat with varying numbers of hyperchromatic spindle cells and mul-tivacuolated lipoblasts. Metastases are rare but retroperitoneal tumors tend to recur.

Dermatofibroma is a benign dermal spindle cell proliferation that most often arises in the extremities. A small, red nodule is seen, which is tender and mobile on exami-nation.

Fibromatosis is a non-neoplastic proliferative connective tissue disorder that can his-tologically resemble a sarcoma. Fibrous tissue infiltrates muscle or other tissue, and may cause a mass lesion. The cut surface is trabeculated. Microscopically, bundles of fibroblasts and collagen are seen.

         Superficial fibromatoses arise from fascia or aponeuroses. Palmar fibroma-tosis is the most common type. Penile fibromatosis is known as Peyronie’s disease.

         Deep fibromatoses (desmoids) occur in extraabdominal sites (children) and abdominal wall and extraabdominal sites (adults). Abdominal desmoids often occur in women within a year of pregnancy. They may also follow surgery or trauma. Intraabdominal fibromatosis is commonly associated with Gardner syndrome.

Fibrosarcoma is a malignant fibrous tumor, commonly seen on the thigh and upper limb. It may arise spontaneously or after therapeutic/accidental irradiation. Micro-scopically, there are uniform spindle cells with a “herringbone” pattern. Metastases are hematogenous, often to the lung.

Undifferentiated pleomorphic sarcoma (previously known as malignant fibrous his-tiocytoma) is a large multilobulated tumor seen in the extremities and retroperito-neum of older adults. Microscopically, they may have a storiform (cartwheel-like) pattern. They recur and metastasize.


Embryonal rhabdomyosarcoma

Leiomyoma is a benign smooth muscle tumor most often seen in the uterus and gastrointestinal tract. Less often it is seen in skin, and only rarely in deep soft tissue.

Leiomyosarcoma of soft tissue is less common than its counterpart in the gastroin-testinal tract and uterus. In soft tissue, it usually arises in the retroperitoneum of older women.

Grossly, the tumor is fleshy and white with hemorrhage and necrosis. Microscopi-cally, the tumor nuclei are blunt ended (“cigar-shaped”). Longitudinal striations can be seen with Masson trichrome staining. The tumor is highly aggressive in the retroperitoneum, where complete resection may not be possible.

Synovial sarcoma occurs in young adults. The knee is a common location. The gross appearance is variable but calcification is common. Microscopically, tumors may be biphasic (epithelial and spindle cells) or monophasic (spindle cell or epithelial).

Benign peripheral nerve sheath tumors

         Schwannoma is an encapsulated nerve sheath tumor with alternating Antoni A and B areas. There is an associa-tion with NF2.


         Neurofibroma is nonencapsulated and may have a solitary, diffuse, or plexi-form pattern. Microscopically, neoplastic cells are interspersed among wavy, loose or dense collagen bundles. There is an association with NF1.

Malignant peripheral nerve sheath tumor may arise from neurofibromas or de novo in a peripheral nerve. It typically occurs in young adults in major nerve trunks (sciatic nerve, brachial plexus, and sacral plexus). Microscopically, it resembles fibro-sarcoma. Recurrence and distant metastases are common.

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