46. Pathogenesis and frequency of soft tissue tumours. Fibrous tumours and tumourlike lesions. Fibrosarcoma. Fibromatoses. Socalled fibrohistiocytic tumors.

Introduction

“Soft tissue” refers to any nonepithelial tissue other than bone, cartilage, CNS, haematopoietic and lymphoid tissues. In practice it refers to muscle, adipose tissue, fibrous tissue, vessels and the peripheral nervous system. Tumors can arise from any of these tissues.

Recall that a sarcoma is a tumor originating from connective tissue cells like fibroblasts and adipocytes.

General

Like most tumors, soft tissue tumors can be classified according to their dignity:

  • Benign soft tissue tumors
    • Resemble mature tissues
    • Little tendency to recur
  • Malignant soft tissue tumors
    • Infiltrative, destructive growth
    • Recurs often
    • Metastasizes

Luckily, benign tumors are 100 times more frequent than malignant ones. Malignant soft tissue tumors account for less than 1% of all malignancies. Treatment is by excision and radiation if necessary. Soft tissue tumor can arise anywhere on the body (as soft tissue is present everywhere).

The grading of soft tissue tumors uses a grading system called FNCLCC. This system is based on the degree of pleomorphism, necrosis and mitotic activity.

Most soft tissue tumors develop without any apparent cause, but irradiation, burn injuries and toxic exposure might be involved. Kaposi sarcoma is associated with HHV-8. Some soft tissue tumors are associated with genetic syndrome like neurofibromatosis type 1, Gardner syndrome and Li-Fraumeni syndrome.

These are the most important soft tissue lesions:

  • Fibrous tumor-like lesions and tumors
    • Nodular fasciitis
    • Myositis ossificans
    • Fibromatosis
    • Fibrosarcomas
    • Fibrohistiocytic tumors
  • Adipose tumors
    • Lipomas
    • Well-differentiated liposarcoma
    • Dedifferentiated liposarcoma
    • Myxoid liposarcoma
  • Smooth muscle tumors
    • Leiomyoma
    • Leiomyosarcoma
  • Skeletal muscle tumors
    • Rhabdomyoma
    • Embryonal rhabdomyosarcoma
    • Alveolar rhabdomyosarcoma
  • Peripheral nerve tumors
    • Neurofibroma
    • Schwannoma
    • Granular cell tumor
    • Malignant peripheral nerve sheath tumor (MPNST)
    • Synovial sarcoma
Fibrous tumorlike lesions (pseudosarcomas)

The fibrous tissue is prone to developing not only real neoplasms but also tumorlike lesions called pseudosarcomas.

Nodular fasciitis is such a pseudosarcoma. It’s a benign, self-limited proliferation of fibroblasts that typically occurs in adults on their palmar side of their forearm. It usually forms a solitary, rapidly growing (over a few weeks) mass. Excision is curative. Should be distinguished from a sarcoma.

Myositis ossificans is another pseudosarcoma characterized by an ossifying mass that usually develops in muscles in the extremities of adults after trauma. The affected area is initially swollen but later evolves into a hard, well-demarcated mass as the periphery ossifies. Excision is curative. It should be distinguished from extraskeletal osteosarcoma.

Fibromatoses

The fibromatoses are a group of intermediate (between benign and malignant) fibroblastic tumors. These tumors infiltrate nearby tissues and often recur after surgical removal. Although they can locally infiltrate, they can never metastasize. We distinguish three important types:

  • Superficial types
    • Palmar/plantar fibromatosis – fibrous growth in the palmar or plantar aponeurosis
    • Penile fibromatosis (Peyronie disease) – fibrous growth in the penis causes abnormal curving and pain
  • Deep type
    • Desmoid tumors – fibrous growth in the extremities, abdominal wall or abdominal cavity

Desmoid tumors are associated with Gardner syndrome, a subtype of familial adenomatous polyposis.

Fibrosarcomas

Fibrosarcoma is a malignant tumor composed of fibroblasts. They most commonly occur in the extremities and retroperitoneal area. They tend to recur, and they can metastasize, often to the lungs and bones. Four subtypes exist:

  • Adult type
  • Infantile type
  • Inflammatory myofibroblastic tumor
  • Dermatofibrosarcoma protuberans (DFSP)

The infantile type is associated with the translocation t(12;15), which causes increased activation of the gene NTRK3. The DFSP type is associated with the translocation t(17;22), which causes overexpression of the gene PDGF (platelet-derived growth factor).

Fibrosarcomas show a characteristic pattern on histology called herringbone pattern (google it so you know what I mean).

Fibrohistiocytic tumors

Fibrohistiocytic tumors are comprised of fibroblasts and histiocytes, which are lipid-filled tissue macrophages. The spectrum of fibrohistiocytic tumors range from self-limiting benign lesions to aggressive high-grade sarcomas.

Benign fibrous histiocytoma or dermatofibroma is a relatively common benign lesion that manifests as a small (< 1 cm), circumscribed brown “spot” in the dermis or subcutaneous fat. They’re usually harmless and no treatment is required, but they can be removed for cosmetic reasons. Google dermatofibroma and you’ll know what they are.

Malignant fibrous histiocytoma or undifferentiated pleiomorphic sarcoma is an aggressive large sarcoma that most commonly occurs in the extremities or retroperitoneum. It’s most common in elderly people. It frequently recurs and metastasizes.


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47. Tumours of adipose tissue and peripheral nerves, synovial sarcoma

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