Classification Principles and Histogenesis
The basis for classifying soft tissue neoplasms is the tissue of origin (histogenesis) and the cell of origin (cytogenesis). Benign variants are traditionally named by adding the suffix "-oma" to the root of the tissue name (e.g., fibroma or lipoma). Malignant counterparts are designated by the general term sarcoma.
Main sources of development:
- Adipose tissue (adipocytes): lipoma and liposarcoma.
- Fibrous tissue (fibroblasts and myofibroblasts): fibroma and fibrosarcoma.
- Fibrohistiocytic elements: histiocytoma and pleomorphic sarcoma.
- Smooth muscle tissue: leiomyoma and leiomyosarcoma.
- Skeletal muscle: rhabdomyoma and rhabdomyosarcoma.
- Blood vessels: angioma and angiosarcoma.
- Other elements: pericytes (pericytoma), cartilage and osteoid tissue (soft tissue chondroma), as well as primitive mesenchymal cells (myxomas, mesenchymomas). A separate group comprises hamartomas and their malignant counterparts.
Adipose Tissue Tumors
These are organ-nonspecific neoplasms typically characterized by various chromosomal rearrangements and gene amplifications.
Lipoma — an encapsulated yellow nodule with a lobular structure. It may be located superficially (up to 5 cm in depth) or deeply. Found in subcutaneous adipose tissue, muscles, and synovial membranes of joints (where it may acquire a papillary surface). Microscopically, it consists of adipocyte lobules separated by fibrovascular septa. Depending on the admixture of other tissues, osteolipomas (with bone tissue), chondrolipomas (with cartilage), fibrolipomas, and myxolipomas are distinguished.
Liposarcoma — a malignant counterpart accounting for up to 55% of all liposarcomas. It most frequently affects deep spaces: the retroperitoneum, paratesticular region, and mediastinum. Macroscopically, it is a white-yellow encapsulated lobular mass capable of multicentric growth. There are two main subtypes:
- Well-differentiated: includes lipoma-like, sclerosing, inflammatory, and spindle cell variants.
- Dedifferentiated: represents a mixture of atypical adipocytes with sarcomatous elements of a different structure.
The immunohistochemical marker S-100 is used for diagnosis. The tumor metastasizes hematogenously to internal organs. Retroperitoneal dedifferentiated liposarcomas carry the worst prognosis.
Fibrous Tissue Tumors
These tumors form from cells showing fibroblastic and myofibroblastic differentiation. This broad group includes both true neoplasms and tumor-like processes.
- Benign and tumor-like lesions: nodular, proliferative, and ischemic fasciitis, proliferative myositis, elastofibroma, and fibrous hamartoma of infancy.
- Malignant (sarcomas): low-grade myofibroblastic sarcoma, myxoinflammatory fibroblastic sarcoma, and others.
Genetic alterations play an important role in the genesis of fibrous tumors: mutations in the APC tumor suppressor gene, monosomies, trisomies, and various translocations.
Risk Factors and Prognostic Evaluation
Sarcomas can arise in areas of prior tissue injury—in scars (including those at bone fracture sites), as well as in close proximity to implanted orthopedic hardware.
Morphometric grading systems are used to assess malignant potential. Key prognostic parameters include:
- The level of maturation (differentiation) of the tumor tissue.
- The number of mitotic figures per unit area.
- The extent of necrotic zones.
Staging is performed using the international TNM system (stages I–IV). It accounts for the size of the primary tumor nodule, regional lymph node involvement, and distant metastases.
Gastrointestinal Stromal Tumors (GIST)
A distinct category of mesenchymal neoplasms with variable clinical behavior. They are located primarily in the gastrointestinal tract (predominantly in the stomach and small intestine).
GIST cells are unique in that they can express markers of various mesenchymal derivatives, including smooth muscle and Schwann cell elements. The main diagnostic criterion is a positive immunohistochemical reaction for the specific marker CD117 (c-kit).