Classification principles and histogenesis
The basis for dividing soft tissue neoplasms is the source tissue (histogenesis) and source cell (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.
- Vessels: angioma and angiosarcoma.
- Other elements: pericytes (pericytomas), 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 characterized by various chromosomal rearrangements and gene locus amplification.
Lipoma — an encapsulated yellow lobular node up to 5 cm deep or superficial. It can be located in the 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 more frequently affects deep spaces: the retroperitoneal region, paratesticular zone, and mediastinum. Macroscopically, this is a white-yellow encapsulated lobular formation capable of multicentric growth. Two main subtypes are distinguished:
- 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 have the worst prognosis.
Fibrous tissue tumors
They form from cells with features of fibroblastic and myofibroblastic differentiation. This broad group includes both true tumors and tumor-like processes.
- Benign and tumor-like lesions: nodular, proliferative, and ischemic fasciitis, proliferative myositis, elastofibroma, as well as 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 damage — in scars (including at the site of bone fractures), as well as in close proximity to implanted orthopedic devices.
Morphometric systems are used to assess malignant potential. Key prognostic parameters:
- Level of maturity (differentiation) of the tumor tissue.
- Number of mitotic figures per unit area.
- Extent of necrosis zones.
Staging is performed according to the international TNM system (stages I–IV). It takes into account the dimensions of the primary tumor node, the presence of regional lymph node metastases, and distant organ involvement.
Gastrointestinal stromal tumors (GIST)
A special type of mesenchymal neoplasm with an unknown clinical prognosis. They are localized predominantly in the gastrointestinal tract (primarily in the stomach and intestines).
GIST cells are unique in that they can express markers of various mesenchymal derivatives, including smooth muscle and Schwannian elements. The main diagnostic criterion is a positive immunohistochemical reaction for the specific marker CD117 (c-kit).
Briefly: answers to topical questions
To which organs do liposarcomas metastasize hematogenously?
Liposarcomas metastasize hematogenously to internal organs, with the lungs being the most frequent localization of distant metastases. Lung involvement is characteristic of most soft tissue sarcomas, due to the features of tumor venous drainage and the capillary network of the pulmonary circulation, which serves as the first "filter" for tumor emboli. The prognosis of the disease significantly worsens with retroperitoneal localization of the primary tumor and a dedifferentiated histological variant.
What histological variants of malignant fibrous tumors exist?
Malignant fibrous tissue tumors include fibrosarcoma and other specific sarcomas. The following histological variants are distinguished:
- Fibrosarcoma — a very common malignant connective tissue tumor consisting of atypical cells of varying size and shape.
- Low-grade myofibroblastic sarcoma — a malignant tumor with signs of myofibroblastic differentiation.
- Myxoinflammatory fibroblastic sarcoma — a variant of malignant soft tissue lesion.
In which hereditary syndromes is the risk of soft tissue sarcomas increased?
The risk of soft tissue sarcomas is increased in a number of genetic disorders. These hereditary syndromes include:
- Li-Fraumeni syndrome — associated with TP53 suppressor gene mutations.
- Hereditary retinoblastoma — associated with Rb suppressor gene mutations.
- Neurofibromatosis (von Recklinghausen's disease) — an autosomal dominant disorder in which the risk of developing neurofibrosarcoma (malignant peripheral nerve sheath tumor) is 10–15%.
To easily memorize the nomenclature, use a simple rule: "Tissue + OMA = Benign", "Tissue + SARCOMA = Malignant". For example: fibrous tissue gives rise to fibroma (benign) and fibrosarcoma (malignant).
Class and exam questions
How common are malignant soft tissue tumors?
They account for only about 1% of all human malignant neoplasms, with no pronounced geographical distribution patterns.
Which histological variant of liposarcoma has the worst prognosis?
The most unfavorable prognosis is characteristic of retroperitoneal dedifferentiated liposarcomas.
Which immunohistochemical marker confirms the diagnosis of GIST?
Detection of the CD117 (c-kit) marker is used to verify gastrointestinal stromal tumors.
Where else can sarcomas develop besides unaltered tissues?
Malignant soft tissue tumors can form in the area of scars, at fracture sites, and near installed bone implants.
What else to study on this topic
- Morphological differences between well-differentiated and dedifferentiated liposarcoma.
- Tumor-like processes of fibrous tissue: fasciitis and hamartomas.
- Features of hematogenous metastasis of soft tissue sarcomas.
- Application of the TNM system for sarcoma staging.
- The role of the APC gene and chromosomal aberrations in the development of fibrous tumors.
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