Soft tissue tumors are a broad group of neoplasms originating from mesenchymal derivatives, primarily the muscle and connective tissues of the musculoskeletal system. Benign variants are hundreds of times more common than malignant ones and have a favorable prognosis, whereas sarcomas are characterized by aggressive infiltrative growth and hematogenous metastasis.
HistogenesisThese tumors originate from mesenchymal derivatives.
SarcomasMalignant tumors exhibit architectural and cellular atypia and produce hematogenous metastases.
Benign TumorsCharacterized exclusively by architectural atypia without cellular polymorphism.
Treatment and PrognosisThe mainstay of treatment is surgical excision; malignant forms frequently recur.
Tumors of Muscle Tissue
These neoplasms arise from striated muscle elements.
Rhabdomyoma: A benign tumor that macroscopically presents as a dense, grayish-white node up to 15 cm in diameter. Microscopically, the tumor consists of large spindle-shaped and round clear cells with well-visible transverse (and sometimes longitudinal) striations in the cytoplasm. Cellular atypia is completely absent, and the stroma consists of a delicate argyrophilic network. The prognosis after excision is favorable.
Rhabdomyosarcoma: A rare but extremely aggressive malignant tumor. It typically locates deep within the muscles of the trunk and extremities. Atypical locations—such as the retroperitoneal fat, mediastinum, or urogenital tract organs—are also possible. Histological preparations show marked cellular polymorphism (cells mimicking embryonic muscle tissue), pronounced nuclear atypia, and the presence of "bizarre" atypical mitoses. The tumor metastasizes extensively early on and frequently recurs, leading to a poor prognosis.
Tumors of Connective Tissue
This group is the most common among all soft tissue neoplasms.
Fibroma: The most frequent benign connective tissue tumor. It grows as a well-circumscribed, encapsulated, dense gray node. It occurs virtually everywhere, including the mammary glands, ovaries, and along nerve trunks (neurofibromas). It consists of bundles of mature spindle-shaped fibroblasts surrounded by a fibrous stroma that may undergo hyalinosis.
Fibrosarcoma: The malignant counterpart, frequently affecting the retroperitoneum and lower extremity tissues (especially the thigh and knee joint). It lacks a capsule and grows infiltratively. On cross-section, the tissue is soft, resembling "reddish fish flesh," with foci of necrosis and hemorrhage. The cells are arranged haphazardly, nuclei are hyperchromatic, and numerous bizarre atypical cells and abnormal mitoses are present. It recurs in more than 50% of cases.
Fibrous Histiocytoma: The benign variant (dermatofibroma) usually arises in the skin (up to 1 cm in size), with spindle cells forming storiform ("whirled") structures. The malignant form can grow up to 20 cm, infiltrates skeletal muscles, and exhibits pronounced polymorphism. Malignant histological variants include pleomorphic, giant cell, angiomatoid, and myxoid types (with myxoid changes in the stroma).
Borderline and Tumor-Like Lesions
This group includes processes that may clinically or morphologically resemble malignant tumors but have a different nature.
Desmoid (Aggressive Fibromatosis): Occupies an intermediate position between benign and malignant tumors because it is capable of destructive, infiltrative growth. It consists of mature fibroblasts with rare mitoses. The abdominal variant frequently arises in women from the rectus abdominis muscle sheath (particularly active during pregnancy).
Pseudosarcomatous Proliferations: Reactive lesions arising in response to trauma or ischemia. An example is nodular fasciitis, a rapidly growing mass frequently located on the forearm. It consists of immature fibroblasts with high mitotic activity but no signs of cellular atypia.
Myositis Ossificans (Heterotopic Ossification): The appearance of circumscribed bone tissue (3–6 cm) near or at a distance from the periosteum, often following trauma. Muscle fibers themselves are not involved. Common locations include the elbow region and thigh.
Fibromatoses: Local proliferation of fibroblasts and collagen fibers. Includes palmar fibromatosis (Dupuytren's contracture—flexion deformity of the 4th and 5th digits), plantar (without contractures), and penile fibromatosis (Peyronie's disease, causing penile curvature).
Mnemonic
To remember desmoid tumor locations, link them by site: abdominal is "female" (abdominal wall, pregnancy), extra-abdominal is "general" (shoulders, chest, thighs in both sexes), and intra-abdominal is "deep" (mesentery, pelvis).
Frequently asked questions
What histological variants of rhabdomyosarcoma are distinguished?
Current classification distinguishes the following histological variants of rhabdomyosarcoma:
Embryonal — the most common variant, resembling primitive mesenchyme with mucoid edematous stroma.
Botryoid — a subtype of embryonal, featuring a compact cambium layer of hyperchromatic cells beneath the epithelium.
Spindle cell — a subtype of embryonal, forming fascicles of elongated cells.
Alveolar — characterized by cellular clusters separated by fibrovascular connective tissue septa.
Pleomorphic — recognized in the nomenclature of malignant tumors.
What histological variants of liposarcoma exist?
The following histological variants of liposarcoma exist:
Well-differentiated — includes lipoma-like, sclerosing, inflammatory, and spindle cell subtypes.
Dedifferentiated — a mixture of atypical adipocytes with sarcomatous elements of different architecture.
Myxoid — recognized in general histological classification.
Round cell — distinguished among the tumor subtypes.
Pleomorphic — also classified as a histological variant of liposarcoma.
Which tumors develop from soft tissue blood vessels?
Vascular tumors develop from blood vessels and include:
Hemangiomas — benign tumors of blood vessels; often congenital and slow-growing.
Angiosarcoma — the malignant vascular tumor variant.
Postradiation angiosarcoma — noted among vascular tumors of the breast.
Epithelioid hemangioma — an intermediate, locally aggressive vascular tumor.
Epithelioid hemangioendothelioma — a malignant vascular tumor.
What are the microscopic features of synovial sarcoma?
The microscopic picture of synovial sarcoma features extreme polymorphism and biphasic, spindle cell, or epithelioid architecture.
The tumor consists of undifferentiated fibroblasts and atypical clear synoviocytes with hyperchromatic nuclei.
Atypical synoviocytes form clusters, nests, and tubular structures.
Atypical fibroblasts together with stromal tissue form fascicles running in various directions.
A characteristic feature is the biphasic structure of the tumor.
How do malignant soft tissue tumors differ from benign ones?
Benign tumors exhibit only architectural atypia. Sarcomas display both architectural and marked cellular atypia, infiltrate surrounding tissues, and metastasize predominantly via hematogenous pathways.
What is Dupuytren's contracture?
It is a form of palmar fibromatosis characterized by localized proliferation of fibroblasts with thickening of the fascia, leading to a slowly progressive flexion contracture of the 4th and 5th digits of the hand.
What are the characteristic histological features of rhabdomyosarcoma?
It is characterized by marked cellular polymorphism (cells mimicking embryonic structures), severe nuclear atypia, and the presence of large cells with multiple "bizarre" mitoses.
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