Tumors of connective tissue

Abstract

Fibrous tissue consists of fibroblasts and an extracellular matrix containing both fibrillary structures (collagen, elastin) and nonfibrillary, gel-like ground substance. Both fibroblasts (spindle-shaped cells) and myofibroblasts, which are modified fibroblasts showing features common to fibroblasts and smooth muscle cells, produce procollagen and collagen. Collagen is the main, noncontractile component of the extracellular matrix, elastin is the main, contractile component of elastic fibers. The amorphous ground substance of fibrous tissue contains glycosaminoglycans (mucopolysaccharides), the most common types being hyaluronic acid, chondroitin 4- and 6-sulfates, and proteoglycans. [56] Fibrous tissue can be loose or dense depending on the relative amount of the three components. Dense fibrous tissue is seen in tendons, ligaments, and aponeuroses. Fibroblastic/myofibroblastic tumors represent a very large subset of mesenchymal tumors. Many lesions in this category contain cells with both fibroblastic and myofibroblastic features, which may in fact represent functional variants of a single cell type. The relative proportions of these cell types vary not only between individual cases but also within a single lesion over time (often in proportion to cellularity). A significant subset of spindle cell and pleomorphic sarcomas are probably myofibroblastic in type, but, to date, only low-grade forms have been reproducibly characterized. Among lesions formerly known as malignant fibrous histiocytoma (MFH), at least some represent pleomorphic myofibrosarcomas. Principal changes and advances since the 1994 WHO classification have been the characterization of numerous previously undefined lesions, including ischemic fasciitis, desmoplastic fibroblastoma, mammary-type myofibroblastoma, angiomyofibroblastoma, cellular angiofibroma, Gardner fibroma, low-grade fibromyxoid sarcoma, acral myxoinflammatory fibroblastic sarcoma, sclerosing epithelioid fibrosarcoma, and low-grade myofibroblastic sarcoma. Conceptual changes have included the clearer recognition of solitary fibrous tumor in soft tissue and the realization that most cases of so-called hemangiopericytoma belong in this category, as well as the reclassification of lesions formerly labeled myxoid MFH as myxofibrosarcoma and the definitive allocation of the tumors to the fibroblastic category. The new (2002) WHO classification of fibroblastic/ myofibroblastic tumors [20] contains a large number of new and modified lesions. They are categorized into four groups according to their degree of malignancy, i.e., (1) benign, (2) intermediate (locally aggressive), (3) intermediate (rarely metastasizing), and (4) malignant: 1. Benign: Nodular fasciitis Proliferative fasciitis Proliferative myositis Myositis ossificans Fibro-osseous pseudotumor of digits Ischemic fasciitis Elastofibroma Fibrous hamartoma of infancy Myofibroma/Myofibromatosis Fibromatosis colli Juvenile hyaline fibromatosis Inclusion-body fibromatosis Fibroma of tendon sheath Desmoplastic fibroblastoma Mammary-type myofibroblastoma Calcifying aponeurotic fibroma Angiomyofibroblastoma Cellular angiofibroma Nuchal-type fibroma Gardner fibroma Calcifying fibrous tumor Giant cell angiofibroma 2. Intermediate (locally aggressive): Superficial fibromatoses (palmar/plantar) Desmoid-type fibromatoses Lipofibromatosis 3. Intermediate (rarely metastasizing): Solitary fibrous tumor and hemangiopericytoma (including lipomatous hemangiopericytoma) Inflammatory myofibroblastic tumor Low-grade myofibroblastic sarcoma Myxoinflammatory fibroblastic sarcoma Infantile fibrosarcoma 4. Malignant: Adult fibrosarcoma Myxofibrosarcoma Low-grade fibromyxoid sarcoma hyalinizing spindlecell tumor Sclerosing epithelioid fibrosarcoma In the case of several fibroblastic proliferations (elastofibroma, fibroma of tendon sheath, extra-abdominal desmoids,fibromatosis colli), it is unclear whether these constitute reactive fibrosing processes or true neoplasms. Fibromatoses are aggressive, infiltrating lesions, despite their histologically benign character, and aggressive fibromatoses or musculoaponeurotic desmoid tumors are by far the largest group of tumors of fibrous tissue. The terminology of childhood fibromatosis is confusing, and there are many classification systems based on different clinicopathological parameters such as age, localization, histology, and aggressiveness of the lesion [86]. Fibrous tumors of infancy and childhood and soft tissue fibrosarcomas are rare and have only sparsely been reported in the radiological literature. It is a constant finding that the majority of these tumors have a high recurrence rate after surgical resection, and recurrent lesions mostly have a more aggressive behavior than their primary counterparts. Another constant finding is the natural evolution of tumors of fibrous tissue, which are hypercellular in their initial stage and become more collagenous in later stages.Localization of the lesion and age of the patient are major diagnostic factors. Fibroma of tendon sheath, elastofibroma, all types of fibromatosis, and fibromatosis colli are characterized by typical localizations [30, 85].