Miscellaneous Mesenchymal Tumors


Chapter 16

Miscellaneous Mesenchymal Tumors



Lipoma




Definition


Lipoma is a rare primary tumor of bone composed of mature adipose tissue. Lipomas involving bone are divided into three types: intramedullary (intraosseous), intracortical/subperiosteal, and parosteal. Lipomas involving synovial tissue are described in Chapter 20.



Incidence and Location


Lipomas of bone are extremely rare; fewer than 50 cases are described in the literature.



Intramedullary Lipoma


Intramedullary lipomas usually occur in the metaphyseal parts of major long bones of the lower extremity—the femur, tibia, and fibula.36,814,16,17,20,21,23,25,26,28,29,34 The calcaneus is also a common site.1,12,19,21,22,27,31,33 Lipomas also have been reported in the sacrum, coccyx, mandible, maxilla, skull, ribs, and metatarsals.36,37 Pain and swelling are the most frequent presenting symptoms, but most of these lesions are asymptomatic and may only be incidentally noted on radiographs.



Radiographic Imaging


Radiographically, intramedullary lipoma is seen as a lytic lesion with trabeculation (Figs. 16-1 and 16-2). Bone expansion with a thinned but intact cortex may be present (Fig. 16-2). Central, niduslike calcification is a frequent radiographic feature of intramedullary lipoma (Figs. 16-2 and 16-3). Magnetic resonance imaging may show no discernible signal alteration because lipoma is composed of mature adipose tissue with signal characteristics similar to normal fatty marrow (Fig. 16-1). Lipomas with extensive fat necrosis may appear as low signal density lesions with signal void in areas of calcifications (Fig. 16-4).








Microscopic Findings


Histologically, lipomas consist of mature adipose tissue and do not differ from ordinary lipomas of soft tissue (Fig. 16-4). The presence of fat necrosis with dystrophic calcifications, which correspond to the central, niduslike opacity seen radiographically, is frequently seen (Figs. 16-3 and 16-4).



Intracortical and Subperiosteal Lipoma


Intracortical and subperiosteal lipomas are extremely rare.2,24,28,38 They radiographically present as intracortical or subperiosteal lucent lesions with intralesional trabeculations and foci of dotlike calcifications. Histologically, these lesions consist of mature adipose tissue and, similar to intramedullary lipomas, they do not differ from ordinary lipomas of soft tissue.



Parosteal Lipoma


Parosteal lipomas are not true bone tumors and belong to the category of soft tissue lesions. Because of their association with the bone surface, parosteal lipomas may have some distinctive radiographic and microscopic features. Similar to intramedullary lipomas, parosteal lipomas of bone are extremely rare.7,15,17,18,20,30,32,35 They usually involve the metaphyseal parts of long tubular bones, but they may involve other bones, such as the flat bones of the trunk and the bones of the upper extremities (Fig. 16-5). A characteristic feature of parosteal lipoma (not always seen) is a sclerotic, osseous pedicle that grows from the surface of the bone into the adipose tissue. This bony excrescence can dominate the lesion and can result in radiographic misdiagnosis of the lesion as an osteochondroma. Foci of cartilaginous metaplasia with formation of a cartilaginous cap may be present.




Liposarcoma




Definition


Liposarcoma is an extremely rare primary sarcoma of bone; it is composed of immature adipose tissue.



Incidence and Location


Liposarcoma of bone is one of the rarest of primary bone tumors.3948 In 1982, Addison and Payne accepted only six examples of this entity in bone from previously published reports.24 The patients ranged in age from 15 to 53 years but usually had symptoms during the third and fourth decades of life. Reported cases of intraosseous liposarcoma involved the major long tubular bones, such as the femur, tibia, and humerus.



Radiographic Imaging


Radiographic features are nonspecific and show a bone-destructive process indicative of a malignant tumor.



Microscopic Findings


Microscopically, liposarcomas in bone represent high-grade pleomorphic or round-cell (signet-ring) lesions. Some of these tumors may have features of myxoid liposarcoma.



Differential Diagnosis


The high-grade pleomorphic liposarcoma should be differentiated from malignant fibrous histiocytoma. In the evaluation of adipose tissue and differentiation of intraosseous lesions, the presence of residual nonneoplastic tissue of fatty marrow infiltrated by a nonlipomatous tumor should be ruled out before the lesion is classified as a liposarcoma. It is our impression that at least some of the cases previously reported as primary liposarcomas of bone would be currently classified as malignant fibrous histiocytomas.



Clinical Behavior


Approximately 50% of patients who were reported to have primary liposarcoma of bone had metastases after amputation.



Vascular-Cartilaginous Hamartoma of the Chest Wall




Definition


Vascular-cartilaginous hamartoma is a rare and distinct clinical, radiologic, and pathologic entity that presents at birth or early infancy as an expansile intraosseous lesion involving the central portion of one or more ribs.



Incidence and Location


McLeod and Dahlin56 reported nine cases from the Mayo Clinic’s consultation files. The lesion typically involves the chest wall and most likely originates within the ribs.4959



Gross Findings


The gross findings of these tumors are those of smooth, well-circumscribed, red-brown masses that arise from the surface of the rib. The cut surface is marked by the presence of many dilated channels filled with blood. These spaces are separated by a friable, red-brown stroma that contains variable amounts of gritty, shiny, white, cartilaginous tissue.




Microscopic Findings


Microscopically, these lesions show a mixture of cellular fibrous tissue that contains many blood-filled, endothelial-lined vascular spaces; numerous islands of randomly distributed mature hyaline cartilage; and reactive new bone (Figs. 16-6 and 16-7). The cartilage islands represent well-developed nodules of cartilaginous tissue that are well demarcated from the surrounding vascular component and occasionally form irregular clusters. The vascular component is similar to conventional hemangioma and may contain large, highly cellular areas composed of spindle cells forming narrow vascular channels. The spindle-cell areas may show prominent mitotic activity, but atypical mitoses are absent. Larger-caliber vessels are also present and may form clusters mimicking common (cavernous) hemangioma. Vascular-cartilaginous hamartoma frequently shows features of superimposed secondary aneurysmal bone cyst.49,5456 In fact, aneurysmal bone cyst features can dominate the lesion. Some of the lesions may contain coalescent nodules of mature hyaline cartilage that obscure the spindle and vascular component of the lesion (Fig. 16-9).







Melanotic Neuroectodermal Tumor of Infancy




Definition


Melanotic neuroectodermal tumor of infancy is a rare, distinctive neoplasm that frequently affects children younger than age 1 year. The tumor is composed of nests of cells that have melanin pigment and is also referred to as retinal anlage tumor, melanotic progonoma, or melanotic ameloblastoma.



Incidence and Location


Melanotic neuroectodermal tumors usually involve the maxilla or mandible but also occur in the soft tissue, the epididymis, and the mediastinum or may present as intracranial (dural or brain) masses.60,6264,6769,71,72,79,80,82,85 They may present as lesions in the long bones. The lesion can be clinically associated with increased levels of catecholamines and urinary excretion of vanillylmandelic acid.61,70



Radiographic Imaging


Radiographically, the tumor presents as a well-demarcated, lytic bone lesion.63,72,80 The preoperative diagnosis can be suspected radiographically if the lesion presents in typical locations—the maxilla or mandible—in a child during the first year of life.



Gross Findings


The lesion is fibrous and tan, brown, or black, depending on the proportion of the pigmented component.



Microscopic Findings


Microscopic features are characteristic and consist of two basic cell populations: small neuroblastic cells and larger, melanin-containing epithelial cells arranged in a distinct architectural pattern.65,66,7378,81 The tumor cells form nests within a predominantly fibrous stroma. The nests are delineated by epithelial, pigment-containing cells (Figs. 16-10 to 16-14). The central portion of these structures with alveolar or tubular arrangements is filled by small, loosely arranged neuroblastic cells in a neurofibrillary matrix (Fig. 16-13, B to D). The neuroblastic cells occasionally form larger, more diffuse areas in which epithelial alveolar structures are loosely distributed. This may create a more disorganized pattern of neuroblastic cells and solid ephithelial proliferations. The presence of heterologous elements (i.e., mesenchymal spindle cells and woven bone) has also been described but is extremely rare.83







The tumor cells, particularly the epithelial pigment–containing cells, are positive for keratin and occasionally positive for epithelial membrane antigen and the HMB-45 marker. In addition, both components (i.e., small neuroblastic cells and epithelial cells) are positive for neuron-specific enolase and Leu-7 and occasionally are positive for synaptophysin. Glial fibrillary acidic and S-100 proteins may be focally positive in a small-cell neuroblastic component. This immunohistochemical pattern is consistent with the postulated neuroectodermal derivation of these tumors.73,84,86,92,81



Treatment and Behavior


Melanotic neuroectodermal tumors of infancy generally behave as benign lesions, but local recurrence is possible. In addition, approximately 6% of patients develop distant metastases. The development of distant metastases cannot be correlated with any specific microscopic features, the distribution of DNA ploidy, or tumor proliferation parameters.



Perivascular Epithelioid Cell Tumor




Definition


Perivascular epithelioid cell tumor (PEComa) is a rare distinct neoplasm of bone made up of epithelioid clear cells rich in glycogen and characterized by coexpression of muscle and melanocytic markers. This term has been applied to an expanding family of tumors that are presumed to originate from microscopically and phenotypically unique perivascular epithelioid cells, although there is no known normal cellular counterpart to these cells, and precursor lesions for PEComas have not been identified.8891,99,101 It includes angiomyolipoma (both renal and extrarenal), lymphangiomyomatosis, clear cell sugar tumor of the lung, and a variety of spindled and epithelioid neoplasms involving various organs, soft tissue, and bone with a similar phenotype.88,92,93,96,98,100,105,108110,114,116 The unifying concept proposed for this family of tumors evolved over the past century and was developed by identifying unique clear cell features as well as the smooth muscle and melanocytic characteristics of the tumor cells. A distinct type of benign clear cell tumor referred to as clear cell sugar tumor was described in the lung in 1963 by Liebow and Castleman.107 The name refers to the clear cytoplasm of the cells, which is rich in glycogen. Despite their epithelioid appearance, the clear cells are negative for epithelial markers and exhibit distinctive coexpression of muscle and melanoma markers.88,89,97,100,105,106,112,115 The same immunophenotypic features are also present in a subset of cells in lymphangiomatosis and angiomyolipoma. On the basis of the common and distinctive phenotype, Bonetti et al, Pea et al, and Zamboni et al defined a family of lesions characterized by the presence of this peculiar cell type, the perivascular epithelioid cell (PEC), and designated the group of tumors with similar features as PEComas.88,89,90,118 These tumors have also been referred to as PEST (primary extrapulmonary sugar tumors).114 Patients with one or more of the PEC family of lesions may have the tuberous sclerosis complex. The tuberous sclerosis complex is typically associated with angiomyolipomas and lymphangiomyomatosis, but PEComas in other anatomic sites can develop in this syndrome.91 Two-step inactivation of the TSC1 and TSC2 genes activating the Rheb/mTOR/p70S6K pathway underlie the development of PEComas in tuberous sclerosis.101,117 Comparative genomic hybridization studies of PEComas disclose frequent losses and gains of multiple chromosomes, implicating major chromosomal rearrangements.111 Some PEComas show rearrangements of the transcription factor E3 (TFE3) gene and overexpression of the TFE3 protein immunohistochemically.87,95,104,113



Incidence and Location


Our experience is restricted to four cases of primary clear cell sugar tumor of bone seen in consultation. These presented in the calcaneus of a 9-year-old boy, the humerus of a 10-year-old girl, the patella of a 29-year-old man, and the proximal phalanx of the middle finger of the left hand in a 33-year-old woman. One of the cases showed a clonal cytogenetic abnormality consisting of reciprocal chromosomal translocation t(X;2) (q11-12; q34-35).


May 31, 2017 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Miscellaneous Mesenchymal Tumors
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