Rhabdomyosarcoma

Embryonal Rhabdomyosarcoma

Embryonal rhabdomyosarcoma (without other distinguishing features) accounts for approximately 60% of all rhabdomyosarcomas, occurring in 2.6 per million children younger than 15 years in the United States. It mostly affects children younger than 10 (mean age: almost 7 years), but it also occurs in adolescents and young adults. In contrast, it is uncommon in patients older than 40. There is a slight male predominance. The most common site of involvement is the head and neck, particularly the orbit and parameninges ( Table 19.3 ). After the head and neck, this tumor is most commonly found in the genitourinary tract, followed by the deep soft tissues of the extremities and the pelvis and retroperitoneum.

Histologically, embryonal rhabdomyosarcoma closely resembles various stages in the embryogenesis of normal skeletal muscle. However, its pattern is much more variable, ranging from poorly differentiated tumors that are difficult to diagnose without immunohistochemical examination, to well-differentiated neoplasms that resemble fetal muscle. Features common to most include (1) varying degrees of cellularity, alternating between densely packed, hypercellular areas and loosely textured, myxoid areas ( Figs. 19.1 to 19.4 ); (2) a mixture of poorly oriented, small, undifferentiated, hyperchromatic, round or spindle-shaped cells, along with a varying number of differentiated cells with eosinophilic cytoplasm, characteristic of rhabdomyoblasts ( Figs. 19.5 to 19.7 ); and (3) a matrix containing little collagen and varying amounts of myxoid material. Cross-striations are discernible in 50% to 60% of cases.

The least well-differentiated examples of embryonal rhabdomyosarcoma correspond in appearance to developing muscle at 5 to 8 weeks’ gestation. For the most part, they consist of small, round or spindle-shaped cells with darkly staining hyperchromatic nuclei and indistinct cytoplasm. The nuclei vary slightly in size and shape (more so than those of alveolar rhabdomyosarcoma), have one or two small nucleoli, and usually exhibit a high rate of mitotic activity. Differentiated rhabdomyoblasts are either absent entirely or are confined to a few small areas, making it mandatory to examine multiple sections from different portions of the tumor; adjunctive diagnostic procedures are required to confirm the diagnosis in virtually all cases (discussed later).

Better-differentiated examples have, in addition to the primitive or undifferentiated cellular areas, larger round or oval eosinophilic cells, characteristic of rhabdomyoblasts ( Figs. 19.6 to 19.8 ). The cytoplasm of these cells contains granular material or deeply eosinophilic masses of stringy or fibrillary material, concentrically arranged near or around the nucleus. Cross-striations are rare in the round cells; if present, they are usually confined to narrow bundles of concentrically arranged myofibrils at the circumference of the rhabdomyoblast ( Fig. 19.8 ). Degenerated rhabdomyoblasts with a glassy or hyalinized, deeply eosinophilic cytoplasm and pyknotic nuclei, but without cross-striations, are a frequent feature of this tumor.

Cross-striations are more readily discernible in embryonal rhabdomyosarcomas with a more prominent spindle cell component, tumors that might be regarded as the morphologic equivalent of normal muscle at 9 to 15 weeks of intrauterine development ( Figs. 19.9 and 19.10 ). These neoplasms are composed mainly of a mixture of undifferentiated cells and differentiated fusiform or elongated cells that are readily identifiable as rhabdomyoblasts on light microscopy. The rhabdomyoblasts range from slender spindle-shaped cells with a small number of peripherally placed myofibrils, to large eosinophilic cells with a strap, ribbon, tadpole, or racket shape, one or two centrally positioned nuclei, and prominent nucleoli, with or without cross-striations. Cross-striations in neoplastic cells differ from those in residual or entrapped muscle cells by their more irregular distribution and because they often traverse only part of the tumor cell. Intracellular granules may be confused with cross-striations, but their granular nature is readily apparent after a careful examination of the cell under oil immersion. Sometimes, the strap-shaped cells are sharply angulated, and form a diagnostically useful zigzag or “broken straw” pattern. Most of these tumors have only a moderate degree of cellular pleomorphism.

Defined similarly in Wilms tumor, anaplasia in rhabdomyosarcoma consists of large, lobate, hyperchromatic nuclei (at least three times the size of neighboring nuclei), with or without large, atypical mitotic figures. Embryonal rhabdomyosarcomas with a prominent degree of cellular pleomorphism (anaplasia) are rare. While some cases are difficult to distinguish from adult pleomorphic rhabdomyosarcomas ( Fig. 19.11 ), the more common cross-striations in childhood tumors and the identification of areas of more typical embryonal rhabdomyosarcoma facilitate the diagnosis. Survival in patients with diffuse anaplasia in embryonal rhabdomyosarcoma is similar to the unfavorable survival of patients with alveolar rhabdomyosarcoma.

There are also extremely well-differentiated embryonal rhabdomyosarcomas, whose features include almost entirely rounded, spindle-shaped, or polygonal rhabdomyoblasts; abundant eosinophilic cytoplasm; and frequent cross-striations. Some of these differentiated tumors are found in recurrent or metastatic neoplasms after prolonged therapy ( Fig. 19.12 ), possibly because of the selective destruction of undifferentiated tumor cells.

Glycogen is demonstrable in most rhabdomyosarcomas, regardless of type. Removal of the glycogen during fixation results in multivacuolated cells or spider cells, which are large rhabdomyoblasts with narrow strands of cytoplasm connecting the center of the cell with its periphery. The centrally located nuclei and the irregular shape of the cytoplasmic vacuoles help distinguish these cells from the more rounded, lipid-filled vacuoles of lipoblasts. In contrast to alveolar rhabdomyosarcoma, multinucleated giant cells are rare in embryonal rhabdomyosarcomas.

Occasionally, the embryonal rhabdomyosarcoma displays, in addition to its rhabdomyoblastic component, foci of immature cartilaginous ( Fig. 19.13 ) or osseous tissue, or both. These tumors occur at any age and any location, but seem to be more common in the genitourinary tract and retroperitoneum.

Embryonal Rhabdomyosarcoma, Botryoid Type

Botryoid rhabdomyosarcoma accounts for approximately 6% of all rhabdomyosarcomas. The botryoid variant (Greek botrys, “bunch of grapes”) is characterized grossly by its polypoid (grapelike) growth. Microscopically, it demonstrates a relative sparsity of cells and abundance of mucoid stroma, often resulting in a myxoma-like picture. Most botryoid rhabdomyosarcomas are found in mucosa-lined, hollow organs, such as the nasal cavity, nasopharynx, bile duct, urinary bladder, and vagina ( Fig. 19.14 ). Tumors of this type may also be encountered in areas where the expanding neoplasm reaches the body surface, as in some rhabdomyosarcomas of the eyelid or anal region. Clearly, its unrestricted growth in body cavities or on body surfaces accounts for its characteristic edematous and botryoid appearance.

Although a grapelike configuration has traditionally been a defining feature of the botryoid variant, the ICR scheme does not require this characteristic gross appearance. According to the ICR criteria, a cambium layer, characterized by a subepithelial condensation of tumor cells separated from an intact surface epithelium by a zone of loose stroma, must be present to recognize this variant ( Figs. 19.15 to 19.19 ). The tumor cells should form a distinct zone that is several layers thick, although the thickness may vary in extent in different areas of the tumor. The cells range from primitive small cells to cells with clear-cut rhabdomyoblastic differentiation ( Fig. 19.19 ). Cells with stellate cytoplasmic processes are often prominent. The stroma is typically loosely cellular with a myxoid appearance, including a hypocellular zone that separates the surface epithelium from the underlying cambium layer. The surface epithelium may be hyperplastic or may undergo squamous changes, sometimes mimicking a carcinoma.

On immunohistochemistry (IHC), there is usually strong staining for myogenic antigens, particularly in cells showing light microscopic evidence of rhabdomyoblastic differentiation. Using cytogenetics, Palazzo et al. reported deletion of the short arm of chromosome 1 and trisomies of chromosomes 13 and 18. A second reported case showed a hyperdiploid clone with a complex karyotype, including numerous chromosomal gains. Manor et al. described a patient with trisomy 8. Aberrations of 11q21 were reported in several cases of botryoid rhabdomyosarcoma.

Alveolar Rhabdomyosarcoma

Alveolar rhabdomyosarcoma is the second most common subtype, accounting for approximately 31% of all rhabdomyosarcomas. This variant tends to arise at a slightly older age than embryonal and botryoid rhabdomyosarcomas, with a peak incidence at 10 to 25 years. It has a predilection for the deep soft tissues of the extremities, although the tumor may arise in many other sites, including the head and neck, genitourinary tract, and gynecologic sites.

Histologically, alveolar rhabdomyosarcoma is mainly composed of ill-defined aggregates of poorly differentiated round or oval tumor cells that frequently show central loss of cellular cohesion and formation of irregular alveolar spaces ( Figs. 19.20 to 19.27 ). The individual cellular aggregates are separated and surrounded by a framework of dense, frequently hyalinized fibrous septa that surround dilated vascular channels. Characteristically, the cells at the periphery of the alveolar spaces are well preserved and adhere in a single layer to the fibrous septa in a manner somewhat reminiscent of an adenocarcinoma or papillary carcinoma. The cells in the center of the alveolar spaces tend to be more loosely arranged, or freely floating ( Figs. 19.24 and 19.25 ); they are often poorly preserved and show evidence of degeneration and necrosis. In rare instances, viable cells are virtually absent, and the tumor consists merely of a coarse, sievelike or honeycomb-like meshwork of thick, fibrous trabeculae. The trabeculae surround small, loosely textured groups of severely degenerated cells with pyknotic nuclei and necrotic cellular debris.

Solid forms of this tumor, which lack an alveolar pattern entirely. Instead, they are composed of densely packed groups or masses of tumor cells that resemble the round cell areas of embryonal rhabdomyosarcoma, but demonstrate a more uniform cellular picture ( Figs. 19.21 and 19.24 ). These solidly cellular areas are more often encountered at the periphery of the tumor, and probably represent the most active and most cellular stage of growth. In most cases, examination of the solid tumor shows, in addition to the uniform cellular pattern, incipient alveolar features. Even in the solid areas, there is a regular arrangement of fibrous septa that surround the primitive round cells. Also, in rare cases the cells have abundant pale-staining, glycogen-containing cytoplasm and vaguely resemble clear cell carcinoma or clear cell malignant melanoma ( clear cell rhabdomyosarcoma ).

The individual cells in both alveolar and solid portions of the tumor have round or oval hyperchromatic nuclei with scant amounts of indistinct cytoplasm. Bulbous or club-shaped cells, sometimes with deeply eosinophilic cytoplasm, are often seen protruding from the fibrous walls into the lumen of the alveolar spaces. Mitotic figures are common. Neoplastic rhabdomyoblasts that display pronounced stringy or granular eosinophilic cytoplasm are less common in alveolar than in embryonal rhabdomyosarcomas, but are still present in up to 30% of cases. Most of the rhabdomyoblasts in the alveolar spaces have a round or oval configuration ( Fig. 19.26 ); those located in or attached to the fibrous septa tend to be strap or spindle shaped. If present, cross-striations are almost exclusively found in the spindle-shaped cells.

Multinucleated giant cells are a prominent and diagnostically important feature ( Figs. 19.23 and 19.25 ). Usually, the giant cells have multiple, peripherally placed nuclei, as well as pale-staining or weakly eosinophilic cytoplasm, without cross-striations. Transitional forms between rhabdomyoblasts and giant cells suggest that the latter are formed by cellular fusion. Collagen formation is usually confined to the intervening septa, but occasionally, large portions of the tumor are obliterated by extensive fibroplasia. Some cases contain areas that are indistinguishable from conventional embryonal rhabdomyosarcoma, and were previously considered to be variants of alveolar rhabdomyosarcoma. However, as discussed later, most of these tumors lack PAX-FOXO1A fusions and seem to be more closely related both clinically and biologically to embryonal rhabdomyosarcoma.

Most alveolar rhabdomyosarcomas originate in muscle tissue, and entrapment of normal muscle fibers is common. These fibers are apt to be mistaken for neoplastic rhabdomyoblasts with cross-striations, a feature that sometimes results in the correct diagnosis for the wrong reason.

Metastatic alveolar rhabdomyosarcomas in lymph nodes, lung, and other viscera also display a distinct alveolar pattern ( Fig. 19.27 ), making it unlikely that this pattern is merely the result of infiltrative growth along the fibrous framework of the involved musculature. Diffuse bone marrow metastases may be mistaken for leukemia.

The immunoprofile of alveolar rhabdomyosarcoma is similar to that of other rhabdomyosarcomas (see later). Because the differential diagnosis includes numerous other small round cell tumors, a large battery of immunostains (and perhaps even molecular genetic analysis) is often required to exclude other entities. Expression of keratins and neuroendocrine markers (e.g., synaptophysin, CD56) is common in alveolar rhabdomyosarcomas and may represent a significant diagnostic pitfall, particularly when the differential diagnosis includes small cell carcinoma (e.g., in older adults or in mucosal/visceral locations).