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During the 1930s and 1940s, the diagnosis of adult or pleomorphic rhabdomyosarcoma was increasingly made, and most of the rhabdomyosarcomas reported during this period were of this type. These tumors occurred mainly in the muscles of the lower extremity and affected older patients. They displayed a striking degree of cellular pleomorphism, but cells with cross-striations were typically absent. It subsequently became apparent that virtually all of these tumors were other types of pleomorphic sarcoma, including malignant fibrous histiocytoma (MFH)—now known as undifferentiated pleomorphic sarcoma (UPS). The redefinition and acceptance of MFH as a diagnostic entity also casted doubt on the existence of pleomorphic rhabdomyosarcoma.
It also became evident that many childhood sarcomas formerly diagnosed descriptively as “round cell” or “spindle cell” sarcomas were rhabdomyosarcomas of alveolar or embryonal type. Knowledge of these tumors was fostered by the introduction of newer, more effective therapies. Before 1960, childhood rhabdomyosarcoma was an almost uniformly fatal neoplasm that recurred and metastasized in a high percentage of cases. During the last six decades, however, it has been shown that this tumor responds to multimodality therapy—encompassing biopsy or conservative surgery, multiagent chemotherapy, and radiotherapy—and that many children treated by these modalities remain free of recurrent and metastatic disease. The numerous reports of the Intergroup Rhabdomyosarcoma Study (IRS) (now recognized as the Soft Tissue Sarcoma Committee of the Children’s Oncology Group) have contributed greatly to our understanding of childhood rhabdomyosarcomas, especially the effect of the various treatment modalities on the survival of patients with this tumor.
As with other sarcomas, evidence is lacking to suggest that rhabdomyosarcoma actually arises from skeletal muscle cells. In fact, these tumors often arise at sites where striated muscle tissue is normally absent (e.g., common bile duct, urinary bladder), or scant (e.g., nasal cavity, middle ear, vagina).
Little is known about the underlying cause of the rhabdomyoblastic proliferations and the stimulus that induces their growth. Genetic factors are implicated by the rare occurrence of the disease in siblings, the occasional presence of the tumor at birth, and the association of the disease with other neoplasms in the same patient. Rhabdomyosarcoma has been described in conjunction with congenital retinoblastoma, familial adenomatous polyposis, multiple lentigines syndrome, type 1 neurofibromatosis, Costello syndrome, Noonan syndrome, and Beckwith-Wiedemann syndrome, among a host of others. Germline mutations in the DICER1 gene predispose affected patients to a broad range of tumors (DICER1 syndrome), including embryonal rhabdomyosarcoma. A 2009 report from the Children’s Oncology Group found an association between first-trimester x-ray exposure and embryonal rhabdomyosarcoma.
Rhabdomyosarcoma is not only the most common soft tissue sarcoma in children under 15 years of age, but also one of the most common soft tissue sarcomas of adolescents and young adults. Rhabdomyosarcoma accounts for an estimated 4.5% of all childhood cancers, with an annual incidence of 6 cases per 1 million per year. It is rare in persons older than 45 and accounts for an estimated 2% to 5% of all adult sarcomas, but it is probably lower than that. There is a bimodal distribution for age at presentation, with the first peak occurring between 2 and 6 years and a second peak between 10 and 18 years. This reflects the peak incidence of embryonal and alveolar rhabdomyosarcomas, respectively.
Arthur Purdy Stout was the first to delineate rhabdomyosarcoma as a distinct entity, and Horn and Enterline devised the first rhabdomyosarcoma classification scheme in 1958. This scheme, also known as the conventional scheme , recognized embryonal, botryoid, alveolar, and pleomorphic subtypes. Most patients in that series died of rhabdomyosarcoma, and the authors were unable to identify any prognostic differences among the four histologic subtypes. This scheme was adopted by the World Health Organization (WHO) Classification of Soft Tissue Tumors and served as the basis for the numerous IRS series to follow, with minor modifications ( Box 19.1 ).
Sarcoma, not classified
Small round cell sarcoma, type indeterminate
Extraosseous Ewing sarcoma
Subsequently, Palmer et al. devised a classification scheme based on tumor cytology rather than tumor architecture. This scheme, known as the cytohistologic scheme , identified two major unfavorable histologic subtypes: the monomorphous round cell type and the anaplastic type. This was the only classification that was not based on the Horn and Enterline scheme; rather, it was devised solely on nuclear morphology.
In 1989 the International Society of Pediatric Oncology (SIOP), including collaborators from 30 European countries, developed a classification scheme that emphasized the relationship between clinical behavior and cellular differentiation in rhabdomyosarcoma subtypes with and without alveolar morphology ( Box 19.2 ). Based on a review of 513 rhabdomyosarcomas from the SIOP tumor registry, collaborators found that an alveolar architecture independently was not prognostically significant. Loose botryoid and dense well-differentiated rhabdomyosarcomas had a better prognosis than loose nonbotryoid, dense poorly differentiated, and alveolar rhabdomyosarcomas. SIOP also delineated embryonal sarcoma as a spindle cell tumor composed of peripheral mesenchymal cells, with no evidence of myoblastic differentiation.
Adult (pleomorphic) rhabdomyosarcoma
Other specified soft tissue tumors
Sarcoma, not otherwise specified
In 1992, collaborators at the Pediatric Branch of the National Cancer Institute (NCI) developed a modification of the conventional scheme, based on their review of 159 rhabdomyosarcomas ( Box 19.3 ). This scheme recognized the favorable prognosis of conventional embryonal rhabdomyosarcoma and three subtypes (pleomorphic, leiomyomatous, and those with aggressive histologic features) and the unfavorable prognosis of alveolar rhabdomyosarcoma. Most important, it also delineated the solid variant of alveolar rhabdomyosarcoma, composed of round tumor cells identical to those in conventional alveolar rhabdomyosarcoma but lacking the characteristic alveolar architecture. These authors found that tumors with any degree of alveolar architecture or cytology had an unfavorable prognosis, regardless of extent.
Embryonal rhabdomyosarcoma (favorable)
Aggressive histologic features
Alveolar rhabdomyosarcoma (unfavorable)
From 1987 to 1991, the IRS committee conducted a comparative study of the various rhabdomyosarcoma classification systems to determine their reproducibility and prognostic significance. The 800 representative rhabdomyosarcomas were reviewed and classified by 16 pathologists, using the conventional, SIOP, NCI, and cytohistologic systems. The highest degree of interobserver and intraobserver reproducibility was achieved using a modification of the conventional system, with fair to good observer agreement ( Table 19.1 ). In addition, the histologic subtypes of the modified conventional system demonstrated a highly significant relationship to survival. Based on the reproducibility and prognostic significance of this system, the IRS proposed a classification scheme, the International Classification of Rhabdomyosarcoma (ICR), which essentially was a modification of the conventional scheme with elements of the SIOP and NCI systems ( Box 19.4 ). The botryoid and spindle cell variants of embryonal rhabdomyosarcoma had a superior prognosis, conventional embryonal rhabdomyosarcoma had an intermediate prognosis, and alveolar rhabdomyosarcoma and undifferentiated sarcoma had a poor prognosis. In addition, this classification scheme included those rhabdomyosarcoma subtypes in which the prognosis had yet to be determined ( rhabdomyosarcoma with rhabdoid features ). Similar to the NCI scheme, the ICR classified a tumor as the alveolar subtype if there were any alveolar architecture or cytology. Pleomorphic rhabdomyosarcoma was excluded, given its extreme rarity in children. The classification has been modified to include the anaplastic variant of rhabdomyosarcoma. Anaplasia (cell pleomorphism, dedifferentiation) is a histologic feature that may be found in any histologic subtype of rhabdomyosarcoma, but is most common in embryonal rhabdomyosarcoma. The most recent WHO classification recognizes four major rhabdomyosarcoma subtypes: embryonal, alveolar, pleomorphic, and spindle cell/sclerosing . As discussed later, molecular genetic features, including fusion status, will undoubtedly play a much larger role in risk stratification and therapeutic strategies.
|System||Interobserver Average Kappa (K)||Intraobserver Average Kappa (K)|
Spindle cell rhabdomyosarcoma
Subtypes whose prognosis is not presently evaluable
Rhabdomyosarcoma with rhabdoid features
Each of the rhabdomyosarcoma subtypes occurs in a characteristic age group. For example, embryonal rhabdomyosarcomas affect mainly, but not exclusively, children between birth and 15 years of age. On the other hand, alveolar rhabdomyosarcoma tends to affect older patients, with peak ages of 10 to 25 years. Rhabdomyosarcomas are uncommon in patients older than 40. The spindle cell type comprises a significant percentage of adult rhabdomyosarcomas (25/57 = 44%), but these differ somewhat from those described in childhood (see next section). There is some correlation between tumor location and age; for example, rhabdomyosarcomas of the urinary bladder, prostate, vagina, and middle ear tend to occur at a younger age (median: 4 years) than those in the paratesticular region or the extremities (median: 14 years for both).
Males are affected more often than females by approximately 1.5 to 1.0, but the male predominance is less pronounced during adolescence and young adulthood and for rhabdomyosarcomas of the alveolar type. Blacks seem to be affected less often than whites.
Although rhabdomyosarcomas may arise anywhere in the body, they occur predominantly in three regions: the head and neck, genitourinary tract and retroperitoneum, and upper and lower extremities. Each rhabdomyosarcoma histologic subtype may occur in virtually any location, but each subtype has a site predilection, as discussed in the specific sections.
The head and neck area is the principal location of rhabdomyosarcoma; 970 (26%) of 3717 tumors from IRS-I, IRS-II, and IRS-III occurred in this location ( Table 19.2 ). In the head and neck, parameningeal tumors are the most common. Parameningeal rhabdomyosarcomas should be distinguished from the other rhabdomyosarcomas arising in the head and neck because of their potential for intracranial extension and seeding, and therefore less favorable clinical course.
|Head and neck||970||26|
The orbit is the second most common head and neck site of rhabdomyosarcoma, accounting for 7% of cases from the IRS series. Most rhabdomyosarcomas in this location are of the embryonal subtype. For example, 221 (90%) of 245 orbital tumors from IRS-I through IRS-IV were of the embryonal subtype, although rare botryoid-type embryonal rhabdomyosarcomas and alveolar rhabdomyosarcomas also arise in the orbit. Rhabdomyosarcoma may also involve other head and neck sites, including the nasal cavity and nasopharynx, followed in frequency by the ear and ear canal, paranasal sinuses, soft tissues of the face and neck, and oral cavity (including the tongue, lip, and palate).
After the head and neck, the genitourinary tract is the second most common site for rhabdomyosarcoma. In the IRS series, 650 (17%) of 3717 cases arose in this general region. Histologically, most tumors arising in this location are of the embryonal subtype. The most common location is the paratesticular region, often of the embryonal or spindle cell/sclerosing subtypes. They may also involve the spermatic cord and epididymis, but usually are separate from the testis proper.
The retroperitoneum and pelvis are other sites of involvement. Approximately 45% of tumors in these sites are of the embryonal subtype, but up to 15% are alveolar rhabdomyosarcomas. In general, effective therapy of rhabdomyosarcomas in the retroperitoneum and pelvic region is more difficult than that of paratesticular rhabdomyosarcomas.
Approximately 5% of rhabdomyosarcomas arise in the urinary bladder or prostate. In fact, rhabdomyosarcoma is the most common bladder tumor in children under 10 years of age. Almost all pediatric tumors arising in this location are embryonal or botryoid rhabdomyosarcomas. Those with a botryoid histology typically grow into the lumen of the urinary bladder as a grapelike, richly mucoid, multinodular or polypoid mass, with a broad base that can cause an obstruction of the internal urethral orifice and prostatic urethra. This in turn results in incontinence and difficulty with urination. Interestingly, however, adult rhabdomyosarcomas of the urinary bladder are more often of the alveolar type, sometimes with anaplastic features, which can cause morphologic confusion with small cell carcinoma. Rarely, rhabdomyosarcomas arise in other genitourinary sites, including the fallopian tube, uterus, cervix, vagina, labium and vulva, and perineum and perianal region. Tumors in these locations are often (but not always) of the botryoid subtype. Rhabdomyosarcomas that arise in gynecologic organs in adults are morphologically similar to those in pediatric patients, but they seem to behave more aggressively.
Unlike adult soft tissue sarcomas, rhabdomyosarcomas involve the extremities much less often. Only 14.6% of cases from the Armed Forces Institute of Pathology (AFIP) series occurred in this location, with a similar incidence in the upper and lower extremities; alveolar rhabdomyosarcomas outnumbered embryonal rhabdomyosarcomas by 4 to 3, similar to IRS-I and IRS-II. Most pleomorphic rhabdomyosarcomas arise in the deep soft tissues of the extremities of adults.
Unusual rhabdomyosarcomas arise outside the aforementioned sites. Tumors originating in the hepatobiliary tract usually arise from the submucosa of the common bile duct; most are botryoid type with typical myxoid, grapelike gross and microscopic appearances.
Rhabdomyosarcoma displays few characteristic features grossly. As with other rapidly growing sarcomas, the appearance of the tumor reflects the degree of cellularity, relative amounts of collagenous or myxoid stroma, and presence and extent of secondary changes (e.g., hemorrhage, necrosis, ulceration). In general, tumors growing into body cavities, such as those in the nasopharynx and urinary bladder, are fairly well circumscribed, multinodular, or distinctly polypoid. On cross-section, they show a glistening, gelatinous, gray-white surface, with patchy areas of hemorrhage or cyst formation. Deep-seated tumors involving or arising in the musculature are usually less well defined and almost always infiltrate the surrounding tissues. They are firmer and rubbery, and have a mottled, gray-white to pink-tan, smooth or finely granular, often bulging surface. Deep-seated rhabdomyosarcomas rarely become large, averaging 3 to 4 cm in greatest diameter. There are often areas of focal necrosis and cystic degeneration.
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.
|Head and neck||411 (71%)||76 (13%)||13 (2%)||77 (13%)||577|
|Genitourinary||246 (71%)||8 (2%)||70 (20%)||1 (<1%)||23 (7%)||348|
|Extremities||76 (24%)||156 (50%)||5 (2%)||74 (24%)||311|
|Trunk||27 (19%)||43 (30%)||3 (2%)||71 (49%)||144|
|Pelvis||45 (48%)||19 (20%)||29 (31%)||93|
|Retroperitoneum||44 (59%)||14 (19%)||1 (1%)||16 (21%)||75|
|Perineum/anus||13 (33%)||19 (48%)||1 (2%)||1 (2%)||6 (15%)||40|
|Other sites||15 (39%)||9 (24%)||4 (11%)||10 (26%)||38|
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.
The molecular underpinnings of embryonal rhabdomyosarcoma are complex and characterized by various chromosomal gains and losses. Whole chromosome gains (chromosomes 2, 8, 11, 12, 13, and 20) are relatively common, but some show whole chromosome losses, including monosomy 10 and 15. The most characteristic finding is loss of heterozygosity (LOH) for multiple, closely linked loci at chromosome 11p15.5. This molecular alteration results in inactivation of growth factors and tumor suppressor genes, including GOK , H19 , CDKN1C , HOTS, and IGF2 . In a comprehensive analysis of rhabdomyosarcoma by comparative genomic hybridization (CGH), Shern et al. found a higher number of oncogenic mutations in embryonal rhabdomyosarcoma than in the alveolar subtype, including alterations in NRAS , KRAS , HRAS , FGFR4 , PIK3CA , CTNNB1 , FBXW7, and BCOR . The majority of these tumors show alterations of the receptor tyrosine kinase/ RAS/PIK3CA axis, providing potential opportunities for therapeutic intervention. Aberrations of the ALK gene, although identified in some alveolar rhabdomyosarcomas, are also frequently seen in embryonal rhabdomyosarcomas and correlate with metastatic disease and poor disease-specific survival. They also offer an opportunity for targeted therapy with ALK inhibitors, such as ceritinib. Genes involved in the hedgehog pathway, including GLI1 and PTCH1 , have also been implicated in the pathogenesis of this tumor.
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 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).
Alveolar rhabdomyosarcoma is characterized by distinctive cytogenetic abnormalities that allow its distinction from other rhabdomyosarcoma subtypes and other round cell neoplasms in the differential diagnosis. Approximately 60% of cases have a t(2;13)(q35;q14) translocation, which results in the generation of two derivative chromosomes: a shortened chromosome 13 and an elongated chromosome 2. The breakpoints occur within the PAX3 gene on chromosome 2 and the FOXO1A gene (formerly FKHR ) on chromosome 13, resulting in a PAX3-FOXO1A fusion gene on chromosome 13 and a FOXO1A-PAX3 fusion gene on chromosome 2. Both these genes encode transcription factors that regulate the expression of specific target genes. The chimeric gene encodes for a chimeric protein that excessively activates expression of genes with PAX3 -binding sites, including MCYN . The PAX3-FOXO1A fusion appears to be more sensitive and specific than the FOXO1A-PAX3 fusion in detecting this tumor.
Approximately 20% of alveolar rhabdomyosarcomas are associated with a variant translocation, t(1;13)(p36;q14), which juxtaposes the PAX7 gene on 1p36 with the FOXO1A gene on 13q14. A high degree of homology exists between PAX3 and PAX7 . Therefore, it is likely that the fusion proteins act as aberrant transcription factors to regulate a common set of target genes involved in the pathogenesis of alveolar rhabdomyosarcoma. In addition to cytogenetic examination, these molecular abnormalities can be detected by RT-PCR or FISH using either frozen or paraffin-embedded tissues, with a high degree of specificity. Overall, about 80% of tumors diagnosed histologically as alveolar rhabdomyosarcoma are found to have the PAX3-FOXO1A or PAX7-FOXO1A fusion ( Table 19.4 ).
|Barr||16/21 (76%)||2/21 (10%)||1/30 (3%)||1/30 (3%)|
|De Alava||7/13 (54%)||2/13 (15%)||0/9 (0%)||0/9 (0%)|
|Downing||20/23 (87%)||2/12 (17%)|
|Arden||8/13 (62%)||1/13 (8%)||0/11 (0%)||0/11 (0%)|
|total||51/70 (73%)||5/47 (11%)||3/62 (5%)||1/50 (2%)|
Fusion-positive alveolar rhabdomyosarcomas frequently show genomic amplifications. In addition to amplification of PAX7-FOXO1A , about 20% of cases show MYCN amplification on 2p24. About 25% of cases show amplification of a region of 12q13-14, which includes a number of genes including CDK4 . The majority of PAX7-FOXO1A –positive cases show amplification of a region of 12q31 that includes the MIR17HG gene.
Approximately 20% of cases diagnosed histologically as alveolar subtype lack PAX3- or PAX7-FOXO1A fusions. Some of these fusion-negative cases could be “low expressors,” in which the fusion is actually present in rare cells only, or there are cryptic genomic fusions that cannot be detected by standard techniques. In addition, some cases of fusion-negative alveolar rhabdomyosarcoma are incorrectly classified, although true fusion-negative tumors clearly exist. In a large group of alveolar rhabdomyosarcomas blinded to fusion status, Parham et al. found that histologic parameters were of limited utility in predicting fusion status. However, almost half the fusion-negative cases had a totally solid architecture (vs. none with a PAX3-FOXO1A fusion and only rarely with PAX7-FOXO1A fusions). In a report from the Soft Tissue Sarcoma Committee of the Children’s Oncology Group, 33% of cases originally diagnosed as alveolar rhabdomyosarcoma were rediagnosed as embryonal rhabdomyosarcoma after re-review. Many of these reclassified cases were actually embryonal rhabdomyosarcomas with dense cellularity that were misdiagnosed as solid alveolar rhabdomyosarcoma. All reclassified cases proved to be fusion negative. Nishio et al. found some of these true fusion-negative cases to have a mixed histology with both alveolar and embryonal areas.
Interestingly, gene expression array analyses have found distinct differences between fusion-positive and fusion-negative cases. In fact, the fusion-negative cases show overlap with cases classified as embryonal rhabdomyosarcoma. In a recent consensus paper, Borinstein et al. made a strong recommendation to test for FOXO1A fusions on all patients with alveolar or embryonal histology, since this would have a major impact on the therapeutic decision making. Fusion-positive and fusion-negative cases have also been found to have distinct methylation profiles. Sun et al. found an 11-gene DNA methylation signature that could accurately classify fusion-positive and fusion-negative cases. As discussed later, the pattern of myogenin immunoreactivity also differs in fusion-positive and fusion-negative cases, and an IHC panel incorporating myogenin, AP2β, NOS-1, and HMGA2 has been proposed as a surrogate marker of fusion status. Further, a subgroup of fusion-negative cases have been found to have alternate fusions, including novel fusions of PAX3 with NCOA1 , NCOA2 or FOXO4, as well as FOXO1-FGFR1 . In rare cases, FGFR4 serves as an alternate fusion partner.
A number of other genes have been implicated in the pathogenesis of alveolar rhabdomyosarcoma. ALK gene copy number gain and IHC expression of the ALK protein have been reported in the majority of alveolar rhabdomyosarcomas. Inactivating mutations of CDKN2A/CDKN2B and TP53 , as well as activating mutations of FGFR4 , are found in a subset of alveolar rhabdomyosarcomas.