Lipid-rich rhabdomyosarcoma--a potential source of diagnostic confusion

Haploinsufficiency of the lysosomal sialidase NEU1 results in a model of pleomorphic rhabdomyosarcoma in mice

Rhabdomyosarcoma, the most common pediatric sarcoma, has no effective treatment for the pleomorphic subtype. Still, what triggers transformation into this aggressive phenotype remains poorly understood. Here we used Ptch1+/-/ETV7TG/+/- mice with enhanced incidence of rhabdomyosarcoma to generate a model of pleomorphic rhabdomyosarcoma driven by haploinsufficiency of the lysosomal sialidase neuraminidase 1. These tumors share mostly features of embryonal and some of alveolar rhabdomyosarcoma. Mechanistically, we show that the transforming pathway is increased lysosomal exocytosis downstream of reduced neuraminidase 1, exemplified by the redistribution of the lysosomal associated membrane protein 1 at the plasma membrane of tumor and stromal cells. Here we exploit this unique feature for single cell analysis and define heterogeneous populations of exocytic, only partially differentiated cells that force tumors to pleomorphism and promote a fibrotic microenvironment. These data together with the identification of an adipogenic signature shared by human rhabdomyosarcoma, and likely fueling the tumor's metabolism, make this model of pleomorphic rhabdomyosarcoma ideal for diagnostic and therapeutic studies.

Adipocyte-Like Differentiation in a Posttreatment Embryonal Rhabdomyosarcoma

We describe a 16-year-old boy with rhabdomyosarcoma, consistent with embryonal subtype, of the lower extremity who received systemic neoadjuvant chemotherapy and subsequent excision. Microscopic sections of the postchemotherapy excision demonstrated diffuse, prominent, and immature adipocyte-like differentiation, in addition to skeletal muscle differentiation. Adipocyte-like differentiation was confirmed by a combination of positive Oil Red O and adipophilin immunohistochemical staining. To our knowledge, this represents the first report of an unusual phenomenon of differentiation of a soft tissue rhabdomyosarcoma into adipocyte-like cells after chemotherapy.

DAX-1 Expression in Pediatric Rhabdomyosarcomas: Another Immunohistochemical Marker Useful in the Diagnosis of Translocation Positive Alveolar Rhabdomyosarcoma

Objectives

The aim of this study was to investigate the expression of DAX-1 in a series of pediatric rhabdomyosarcomas (RMS) with known translocation and compare it to Ap2β, known to be selectively expressed in ARMS.

Design

We revised a series of 71 alveolar rhabdomyosarcomas (ARMS), enrolled in the Italian Protocols RMS 79 and 96, and 23 embryonal rhabdomyosarcomas (ERMS) as controls. Before investigating Ap2β and DAX-1, ARMS were reviewed and reclassified as 48 ARMS and 23 non-ARMS.

Results

Translocation positive ARMS showed a characteristic Ap2β/DAX-1+ staining pattern in 78% of cases, while 76% of classic ERMS were negative for both. Ap2β alone was positive in 3.9% of RMS lacking translocation, whereas DAX-1 alone was positive in 25.4%. Conversely, 9% and 6% of translocation positive ARMS were positive only for DAX-1 or Ap2β, respectively. The 23 non-ARMS shared the same phenotype as ERMS but had a higher frequency of DAX-1 expression.

Conclusions

DAX-1 is less specific than Ap2β, however it is a sensitive marker for translocation positive ARMS and can be helpful in their diagnosis if used in combination with Ap2β.

Myogenic tumors in children and adolescents

Neoplasms of striated and smooth muscle in children are a diverse group of neoplasms that have some unique aspects in contrast to these tumors in adults. Rhabdomyosarcoma is the most common soft tissue sarcoma of infancy and childhood and is relatively common in adolescents. In contrast, smooth muscle tumors are relatively rare, and the various types of rhabdomyoma and smooth and skeletal muscle hamartomas are very uncommon. In recent years, the understanding of the pathologic and genetic aspects of rhabdomyosarcoma has been enhanced by adjunct techniques, such as immunohistochemistry and cytogenetic or molecular genetic analysis. The current classification of rhabdomyosarcoma emphasizes the histologic-prognostic correlations. This article reviews the clinicopathologic features of striated and smooth muscle tumors with an emphasis on the unique aspects of these neoplasms in children and adolescents and the differential diagnosis.

Pleomorphic rhabdomyosarcoma showing smooth-muscle and fibrohistiocytic differentiation: a single case report

Rhabdomyosarcoma has traditionally been subclassified into alveolar, embryonal, and pleomorphic variants. Less commonly, spindle-cell, neuroendocrine, sclerosing, and lipid-rich or clear-cell subtypes are seen. The author recently encountered a myogenic sarcoma, with all the common markers of rhabdomyosarcoma, but expressing the unusual features of alpha-smooth-muscle actin and abundant rough endoplasmic reticulum (rER). This myogenic sarcoma, therefore, exhibited four lines of differentiation, and is documented here. The patient was a 65-year-old man with an inguinal soft tissue mass. Following surgical excision, the patient was given radiotherapy and was well without disease after 6 years. The tumor was positive for vimentin, desmin, alpha-smooth-muscle actin, alpha-sarcomeric actin, myogenin, MyoD1, and CD68. Cytoplasm was dominated by abundant rER intermingled with lipid droplets and lysosomes. Cell surfaces exhibited microvillous processes and focal adhesions, but no lamina. Subplasmalemmal smooth-muscle-type myofilaments with focal densities and rare sarcomeric filaments were seen. The low level of expression of some markers was interpreted as consistent with a poorly differentiated tumor. Given the four lines of differentiation--striated muscle, smooth muscle, fibroblastic, and histiocytic--a name reflecting its phenotype would be pleomorphic rhabdomyosarcoma showing smooth-muscle and fibrohistiocytic differentiation.

Rhabdomyosarcomas in adults and children: an update

CONTEXT

Rhabdomyosarcomas comprise a relatively common diagnostic entity among childhood cancers and a relatively rare one among adult tumors. They may possess a variety of histologies that generally differ among age groups. These lesions appear to be separate biologic entities as well as morphologic categories, with embryonal tumors having genetic lesions related to loss of heterozygosity and aberrant parental imprinting, alveolar tumors containing genetic fusions between PAX and forkhead genes, and pleomorphic tumors showing an accumulation of genetic lesions similar to other adult high-grade sarcomas.

OBJECTIVE

To present guidelines for diagnosis of rhabdomyosarcoma and recent finding concerning the biology and classification of these lesions.

DATA SOURCES

Review of recent and older published literature and distillation of the authors' experience.

CONCLUSIONS

Infants and young children tend to have embryonal rhabdomyosarcomas, adolescents and young adults tend to have alveolar rhabdomyosarcomas, and older adults tend to have pleomorphic rhabdomyosarcomas, although there is some overlap. Newer rare entities, including spindle cell rhabdomyosarcoma and sclerosing rhabdomyosarcoma, have been described in children and adults. Fusion-positive tumors have a distinct molecular signature with downstream activation of a number of myogenic and tumorigenic factors. Genetic testing may be successfully used for diagnosis and may guide therapy in future clinical trials. Differential diagnosis has become simpler than in previous years, because of use of myogenic factors in immunohistochemistry, but classification based solely on histologic features remains challenging.

Clear cell rhabdomyosarcoma

A clear cell rhabdomyosarcoma was studied by light microscopy, histochemistry, immunohistochemistry, and electron microscopy. It was a large, painful left parapharyngeal mass in a 10-year-old boy with intracranial extension and cervical metastatic enlarged lymph nodes. Tumor tissue was macroscopically grayish. At microscopic examination, the architecture was diffuse and focally alveolar. Tumor cells were of three types. Most cells were large, round or polygonal, with abundant clear vacuolated cytoplasm. Fibrils were sometimes found to be present around the nucleus. Nuclei often had irregular outlines and multiple nucleoli. Mitotic activity was high. Some round or elongated cells had eosinophilic fibrillar cytoplasm and were found to have a few double striations. A few cells were round and medium sized with a high nucleocytoplasmic ratio. Periodic acid-Schiff stain demonstrated huge amounts of intracytoplasmic glycogen in clear cells. Tumor cells showed positive immunostaining for muscle markers (desmin, muscle actins, dystrophin). Electron microscopy showed large lakes of glycogen, lipid droplets, and striated muscle features.

Lipid rich rhabdomyosarcoma

A lipid rich rhabdomyosarcoma of the paratesticular region was studied by light microscopy, histochemistry, immunohistochemistry and electron microscopy. The tumour was composed of primitive looking, vacuolated, and pleomorphic cells. Lipid was present in varying amounts in all cells but was especially abundant in the vacuolated and pleomorphic cells. Some cells showed eosinophilic fibrillary cytoplasm but cross-striations were not seen. Tumour cells were positive for desmin, muscle specific actin, and vimentin. A few cells were myoglobin positive. At electron microscopy, the presence of lipid was confirmed, while thick and thin filaments, Z disks, lamina and glycogen were observed, thereby confirming striated muscle differentiation. Although moderate amounts of lipid can be expected in almost any tumour, lipid rich rhabdomyosarcomas have received little attention. The present report provides a comprehensively examined case of such a tumour initially presenting diagnostic difficulty because of its possible confusion with liposarcoma.