Micrograph of a metastatic<unk><unk>  with the characteristic cytoplasmic clearing on H&E staining, which was showing to be PAS positive The definitive diagnosis is based on histomorphologic findings, immunohistochemistry and molecular pathology.<unk><unk>  is a small-blue-round-cell tumor that typically has a clear cytoplasm on H&E staining, due to glycogen. The presence of the glycogen can be demonstrated with positive PAS staining and negative PAS diastase staining. The characteristic immunostain is CD99, which diffusely marks the cell membrane. However, as CD99 is not specific for<unk><unk> , several auxiliary immunohistochemical markers can be employed to support the histological diagnosis. Morphologic and immunohistochemical findings are corroborated with an associated chromosomal translocation, of which several occur. The most common translocation, present in about 90% of<unk><unk>  cases, is t(11;22)(q24;q12), which generates an aberrant transcription factor through fusion of the<unk> R1 gene with the FLI1 gene. The pathologic differential diagnosis is the grouping of small-blue-round-cell tumors, which includes lymphoma, alveolar rhabdomyosarcoma, and desmoplastic small round cell tumor, among others. Medical imaging X-ray of a child with<unk><unk>  of the tibia On conventional radiographs, typical findings of<unk>  consist of multiple confluent lytic bone lesions that have a "moth eaten" pattern due to permeative destruction of bone. There will also be a displaced periosteum as the new sub-periosteal layer of bone begins to grow on top of the tumor. This raised or displaced periosteum is consistent with the classic radiographic finding of the Codman triangle. The proliferative reaction of bone can also create delicate laminations constituting the periosteal layers and giving the radiographic appearance of an onion peel. Plain films add valuable information in the initial evaluation or screening. The wide zone of transition (e.g. permeative) is the most useful plain film characteristic in differentiation of benign versus aggressive or malignant lytic lesions. Magnetic resonance imaging slice showing<unk><unk>  of the left hip (white area shown right) Magnetic resonance imaging (MRI) should be routinely used in the work-up of malignant tumors. It will show the full bony and soft tissue extent and relate the tumor to other nearby anatomic structures (e.g. vessels). Gadolinium contrast is not necessary as it does not give additional information over noncontrast studies, though some current researchers argue that dynamic, contrast-enhanced MRI may help determine the amount of necrosis within the tumor, thus help in determining response to treatment prior to surgery. Computed axial tomography (CT) can also be used to define the extraosseous extent of the tumor,<unk> pecially in the skull, spine, ribs, and pelvis. Both CT and MRI can be used to follow response to radiation and/or chemotherapy. Bone scintigraphy can also be used to follow tumor response to therapy. In the group of malignant small round cell tumors that includes<unk><unk> , bone lymphoma, and small cell osteosarcoma, the cortex may appear almost normal radiographically, while permeative growth occurs throughout the Haversian channels. These tumors may be accompanied by a large soft-tissue mass while almost no bone destruction is visible. The radiographs frequently do not show any signs of cortical destruction. Radiographically,<unk><unk>  presents as "moth-eaten" destructive radiolucencies of the medulla and erosion of the cortex with expansion. Differential diagnosis Other entities with similar clinical presentations include osteomyelitis, osteosarcoma <unk> pecially telangiectatic osteosarcoma), and eosinophilic granuloma. Soft-tissue neoplasms such as pleomorphic undifferentiated<unk>  (malignant fibrous histiocytoma) that erode into adjacent bone may also have a similar appearance. Accumulating evidence suggests that<unk> R1-NFATc2 positive<unk> s, which were previously considered to possibly belong to the<unk>  family of tumors, differ from<unk><unk>  in their genetics, transcriptomes, and epigentic and epidemiological profiles, indicating that they might represent a distinct tumor entity.
Ewing's sarcoma
Osteosarcoma
Osteoblastoma
Eosinophilic granuloma
Enchondroma
Neuroblastoma
Synovial sarcoma
Osteomyelitis