This is a light microscopy image of a bone tumor biopsy stained with Hematoxylin and Eosin (H&E). The image demonstrates the classic small round blue cell morphology of Ewing sarcoma. The tumor is composed of uniform, small, primitive-appearing cells with scant pale eosinophilic or clear cytoplasm and rounded to oval nuclei with evenly dispersed chromatin and small or inconspicuous nucleoli. Cytoplasmic borders are indistinct, producing a syncytial appearance. Mitotic activity is variable and may be increased. Intracytoplasmic glycogen is common and can be highlighted with periodic acid–Schiff (PAS) staining; this feature supports the diagnosis of this entity. The tumor cell nests may arrange around delicate vasculature to form pseudorosettes; true Homer-Wright rosettes may be present in some cases. The stroma is a delicate vascular network; necrosis is frequent, appearing as large geographic areas or smaller foci. Together these features produce a dense, sheets-and-nests pattern with a high nuclear-to-cytoplasmic ratio typical of small round blue cell tumors. The image is diagnostic when integrated with immunohistochemical and molecular studies (e.g., CD99 positivity, EWSR1 rearrangement) but the morphology shown is highly characteristic of Ewing sarcoma. This histology is pathognomonic in the appropriate clinical context and is essential for prognosis and treatment planning.

Imaging modality: Histopathology using light microscopy of formalin-fixed, paraffin-embedded bone tissue stained with Hematoxylin and Eosin (H&E). The slide displays a skeletally derived lesion with features classic for Ewing sarcoma. At high cellular density, there are sheets of small round blue cells with scant cytoplasm and round to oval nuclei showing hyperchromasia and brisk mitotic activity. The tumor cells are arranged in uniform clusters, separated by delicate fibrous strands, creating a highly cellular, often undulating landscape. Areas of geographic necrosis are frequently seen within the tumor, producing pink, acellular zones that disrupt cohesive cell architecture. Cytoplasmic scantiness and high nuclear-to-cytoplasmic ratio are typical; occasional rosette-like formations (Homer-Wright) may be present but are not required for diagnosis. The overall histologic pattern corresponds to a high-grade small round blue cell neoplasm in bone. Although not shown in this image, CD99 membrane positivity is commonly observed in Ewing sarcoma and, together with molecular testing for EWSR1-FLI1 fusion (t(11;22)) or related rearrangements, strengthens the diagnosis. Clinically, this histology correlates with aggressive bone lesions in children and adolescents and has important differential considerations including lymphoma, rhabdomyosarcoma, neuroblastoma, and osteosarcoma. Correlation with radiology and immunohistochemistry is essential for definitive diagnosis and prognostic assessment in clinical practice overall.

This light-microscopy histology slide shows Desmoplastic small round cell tumor (DSRCT) morphology. Hematoxylin and eosin (H&E) staining reveals sharply demarcated nests and anastomosing trabeculae of small round blue cells with scant cytoplasm and hyperchromatic nuclei, embedded in a densely desmoplastic fibrous stroma. The tumor cells exhibit brisk mitotic activity and occasional rosette-like patterns. The desmoplastic background contains abundant collagen with prominent vascularity, contributing to the fibrous, hyalinized appearance. While not visible by this stain alone, typical immunophenotype includes coexpression of epithelial and mesenchymal markers (cytokeratins, EMA) and desmin in a dot-like perinuclear pattern; WT1 positivity supports the diagnosis. Molecular confirmation commonly shows EWS-WT1 fusion from t(11;22)(p13;q12). Clinically, DSRCT is an aggressive small round cell sarcoma arising primarily in the abdomen or pelvis with potential for peritoneal spread and multicentric involvement; prognosis is poor despite multimodal therapy. This image is valuable for educational purposes, illustrating the characteristic nest-desmoplastic matrix contrast and serving as a reference in differential diagnosis against Ewing sarcoma/PNET, neuroblastoma, and rhabdomyosarcoma. It underscores the need for ancillary studies to confirm diagnosis in pathology practice. This image emphasizes classic DSCRT morphology and the essential role of molecular testing in confirming the EWS-WT1 translocation. This makes it valuable for research and education.

This histopathology image represents a small round blue cell tumor of the renal pelvis, consistent with primitive neuroectodermal tumor (PNET)/Ewing family tumor. Acquired from a renal pelvis specimen showing invasion into the urothelial lining, the section is stained with hematoxylin and eosin and viewed under light microscopy at low to intermediate magnification. The tumor is organized in broad sheets of uniform, round cells with scant cytoplasm and hyperchromatic nuclei, yielding a classic high nuclear-to-cytoplasmic ratio. Mitotic figures are frequent and apoptotic activity may be evident; the surrounding renal parenchyma shows desmoplastic response and focal invasion of the mucosa, with the urothelium on the left preserved in outline. The lesion’s morphology mirrors other Ewing sarcoma family tumors and underlines the close morphologic and genetic kinship to EWSR1-associated neoplasms. Immunophenotyping (where performed) would typically show CD99 (MIC2) expression and may reveal EWSR1 gene rearrangement (e.g., EWSR1-FLI1) by molecular testing. Clinically, such tumors are aggressive and require multidisciplinary management. This image highlights the diagnostic significance of recognizing small round blue cell morphology in the renal pelvis, differentiating from Wilms tumor, lymphoma, rhabdomyosarcoma, and neuroblastoma, and underscores the role of histology complemented by molecular diagnostics for accurate classification and staging.