What is the clinical evidence from this study?

Study design: Case Report. Population: Undifferentiated small round cell tumors (n=3). Intervention: Immunohistochemistry and molecular diagnostic evaluation.

What does this research mean for the field?

Immunohistochemistry and molecular techniques are essential for accurately classifying pediatric small round cell tumors, with CD99 serving as an effective initial screening marker. Novelty: ClaimNovelty.INCREMENTAL. Consensus alignment: ConsensusAlignment.NEUTRAL.

What question did this study set out to answer?

The aim is to highlight the diagnostic challenges and the clinicopathological spectrum of undifferentiated small round cell tumors in children.

May 16, 2026Open Access

Undifferentiated Small Round Cell Tumor in Children

Key Result

Diagnostic evaluation using immunohistochemistry and molecular techniques successfully classified three pediatric small round cell tumors, identifying CD99 as a useful initial screening marker.

Key Points

The aim is to highlight the diagnostic challenges and the clinicopathological spectrum of undifferentiated small round cell tumors in children.
Analyzed three pediatric cases over one year at our institution.
Utilized histopathological examination, immunohistochemistry, and molecular studies for diagnosis.
Focused on morphological features, IHC markers, and treatment implications for each tumor.
Ewing’s sarcoma was the most frequently diagnosed with CD99 positivity.
Fibrous medulloblastoma exhibited high nuclear-to-cytoplasmic ratio and positivity for synaptophysin.
Peripheral primitive neuroectodermal tumor demonstrated positivity for CD99 and Vimentin.

Study Design

Type

Case Report (n=3)

Multicenter

Structured PICO

Population

3 pediatric patients with undifferentiated small round cell tumors

Intervention

Diagnostic workup including histopathology, immunohistochemistry (IHC), and molecular studies

Outcome

Accurate diagnosis and classification of tumors

Accurate diagnosis of pediatric undifferentiated small round cell tumors requires a combination of histopathology, IHC, and molecular studies to guide appropriate multimodal therapy.

Abstract

Undifferentiated small round cell tumors are rare aggressive pediatric neoplasms with overlapping morphology. Accurate diagnosis requires a combination of histopathological, immunohistochemistry (IHC), and molecular studies. Early detection is crucial for appropriate triaging and timely management in this age group. Here, we are discussing the clinicopathological spectrum of three pediatric small round cell tumors that we encountered in our institution in a span of one year. Morphological overlap makes it difficult to distinguish them on routine histology alone. IHC and molecular techniques (e.g., EWSR1 translocation in Ewing’s sarcoma) are crucial for accurate classification. Correct diagnosis directly impacts treatment strategies and prognosis. For example, Ewing’s sarcoma requires multimodal therapy (chemotherapy, surgery, and radiotherapy), while medulloblastoma management is centered around neurosurgery, craniospinal irradiation, and chemotherapy. In our experience, Ewing’s sarcoma family tumor was most frequently encountered, making CD99 a good screening marker for the initial evaluation of pediatric undifferentiated small round cell tumor. Case 1: An 8-year-old boy presented with a mass involving the left arm in the humerus bone. Diagnostic workup with bone biopsy revealed a small round cell tumor with Homer-Wright Rossette. Tumor cells showed strong diffuse positivity for CD99. Diagnosis – Ewing’s sarcoma Image 1a-d.Image 1: (a) X-ray image showing an aggressive bone lesion, moth-eaten appearance in the humerus with a typical sunburst appearance. (b) H and E images in different magnification showing a small round tumor cells in sheetsImage 1: (c) Homer Wright Rosettes and (d) tumor cells are showing diffuse and strong membranous positivity for CD99Case 2: An 8-year-old girl presented with headache and vomiting for 2 months. Contrast-enhanced magnetic resonance imaging of the brain showed a 4th ventricular tumor with mass effect. Histopathological examination revealed densely packed round cells arranged in lobules separated by thick fibrous septae. Tumor cells had a high nuclear-to-cytoplasmic ratio. Tumor cells on IHC were positive for synaptophysin. Diagnosis – Fibrous medulloblastoma Image 2 a-e.Image 2: (a) Hematoxylin and eosin (H and E) image showing normal brain parenchyma. (b-d) H and E sections showing a small round tumor infiltrating the brain parenchyma, with tumor cells arranged in lobules separated by thick fibrous septae. High power images exhibiting a high nuclear-to-cytoplasmic ratio. (e) Tumor cells showing diffuse and strong positivity for synaptophysinCase 3: A 14-year-old boy presented with right-sided chest wall swelling with dyspnea. An excisional biopsy was done from the 9th costochondral junction. H and E revealed a small round cell tumor in diffuse sheets with few showing clear cytoplasm and rich vascularity. No rosettes could be discerned. Tumor cells showed cytoplasmic positivity for the PAS stain. On IHC, tumor cells were positive for CD99 and Vimentin. Diagnosis – Peripheral primitive neuroectodermal tumor Image 3a-f.Image 3: (a-c) H and E images showing sheets of tumor cells, many showing clearing of cytoplasmImage 3: (d) Tumor cells showing cytoplasm positivity for Periodic acid–Schiff stain. (e) Tumor cells are diffusely and strongly positive for CD99Image 3: (f) Tumor cells are diffusely positive for VimentinDeclaration of patient consent The authors certify that they have obtained patient consent forms for images and other clinical information to be reported in the journal. The patients understand that their name sand initials will not be published and due efforts will be made to conceal his identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.