• Sheets and clusters of small to intermediate-sized round to oval cells
• High nuclear to cytoplasmic ratio
• Nuclear moulding
• Fine granular chromatin pattern
• Inconspicuous nucleoli
• Scant non-descript cytoplasm
• Closely associated fibrous stroma seen on the PAP stain

The morphologic features are those of a round cell tumour, and ancillary tests are required for definitive diagnosis. Taking into consideration the location of the nodule and age of the patient, the following differential diagnoses are more likely:

• Desmoplastic small round cell tumour
  • Characteristic clinical presentation: young adult male (median age 20) with multiple abdominal nodules
  • Nested appearance with prominent fibrous stroma
  • Polyphenotypic immunohistochemical profile: positive for cytokeratin and show dot-like paranuclear staining for desmin
  • Positive for WT1 (antibody targeting the C-terminus)
  • Shows t(11:22)(p13;q12) EWSR1-WT1 fusion
• Ewing sarcoma
  • Clear cytoplasm containing glycogen which is positive for PAS stain
  • Diffuse membranous positivity for CD99
  • Diffuse nuclear expression for NKX2.2
  • Partially positive for cytokeratins
  • Shows t(11:22)(p24;q12) EWSR1-FLI1 fusion
• Lymphoma, including lymphoblastic lymphoma
  • Cellular discohesion
  • Background lymphoglandular bodies
  • Positive for lymphoid markers: CD45 and TdT (for lymphoblastic lymphoma)
• Rhabdomyosarcoma (embryonal vs alveolar subtype)
  • Focal rhabdoid differentiation
  • Cytologic uniformity suggest alveolar subtype and the presence of PAX3/7-FOXO1 gene fusion
    • Fusion-negative tumours have better prognosis
  • Positive for skeletal muscle markers: desmin, myogenin, myoD1
• Round cell tumours of the abdominal viscera including neuroblastoma, hepatoblastoma and pancreatoblastoma
  • Neuroblastoma
    • May have rosettes, neuropil and ganglionic differentiation
    • Positive for PHOX2B, synaptophysin, chromogranin A, NSE
  • Hepatoblastoma
    • Alternating light and dark areas with trabecular pattern in fetal type (most common)
    • Positive for hepatocyte markers (Hep-Par1) ad glypican-3 in fetal type
    • Embryonal component may stain for glypican-3 and SALL-4
    • Small cell undifferentiated variant may show INI-1 loss
  • Pancreatoblastoma
    • Apart from neuroendocrine elements, shows at least ductal or acinar differentiation
    • Presence of squamoid nests a key defining feature

In an older adult patient, small cell carcinoma should also be considered.

The cell block shows a nondescript round cell tumour with focal necrosis. The cytologic features are better appreciated on the smears. The tumour cells show paranuclear dot-like staining for desmin and focal positivity for NSE.

Desmoplastic small round cell tumour