Nature of tumour:

1. It is a highly malignant tumour resulting in death of 90% patients.

2. Serosal involvement including pleura suggests a possibility of a primitive tumour of mesothelial origin.

3. Fusion of Ewing’s sarcoma gene on chromosome 22 and the Wilm’s tumour gene on chromosome 11, suggest chromosomal translocation t(11;22).

Incidence: 

1. Age: Adolescent and young adults (mean age 22 yrs). 

2. Sex:  Male:Female ratio is 4:1.

Clinically:

1. Abdominal distension, pain and palpable abdominal, pelvic or scrotal mass, often associated with ascites.

2. Lymph node involvement is rare but occasionally they may be the first manifestation of the disease.

Site:

Wide spread abdominal serosal involvement. Isolated cases occur in limbs , head and neck, and brain.

Other extra-abdominal sites:   Kidney ; Lung ; Pancreas ; Ovary. Sinonasal ; Orbit ; Paratesticular ; Liver ; Pleura ; Thoracic ; Mediastinum ; Parotid gland.

Gross:

1. Characteristic gross features:

i) a single large tumors up to 40 cm. in diameter with smooth or bossellated outer surface. or ii) multiple nodules.

2. Cut surfaces are firm to hard, gray-white with focal myxoid change and necrosis.

3. Direct invasion of intra-abdominal or pelvic viscera is restricted to serosa.

Microscopic features:

1.Tumour cells are usually small, round to oval & monotonous, very scanty cytoplasm, indistinct cell  borders with hyperchromatic nuclei and indistinct nucleoli.

2.Mitotic figures and single necrotic cell are numerous.

3. Tumour cells are arranged in well-defined basaloid nests and trabeculae of varying size and shape, separated by cellular desmoplastic stroma (fibroblasts & myofibroblasts)

4.Peripheral palisading, central necrosis with or without calcification, are present in larger islands.

5. Some cases may show: Rosette-like spaces. Others may show eosinophilic “inclusions” and eccentric nucleus resulting in a rhabdoid appearance.

6. Rarely, tubular or glandular differentiation (with luminal mucin), papillae, vacuolated signet-ring cells may be present

5.Vascular invasion, especially lymphatics is common.

Immunocytochemistry:

Tumour displays simultaneous expression of epithelial (Low molecular weight cytokeratin ,EMA), muscular (desmin) and neural (neuron-specific enolase) markers.    

Image1(desmin):; Image2(cytokeratin) ; Image3 (neuron-specific enolase

1.Cytokeratin reactivity has a diffuse cytoplasmic quality.

2.The tumour is positive for WT1, a feature resulting from gene fusion.

3. Vimentin is strongly expressed but stains for actin and myogenin are negative.

4. Desmin & Vimentin positivity is typically paranuclear and globular.

5. Neural/Neuroendocrine (NSE, Leu7) is positive.

6.CD99 (antigen associated with Ewing’s Sarcoma) is usually negative.

7. Mesothelial marker (Calretinin & thrombodulin) are usually negative.

Differential diagnosis: 

Rhabdomyosarcoma, lymphoma, neuroblastoma, PNET. Immunohistochemical expression of WT1 by desmoplastic small round cell tumor: a comparative study with other small round cell tumors.Am J Surg Pathol. 2000 Jun;24(6):830-6.