Small round blue cell tumours

Concept

Pattern, not a diagnosis: malignant tumours composed of small, round, dark “blue” cells with scant cytoplasm and high N:C ratio.
Strongly associated with paediatric / young adult malignancies.
On H&E alone they look similar → need immunohistochemistry + molecular to subtype.
Think “SRBCT club” and then narrow by age + site + imaging behaviour.

Mental

SRBCT = “this looks like a primitive paediatric malignancy”; final label depends on where it is and what markers/genetics show.

Neuroblastoma
- Age: infant / young child.
- Site: adrenal / sympathetic chain.
- Imaging: calcified suprarenal mass, vessel encasement, crosses midline, bone/marrow mets.
Ewing sarcoma / PNET
- Age: child / teen.
- Site: diaphyseal long bones, pelvis, ribs, chest wall (Askin).
- Imaging: aggressive permeative bone lesion, onion-skin periosteal reaction, large soft-tissue component.
Rhabdomyosarcoma (alveolar/embryonal)
- Age: child / adolescent.
- Site: head & neck (parameningeal), orbit, GU tract, extremity.
- Imaging: soft-tissue mass, often crossing fascial planes; may cause bone erosion but not primarily osseous.
Wilms tumour (blastemal component)
- Age: young child.
- Site: kidney.
- Imaging: large intrarenal mass with claw sign; calcification less common than neuroblastoma.
Lymphoma
- Any age, but SRBCT look especially in kids/teens.
- Site: nodes, abdominal mass, bone.
- Imaging: homogeneous soft-tissue mass, tends to encase rather than invade vessels; often multi-station nodal disease.
Desmoplastic small round cell tumour (DSRCT)
- Age: adolescent / young adult male.
- Site: peritoneal surfaces, pelvis.
- Imaging: multiple peritoneal/omental masses, often with ascites; primary organ not obvious.
Medulloblastoma
- Age: child.
- Site: posterior fossa (midline vermis).
- Imaging: hyperdense posterior fossa mass, often restricting diffusion.

Tip

When you see “small round blue cell tumour”, don’t freeze – immediately anchor:

Then default patterns: