Histopathology of malignancy

With the histopathology of childhood malignancies, many of these cancers fall under the category of “small round blue cell tumours.” Under the microscope, they often look quite similar at first glance—densely packed cells with dark nuclei and little cytoplasm—but each has its own distinguishing features. Some examples include:

• Medulloblastomas usually show sheets of small round cells with very little cytoplasm, dark hyperchromatic nuclei, and brisk mitotic activity.
• With acute lymphoblastic leukaemia, the bone marrow is replaced by lymphoblasts that have a high nuclear-to-cytoplasmic ratio, fine chromatin, and barely visible nucleoli. Immunophenotyping is essential to distinguish whether these blasts are of B-cell or T-cell lineage.
• Acute myeloid leukaemia as larger blasts with more obvious nucleoli and, importantly, may display Auer rods—needle-like inclusions that are quite characteristic.
• Neuroblastoma consists of small round blue cells sitting in a background of delicate fibrillary material called neuropil.
• Rhabdomyosarcoma shows small round cells but some of the cells take on elongated shapes, and sometimes cross-striations, hinting at skeletal muscle differentiation.
• The renal malignancy Wilms tumour has a distinctive triphasic appearance: a blastemal component that looks like sheets of small blue cells, an epithelial component forming primitive tubules or glomeruloid structures, and a stromal component that can even show muscle or cartilage.

So, while many childhood cancers share the same general “blue cell” morphology, the key to diagnosis lies in recognizing these subtle but specific histological patterns, and often confirming them with immunohistochemistry and molecular studies.