ABSTRACT
Neuroblastoma is the most common extra–cranial solid tumour in children. Bone marrow examination is a part of diagnosis and staging workup of neuroblastoma. Chemotherapy forms mainstay of treatment and post–chemotherapy histological changes are an important indicator of prognosis. A 4–year–old male child came with complaints of fever and hepatomegaly. Investigations revealed a retroperitoneal neuroblastoma. Bone marrow was involved by neuroblasts (stage 4S). Platinum–based chemotherapy was started and post–induction phase bone marrow showed differentiation of neuroblasts to ganglion cells and schwannian stroma (tumour load <5%, minimal disease). The patient was lost to follow up thereafter. The patient returned after one year with the reappearance of neuroblasts (tumour load >20%, relapse) in the bone marrow. Use of immunohistochemical markers like chromogranin and S100 are helpful to map the tumour load and identify the tumour cells when they are sparse. Persistence / reappearance of neuroblasts post–chemotherapy or increasing tumour load indicate a relapse.