The main diagnostic consideration for this case is metastatic neuroblastoma, with a retroperitoneal primary.
In isolation, the diffuse marrow infiltration may be seen in leukemia such as ALL, however, this would not account for the retroperitoneal mass.
Further investigation involves CT/MRI of the chest, abdomen and pelvis, MIBG and correlation with urinary catecholamines. In this case, neuroblastoma was confirmed on histology.
Case contributed by Dr Kieran Frawley.