What is the most likely diagnosis?
Considering the age of the patient and the fact that this mass appears to arise form the kidney, Wilm's tumour (aka nephroblastoma), is ths most likely diagnosis. Neuroblastoma is an important differential consideration in this age group, but is much less likely.
Name half a dozen features which are helpful in suggesting that a flank mass in a young child is a Wilms tumour rather than neuroblastoma.
1. Calcification uncommon (10 - 15%) whereas it is seen in the majority of neuroblastomas (90%); 2. displace adjacent structures with out insinuating between them; 3. well circumscribed; 4. claw sign with the kidney; 5. slightly older age group : peak 3 - 4 years of age (compared to <2 years of age for neuroblastoma); 6. extension into IVC / renal vein can be seen.
A very large heterogeneously enhancing soft-tissue density mass appear to arise from the left kidney, and results in hydronephrosis of the collecting system which is located posterior to the mass. The mass is well circumscribed without evidence of direct extension in adjacent organs / structures. No convincing nodal enlargement.