Cystic renal mass in a patient with previous Wilm's tumour.

A 35-year-old female presented to her general practitioner with a 1-month history of moderate to severe continuous right flank pain associated with anorexia and weight loss. In the past, she had undergone partial right nephrectomy for a Wilm’s tumour at the age of 8 months and was closely followed up for 16 years without recurrence. On clinical examination there was a palpable mass in the right flank extending to the right iliac fossa. Initial work-up revealed a normochromic normocytic anaemia, normal white cell count, normal urea and creatinine, and mildly elevated C-reactive protein of 48. Glomerular filtration rate measured 92 ml min, with the right kidney contributing 20% function on dimercaptosuccinc acid scintigraphy. A mid-stream specimen of urine had no white cells, although cultured Escherichia coli. CT (Figure 1) and MRI (Figure 2) imaging was performed. What was the diagnosis?