This is a rare example of a urinary tract metastasis - in this instance from a colorectal primary, following invasion of the distal ureter adjacent to the original anastomosis site.

The tumor seeded via retrograde flow of urine into the left renal pelvis.  This was proven on urine samples from the nephrostomy tube, which identified adenocarcioma of colorectal origin.

Urinary tract metastases are uncommon, with documented cases from breast, lung, stomach and prostate cancers.  Rare reports exist from a primary colorectal tumor.