The ureters beginn at the renal pelvis and run behind the abdominal cavity and behind the peritoneum over the large pelvic vessels down to the bladder. They transport the urine from the kidney into the bladder. The ureters are from within lined with a mucous membrane (urothelium). This mucosa is also found in the renal pelvis and bladder. This mucosa is surrounded by two layers of muscle. Through these muscles the ureter is not but not just a simple tube/pipe, but it can actively carry urine from the kidney into the bladder (peristalsis).
The ureter has three positions where it narrows:
the outlet from the renal pelvis
the crossing with the pelvic vessels
the junction with the bladder
Occasionally, the vessels to the ovary or testes can become a further narrow pass.
Normally, each kidney has its own ureter - even by extra kidney. The ureters can then completely separate run to the bladder (ureter duplex) or unite somewhere along the way (fissus ureter).
Kidneys, pelvis and ureters (Seen from behind); mod. H. Gray: Anatomy of the Human Body
Ureteral tumors: these usually originate from the mucosa. Since the renal pelvis and ureter have the same liner, they are treated likewise. Therefore, please look under malignant diseases of the kidney.