ANSWER: A caruncle is a benign, fleshy growth at the end of the urethra, the tube that carries urine from the bladder to the genitals for removal. Nobody really understands why they occur, but they definitely occur more in women who are postmenopausal and who have low estrogen levels. Treatment with topical estrogen usually is effective at reducing the size. It can be removed surgically if it is bothersome, but since yours isn’t, I wouldn’t recommend surgery. Both urologists and gynecologists treat caruncles.
As far as your concerns about cancer from topical estrogen, the risk is very, very small. In someone with a history of breast cancer, I would consult an oncologist, but absorption of topical estrogen is low. Many authorities recommend using twice-weekly estrogen cream to prevent recurrence, but if you were to stop the estrogen cream, you could restart it if the caruncle started bothering you.