How do doctors distinguish between a cyst and cancer/tumor?

Many teens get cysts on the ovaries. It's not unusual. A cysts is a fluid filled sac whereas a tumor is solid m and a trained Dr can see the difference. If he suspects something else, then he will do a biopsy. When I was a teen, the first the Dr would do is remove the cysts and put in through a bunch of tests to make sure it was not going to cause any harm. However, now because there are so many different types of cysts and tumors, most radiologist can tell the difference. You will be alright. If you are hot happy with what your Dr tells you, you always have the option of a second opinion which you should do anyway. I always get another opinion no matter what they say. Good luck. Are they going to remove it?