Can ultrasound tell the difference between tumors and other things? could an ultrasound mistake a tumor for a cyst, fibroid or other mass?

Ultrasound : Ultrasound is excellent at characterizing and correctly identifying cysts (a cyst is a fluid filled structure). Most cysts appear obviously benign on ultrasound. There are some tumors that are cystic (specifically cystic ovarian tumors), but these usually do not look the same as benign ovarian cysts or ovarian follicles. If a cystic ovarian tumor is discovered very early (while it is very small), it is usually indistinguishable from a benign cyst. If the cyst is imaged a few months later and has not grown, then it can be considered a benign cyst. A uterine fibroid is a benign tumor of the muscle of the uterus (leiomyoma). It would be unusual to mistake a uterine fibroid for any other mass. A leiomyosarcoma is a cancerous tumor which is indistinguishable from a uterine fibroid, but this cancer is exceedingly rare.

Yes and Yes: Ultrasound is just one of many imaging modalities. It can be very accurate at distunguishing abnormalities but it often depends on what you are imaging and the expertise of the ultrasonographer. No test is ever 100% but ultrasound is fairly good at telling between cystic and solid lesions.