Fibroadenomas are a common entity. However, a rare occurrence is the presence of a huge mass in the breast in adolescent females. Called juvenile fibroadenomas, they are a variant of the lesion in the adult female populace. See: http://www.ultrasound-images.com/breast.htm for an ultrasound case study of this disease.
Also known as giant fibroadenoma, they are almost always benign and are not as ominous as they appear. They must be differentiated from the other large tumors of the female breast, such as the Phyllodes tumor which can turn malignant.