<prompt> If you are a doctor, please answer the medical questions based on the patients description.My 13 year old is a B cup breast size and has developed a 2 cm round x 1cm deep hard lump in her left breast. it was sore and red so I took her to the doctors. She is waiting to have an ultrasound done on it. Do you think it might be a cyst. How common is this in young girls?  <response>: Hi there.regarding your concern, breast cysts are very common in women 35yrs of age. But you will also find cases of cysts quite common in adolescent girls between 10-18 and in boys with gynecomastia. If the lump is hard painful, and red are signs of inflamed cyst. Many a time could also be asymptomatic. An ultrasound is the IOC for a breast cyst. If positive then can go ahead for diagnostic tests for any hormonal imbalance. If the cysts is infected antibiotics and ID is advised. most of the time if it is not a cyst then you neednt worry as it could be due to puberty changes and hormonal imbalance. Hence nothing to panic : hope this helps. 