First understand that cancer is rare disease. Risk factors for breast cancer are Increasing age, family history, genetic risk, nulliparity etc. Breast cancer occurrence is 17 year old is almost nill, unless rare congenital disorders which can be symptomatic from child hood. For breast cancer prevention if there is strong family history get MRI Breast from 26 year onwards Otherwise start mammogram examination from 40 years annually once. Get trained in self breast examination and do it regularly Dont worry.

No need of breast cancer screening at this age . Please mention the reason for undergoing an ultra sound scan , if she of high risk (family history of cancers specially breast, ovary , colon , prostate ) category by any mean , then please let me know that to give the correct screening advise that she need to undergo . As such there is no prevention I can’t suggest anything other than following a good life style .