Atypical ductal hyperplasia is not a cancerous condition, just a condition that carries bigger risk for having breast cancer.
Lesion that is diagnosed as ADH on the basis of core needle biopsy findings always needs to be excised to rule out carcinoma.
Some percents of ADH lesions after being examined with excision biopsy are diagnosed like ductal carcinoma in situ (non invasive cancer, cancer that hasn't spread away from the milk duct in breast)
Ductal carcinoma in situ, is more serious condition and usually due to technique (core biopsy) could be missed.

Was this found routinely or you have had some symptoms that indicated use of mammograf?

I had a follow-up digital mammogram which found 'atypical' cells. That's why the core biopsy was done. There was nothing palpable. I called a surgeon who did my friend's mastectomy and I'm seeing him Friday. The nurse said I'd be schedule from the excisional biopsy that day. I just feel sooner is better to get an answer. Thanks for the information. It was very helpful and made me feel better.

You are welcome!
One study has shown that 30% of patients with ADH, can be detected with malignancy on excisional biopsy, but, most of them had the presence of a mass on mammography.
A preliminary report from the pathologist (the doctor who can explain the look of the changed cells) might be available when your surgery is over, but, the final report may take three to four days to be completed.
Even in the worst case when the ductal carcinoma in situ is found, the therapy in most of the cases is breast-conserving, removing only the area of the breast containing the cancer.

Best wishes!
Waiting for your new informations!
Marija