This papilloma shows a clonal population of cells with higher grade nuclei (upper left), consistent with DCIS arising within a papilloma. You might wonder if the single epithelioid cells on the upper right represent invasion. These are probably histiocytes and inflammatory cells and are not invasive ductal cells.
The terminology and classification of papillary lesions of the breast is still in flux. One of the more concise reviews by Mulligan and O'Malley broadly divides these lesions into benign and malignant. In the benign category, there are solitary intraductal papillomas, multiple intraductal papillomas (papillomatosis) and atypical hyperplasia arising within a papilloma.
In the malignant category, there are the following entities: DCIS arising in a papilloma, papillary DCIS, intracystic papillary carcinoma, solid papillary carcinoma, invasive carcinoma arising in an intracystic papillary carcinoma and invasive papillary carcinoma. Needless to say, these entities can be confusing and the authors acknowledge that there are other terminologies being used and this area is still evolving and controversy exists.
I shall only discuss here atypical ductal hyperplasia arising in an intraductal carcinoma. Basically, it is very similar to ADH, in which a population of neoplastic/clonal epithelial cells develop in the papilloma. These cells are round with rigid cell borders. There is an absence of cellular streaming and overlapping. Furthemore, the absence of myoepithelial cells in these atypical areas by immunohistochemistry may also be helpful.
Various criteria have been used. Mulligan and O'Malley prefer the following definitions: if the uniform/atypical popualtion is less than or equal to 3mm, it is ADH. If the lesional area is greater than 3mm, it is DCIS. Note that this criteria is only applicable to low-grade lesions, and if higher-grade areas (with increased cytologic atypia), the diagnosis of DCIS within a papilloma is made regardless of the size of the lesion.
Not surprisingly, papillomas with ADH carry a higher risk of developing breast cancer (5-7 fold) compared to papillomas without atypia (2-3 fold). The patients did not show a significant tendency of developing breast cancer on the ipsilateral breast, thus, papillomas with atypia should be considered a marker of increased risk rather than a precursor lesions (Mulligan and O'Malley).
Mulligan AM, O'Malley FP. Papillary Lesions of the Breast. Adv Anat Pathol 2007;14:108-119.
Ueng SH, et al. Papillary Neoplasms of the Breast: A Review. Arch Pathol Lab Med. 2009;133:893-907.