Thanks everyone for the opinion! My diagnosis will be: Sclerosing adenosis/adenosis tumor. 

Firstly, let me give a general description for adenosis/adenosis tumor. Adenosis is a benign breast lesion arising in terminal ductal lobular unit (TDLU). It is usually found in pre-menopausal women with the mean age 30 years old. Clinically it can present as nodules or even palpable “tumor,” when the adenosis nodules become confluent, which could be more common in pregnant women. Radiation imaging could show stellate area with microcalcification, which is a common feature of adenosis, or mass-like lesion (so-called adenosis tumor or nodular adenosis), hence raising the suspicion for invasive carcinoma. Microscopically adenosis has a characteristic low power view of lobular architecture even though the size of the lobules could be much enlarged due to the multiplication of glands/acini. Another low power feature of adenosis is the zonal growth pattern, with the central part more compressed and appearing more cellular, while the peripheral glands/acini appear more open or dilated. Under medium power view, the glands/acini are generally with regular and uniform contours. Under high power view, the glands/acini always maintain two cell layers: luminal epithelium and myoepithelium. This feature could be better viewed at the intermediate or outermost position of the adenosis. The luminal epithelial cells should have the same features as benign ductal luminal cells, such as long-oval shape, overlapping, nuclear membrane folding and grooving, with nucleoli, granular chromatin, and thick nuclear membrane etc., unless the adenosis is  involved  by DCIS or LCIS. Surrounding the luminal epithelium are the prominent myoepithelial cells and concentric basement membrane. The stroma could show dense inter-anastomosing bands of hyalinized collagen, such as in sclerosing adenosis.  But it is usually not myxoid. Sometimes, the layers of the acinar/glands may not be that obvious because of the distortion or compression by the sclerotic stroma in sclerosing adenosis, especially in the center part. Immunohistochemical stain for myoepithelial markers can be used to clarify this situation. When many adenosis nodules become confluent and merged with each other, the term adenosis tumor or nodular adenosis could be used. It is usually around 2.0 cm, but the “tumors” as large as 6.0 cm have been reported.

The current case presented as 5.0 cm “mass”. However, one can still appreciate a vague nodular growth pattern under low power view. It appears that the nodules become confluent and abutting each other, therefore forming a “mass”. The zonal distribution, even though not that obvious, can still be appreciated in most of the individual nodules. The luminal cells are benign looking, with small overlapping nuclei, long oval shape, folded thick nuclear membrane, and small but distinct nucleoli. Myoepithelial cells are generally prominent surrounding the luminal cells. One can readily appreciate this in the photos provided, even without immunohistochemical stain. The collagen bundles are winding around the acinar/glands, anastomosing with each other.

The most critical differential diagnosis, as pointed out by many friends, is the invasive ductal or lobular carcinoma, when the lesion presented as a mass with an “invasive” growth pattern, especially when one is focused in the center part of the lesion under high power view. As we all know, invasive carcinomas will not maintain a lobular centric morphology with zonal distribution, the cells have all the malignant features, will not have myoepithelial lining, and usually will have DCIS associated with it. One other helpful hint is that the glands of invasive carcinoma will infiltrate into the fatty tissue as naked glands/cell nests, while the glands/acini of adenosis will usually stop at the edge of the fibrous tissue. Sometimes we could see some benign glands in the adipose tissue. But they are certainly not “naked”. They will be always surrounded by myoepithelial cells and basement membrane (other than microglandular adenosis). See fig. 1 and 2. Another notoriously confusing, but not that common feature of adenosis is the perineural invasion which, when present, will be confused with invasive carcinoma. Again, immunohistochemical stain for myoepithelium will be the key to solve the problem.

The second differential diagnosis on the list will be adenosis involved by DCIS, as pointed out by many friends. Other than carefully evaluating the luminal cells to see if they are cancerous, one could also look around in the adjacent breast tissue. You will usually identify the original DCIS in the nearby areas. Of course, one could perform IHC stain for myoepithelium markers too. See fig. 3 and 4.

Other differential diagnosis mentioned in the reference books is tubular carcinoma, which will not be the issue here.

The take home messages are:

1)      Take the low power view first to appreciate the lobular centric shape and zonal distribution of adenosis

2)      Evaluate the layers and cells at the intermediate to outermost zone of the adenosis, not to focus in the center part under high power view.

3)      Look around

To my opinion, I think "pathologybz" could be the one to win the prize. There are a few other friends did equally well as him/her, but he/she is the first to post the answer.