A 75-year-old woman was noted to have a suspicious breast lesion on routine mammography. An excisional biopsy was performed, yielding the tissue seen below.
By routine H&E stain, this appears to be a clearly intraductal lesion. Cytologic features of malignancy were noted and the lesion was interpreted as apocrine DCIS. Diagnosing apocrine DCIS is non-trivial, but is not the primary subject of this case and the photos do not show the full range of cytologic atypia encountered in this lesion. However, a higher power image of one focus of atypia is seen below.
The contributing pathologist performed immunohistochemistry for myoepithelial cells using SMA, CD10, CK5/6, and p63, looking for occult areas of microinvasion. The result, which was the same for all markers, is shown below.
He was startled when there was complete absence of myoepithelial cells around these large duct-like structures with all of the markers employed. The case was sent for consultation with the question of whether this could be an invasive carcinoma with a pushing border.
I have seen several cases in the last month raising exactly this issue. Recent studies have demonstrated that apocrine lesions of the breast, whether metaplastic or neoplastic, typically show a marked diminution or complete lack of an outer myoepithelial layer (AJSP 2011;35:202-11).
The frequency of this finding in apocrine intraductal lesions is not entirely clear and will require further study. It is also unclear why this phenomenon should accompany apocrine proliferations in particular.
It is somewhat better understood that expansile intraductal lesions, regardless of cytologic type and regardless of organ, will often produce "stretching" and some attenuation of the outer myoepithelial layer such that it may well appear discontinuous in some planes of section. In the breast, complete absence, in our experience, has only been encountered with apocrine intraductal lesions. Negative immunohistochemical markers are always problematic when used to support a "positive" result (invasion in this case). Until this phenomenon is much better understood, it is important to remember that the presence of myoepithlial cells in breast proliferations is strong evidence for an in-situ process, but lack of myoepithelial cells does not fully equate with an invasive lesion, esp. when the light microscopic features are typical of an intraductal lesion.