Intracystic
papillary carcinomas (IPC) of the breast have traditionally been considered to be variants of
ductal carcinoma in situ (
DCIS). However, it is not clear if all lesions categorized histologically as IPC are truly in situ
carcinomas, or if some such lesions might represent circumscribed or encapsulated nodules of invasive
papillary carcinoma. Given that the demonstration of a myoepithelial cell (MEC) layer around nests of
carcinoma cells is a useful means to distinguish in situ from invasive
carcinomas of the breast in problematic cases, assessment of the presence or absence of a MEC layer at the periphery of the nodules that comprise these lesions could help resolve this issue. We studied the presence and distribution of MEC at the periphery of the nodules of 22 IPC and, for comparison, 15 benign
intraductal papillomas using immunostaining for 5 highly sensitive markers that recognize various MEC components: smooth muscle
myosin heavy chain,
calponin, p63, CD10, and
cytokeratin 5/6. All 22 lesions categorized as IPC showed complete absence of MEC at the periphery of the nodules with all 5 markers. In contrast, a MEC layer was detected around foci of conventional
DCIS present adjacent to the nodules of IPC. Furthermore, all benign
intraductal papillomas, including those of sizes comparable to those of IPC, showed a MEC layer around virtually the entire periphery of the lesion with all 5 MEC markers. In conclusion we could not detect a MEC layer at the periphery of the nodules of any of 22 lesions categorized histologically as IPC. One possible explanation for this observation is that these are in situ lesions in which the delimiting MEC layer has become markedly attenuated or altered with regard to expression of these
antigens, perhaps due to their compression by the expansile growth of these lesions within a cystically dilated duct. Alternatively, it may be that at least some lesions that have been categorized as IPC using conventional histologic criteria actually represent circumscribed, encapsulated nodules of invasive
papillary carcinoma. Regardless of whether these lesions are in situ or invasive
carcinomas, available outcome data indicate that they seem to have an excellent prognosis with adequate local
therapy alone. Therefore, we believe it is most prudent to continue to manage patients with these lesions as they are currently managed (ie, similar to patients with
DCIS) and to avoid categorization of such lesions as frankly invasive
papillary carcinomas. Given our observations, we favor the term "encapsulated
papillary carcinoma" over "intracystic
papillary carcinoma" for circumscribed nodules of
papillary carcinoma surrounded by a fibrous
capsule in which a peripheral layer of MEC is not identifiable.