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Chapter 11

SMA antibody detects actin filaments in myoepithelial cells, but its specificity is poor since myofibroblasts are also highlighted by SMA. SMM-HC detects structural smooth muscle elements in mammary myoepithelial cells but not in myofibroblasts. 185,293 It is notable that CD10 is expressed uniformly in myoepi- thelial cells from the terminal duct to acini, and that caldes- mon is immunoreactive in myoepithelial cells of large ducts, but is not typically expressed in intralobular ductules and acini. 294 In practical terms, SMM-HC and p63 are the most useful immunostains for the diagnosis of microinvasive carcinoma. At sites of clear-cut microinvasive ductal carcinoma, tu- mor cells are distributed singly or as small groups that have irregular shapes reminiscent of conventional invasive carci- noma with no particular orientation (Figs. 11.66 to 11.69). Sometimes the intralobular or periductal stroma appears less dense at sites of microinvasion than in other areas around these structures. Detecting carcinoma cells in the stroma can be difficult when there is a periductal and stromal inflamma- tory cell reaction. Microinvasion may be suspected at sites where there is a pronounced lymphocytic accumulation near ducts with DCIS (Fig. 11.70). A granulomatous reaction may be elicited at foci of microinvasion. 295 The tumor cells can

resemble histiocytes, and it may require immunostains for cytokeratin to confirm the presence of microinvasion. Dou- ble immunolabeling for cytokeratin and actin is an elegant method for visualizing foci of microinvasion (Fig. 11.42). 296 Microinvasion is most often associated with high-grade DCIS, but it may occur in other types of DCIS. 56 Thorough histologic sectioning is recommended for all cases of high- grade DCIS and for other types of DCIS that form a co- hesive lesion larger than 2 cm. Serial routine H&E-stained sections, supported by IHC, usually provide the best evi- dence of microinvasion. Minimal trimming of the block, with conservation of diagnostic tissue, should be ensured in such cases. Care should also be taken to obtain immu- nostains (including those for cytokeratin, myoepithelial markers, and hormone receptors) early in the evaluation of suspected microinvasion before the sample has been excessively sectioned. At least one H&E slide must always be prepared whenever immunostains are done to study a specimen for microinvasion. Carcinomatous epithelium displaced by needling procedures can usually be distin- guished from intrinsic invasive carcinoma (Fig. 11.71). The presence of minute clusters of carcinomatous epi- thelial cells arranged in a linear manner, typically associ- ated with granulation tissue, fat necrosis, and hemosiderin

FIG. 11.69.  DCIS with microinvasive carcinoma. A: Carci- noma cells in groups and individually in reactive stroma . B: Magnified view (of box in [A] ) showing individual carci- noma cells that were partially obscured by the inflamma- tory reaction. C: Invasive carcinoma cells are highlighted by this CAM5.2 immunostain for cytokeratin.