We investigated 20 spindle cell (sarcomatoid) metaplastic
carcinomas (MCs) without squamous differentiation. In addition, five high-grade
phyllodes tumors were assessed for comparison. Our immunohistochemical antibody panel included pan-
cytokeratin (CK), low molecular weight CK (CK8/18), four basal cell type CKs (34betaE12, CK5/6, CK14, and CK17),
vimentin antibodies, as well as
antibodies to established (SMA, CD10, p63, S-100,
maspin,
calponin, GFAP, SM-
myosin), and novel (CD29, 14-3-3sigma) myoepithelial markers. Sixteen of the 20
tumors (80%) expressed at least two markers of the combination CD10/p63/SMA. S-100 detected 1 case negative for CD10/p63/SMA and 3 cases that only expressed one marker of this combination. While 18 MCs (90%) were positive for CD29, 14-3-3sigma (11 cases) and
maspin (9 cases) were observed in 55% and 45%, respectively.
Antibodies to pan-CK and the basal cell type CKs were strongly reactive in 12
tumors (60%), but in 6 cases (30%) positivity for these markers was weak and only focal; 2 MCs showed no positivity for CK. The stromal component of all
phyllodes tumors was positive for
vimentin, whereas all other investigated markers were absent except for focal p63 and CD10 expression in 1 case each. Our findings convincingly show a myoepithelial immunophenotype in sarcomatoid MCs, which is demonstrated by the presence of basal cell type CKs and the combination of the established myoepithelial markers CD10, p63, SMA, and S-100. We conclude that
tumors with weak or even absent CK expression should only be diagnosed as primary
sarcomas of the breast after exclusion of a myoepithelial immunophenotype. CD29 and 14-3-3sigma represent valuable novel myoepithelial markers in these diagnostically difficult cases.