Invasive ductal carcinoma with in situ pattern: how to avoid this diagnostic pitfall?

Although the microscopic features of invasion are usually readily recognized, occasionally invasive ductal carcinoma may mimic the pattern of comedo ductal carcinoma in situ (DCIS) by forming large cellular nests with circumscribed borders, but lacking a definitive myoepithelial cell layer. In these cases, the histologic pattern may appear deceptively noninvasive and the absence of a myoepithelial layer can be easily overlooked. We prospectively examined 10 cases of high grade DCIS. P63, smooth muscle actin, muscle specific actin and calponin immunohistochemical stains were used to identify the presence of myoepithelial cells. In our study, 20% of apparent high grade DCIS cases did not exhibit a myoepithelial layer surrounding large, solid nests with comedo necrosis. Since invasion is defined by the absence of a myoepithelial layer, these results suggest that a DCIS-like pattern may actually represent invasive disease in some cases. Immunohistochemical studies may be essential in making this distinction and in avoiding the potential diagnostic pitfall.