Fig. 1

Routine histology with hematoxylin and eosin (H&E) stains. a The breast lesion has a variable cellularity. Large areas consist of dense collagenous stroma with a low cellularity (upper right of the picture). These areas are admixed with more cellular areas that have a myxoid appearance (lower left of the picture). (H&E ×50). b A highly cellular, myxoid area of the lesion. There is clustering of fusiform to discrete epithelioid cells with a round, slightly irregular nucleus that contains a small nucleolus. Occasional mitotic figures are seen (arrow). (H&E × 400). c Within the breast lesion, small and normal-looking ducts are entrapped by the proliferative spindle cells. The latter show no obvious atypia or mitotic activity at high magnification. At the periphery, a limited infiltration of bland-looking spindle cells into the surrounding fat tissue is seen. A scattered infiltrate of lymphocytes is also seen. (H&E × 100). d A view of the assessable border of the breast lesion at low magnification. A nodular and sharply delineated margin in relation to the surrounding fat tissue can be seen. This is not typical for FLMC, where a more infiltrative growth pattern is expected at the border. (H&E × 25). e In the center of the lesion, a distinctive area with a relatively high number of bland-looking ducts surrounded by a striking sclerotic stroma is seen. (H&E × 200)