Spindle cell ductal carcinoma in situ - An unusual variant of ductal intra-epithelial neoplasia that simulates ductal hyperplasia or a myoepithelial proliferation

Seventeen examples of a variant of ductal carcinoma in situ (DCIS) composed exclusively or predominantly of spindle cells arranged in fascicles, whorl s. and solid sheets are described. The fascicular arrangement of the spindl e cells simulates the "streaming" phenomenon associated with ordinary intra ductal epithelial hyperplasia (IDH). This process also resembles the myoid, solid form of intraductal myoepithelial proliferation. The women ranged in age from 38 years to 79 years with a mean age of 59.3 years. Five patients presented with a palpable mass. The remaining tumors were discovered using mammography. The radiological appearances of the lesions raised concern fo r carcinoma, but there were no distinctive mammographic findings to suggest an unusual variant of DCIS. Cytological preparations were suspicious for m alignancy in two patients and were reported as malignant in another case. S ixteen patients were treated with wide local excision, and one woman had a partial mastectomy. The tumors measured from 3 nim to 15 mm (mean 8.65 mm). In three cases, minute foci of stromal invasion were associated with the s pindle cell DCIS. In another specimen. a 2.7-cm invasive ductal carcinoma o f no special tv. pe was identified in an area away from the foci of the spi ndle cell DCIS. None of the patients has experienced recurrence or metastas is during the relatively short mean follow-up period of 16.2 months (range 4-77 months). Spindle cell DCIS is distinguished from the streaming pattern of ordinary IDH by its solid growth pattern, lack of secondary spaces or p eripheral fenestrations, uniformity of appearance and distribution of nucle i, cytological atypia in the range of low to intermediate-grade DCIS, and n egative immunoreaction with CK-34 beta E12 (HMW-CK903). When fenestrations are present. they are evident in areas of cribriform DCIS that merge with t he solid, spindle cell areas in hybrid ducts harboring both patterns. This admixture, with conventional cribriform DCIS, and the association with foci of invasive ductal carcinoma in some cases further help recognition and co nfirmation of this lesion as in situ carcinoma. When there is no transition from the spindle cells to recognizable cribriform DCIS, distinction from i ntraductal myoepithelial hyperplasia (myoepitheliosis) requires immunostain s for actin and S-100 protein. Recognition of this pattern of DCIS is impor tant in order to avoid its frequent misclassification as a benign lesion.