Abstract Seventeen examples of a variant of ductal car- cinoma in situ (DCIS) composed exclusively or predom- inantly of spindle cells arranged in fascicles, whorls, and solid sheets are described. The fascicular arrangement of the spindle cells simulates the “streaming” phenomenon associated with ordinary intraductal epithelial hyperpla- sia (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 for carcinoma, but there were no distinctive mammographic findings to suggest an un- usual variant of DCIS. Cytological preparations were suspicious for malignancy in two patients and were re- ported as malignant in another case. Sixteen patients were treated with wide local excision, and one woman had a partial mastectomy. The tumors measured from 3 mm to 15 mm (mean 8.65 mm). In three cases, minute foci of stromal invasion were associated with the spindle cell DCIS. In another specimen, a 2.7-cm inva- sive ductal carcinoma of no special type was identified in an area away from the foci of the spindle cell DCIS. None of the patients has experienced recurrence or me- tastasis 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 peripheral fenestrations, uniformity of appearance and distribution of nuclei, cytological atypia in the range of low to intermediate-grade DCIS, and negative immuno- reaction with CK-34βE12 (HMW-CK903). When fenes- trations are present, they are evident in areas of cribri- form DCIS that merge with the 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 fur- ther help recognition and confirmation of this lesion as in situ carcinoma. When there is no transition from the spindle cells to recognizable cribriform DCIS, distinc- tion from intraductal myoepithelial hyperplasia (myoepi- theliosis) requires immunostains for actin and S-100 pro- tein. Recognition of this pattern of DCIS is important in order to avoid its frequent misclassification as a benign lesion. Keywords Ductal carcinoma in situ · Spindle cell DCIS · Intraductal breast cancer · Mammary ductal intraepithelial neoplasia · Noninvasive breast cancer · Immunohistochemistry Introduction Intraductal proliferations in the breast seldom consist of a pure population of spindled cells. Azzopardi [1] has re- ferred to parallel orientation of neighboring cells in an in- traductal proliferative process as “streaming” [1], and Page and Anderson [7] consider the finding of swirling or streaming as “strong, impelling evidence of benignancy”. Most proliferations with partial spindling of cells fall into the category of ductal hyperplasia. Myoepithelial cells may also assume a myoid, spindle cell configura- tion and proliferate in a solid fashion within multiple ducts. This process has been named myoepitheliosis [12]. Both intraductal epithelial hyperplasia (IDH) and G. Farshid Department of Pathology, Adelaide, South Australia G. Farshid · D.J. Meredith University of Adelaide, Division of Tissue Pathology, Institute of Medical and Veterinary Science, Frome Road, Adelaide, South Australia S. Peiterse Clinpath Laboratories, Kent town, South Australia G. Farshid · S. Peiterse Breastscreen SA, Wayville, South Australia F. Moinfar · F.A. Tavassoli ( ✉ ) Department of Gynecologic and Breast Pathology, Armed Forces Institute of Pathology, Washington, D.C. 20306, USA e-mail: tavassol@afip.osd.mil Fax: +1-202-7823939 Virchows Arch (2001) 439:70–77 DOI 10.1007/s004280100446 ORIGINAL ARTICLE Gelareh Farshid · Farid Moinfar · David J. Meredith Steve Peiterse · Fattaneh A. Tavassoli Spindle cell ductal carcinoma in situ An unusual variant of ductal intra-epithelial neoplasia that simulates ductal hyperplasia or a myoepithelial proliferation Received: 30 August 2000 / Accepted: 16 November 2000 / Published online: 11 May 2001 © Springer-Verlag 2001