Introduction 73 15 46 6 9 17 24 31 51 62 67 70 68 46 60 66 Materials and methods Selection of patients 2 Surgical biopsies and tissue sampling: serial macroscopic sectioning 12 Classification of lesions and review of slides 1 4 7 10 17 19 21 28 38 43 45 47 49 50 56 59 61 69 72 74 75 n 70 71 4 n 61 n Atypical ductal hyperplasia: definition and sizing Among the group of ADH/DIN 1B, we individualized two morphologic types of ADH. Neither had any high-grade cytological atypia or necrosis. ADH “mimicking DCIS” 1 Fig. 1 a d a b c d arrows  ADH corresponding to “mini DCIS” 2 3 4 Fig. 2 a e a b c d e  Fig. 3 a c a b c  Fig. 4 a b a b  41 13 Follow-up of patients with epithelial atypia as a single lesion There were 443 patients with epithelial atypia in one or several SB, without any previous or synchronous carcinoma in the same or contralateral breast and treated by biopsy alone (median follow-up 160 months, 7 to 315). Only 28/443 (6%) were lost to follow-up. Among the 415 other patients, 180 were monitored at our institute and 235 outside by correspondent specialists working in close relationship with our institute. All patients received a clinical examination and mammography once a year. When a new biopsy was necessary, it was performed at our institute. Statistical analyses Comparison of clinical and histologic characteristics was conducted by using the chi-square test. For women with epithelial atypia, the probability of developing in situ or invasive cancer was calculated from the date of the first biopsy to the earliest event: breast cancer (ipsi- or contralateral), death, or last contact (last consultation for the group monitored at our institute and checkpoint date, i.e., 1 March 2004, for the others). Probabilities were calculated according to the Kaplan–Meier method (SPSSv11). Results Occurrence of epithelial atypia in the 2,833 surgical biopsies Epithelial atypia were recorded in 971/2,833 SB (34%). They were found with and without a concomitant cancer in 301/971 (31%) and 670/971 (69%) of the cases, respectively. Thus, isolated epithelial atypia were found in 23% of the cases (670 out of the 2,833 SB). Calcifications were present at histologic examination in 98.6% of SB with cancer and were located in benign, cancerous, and both lesions in 10, 39, and 51% of the cases, respectively. In several cases, cancerous foci without any microcalcifications were located at points distant from those with calcifications detected by needle localization. Types of epithelial atypia Among the 971 SB with epithelial atypia, there were 101 SB with FEA as a single lesion (11%), 342 (35%) with ADH, 223 (23%) with LN, and 305 (31%) with ADH and LN. Thus, ADH was encountered in 647/971 SB (66%). Types of cancers associated with epithelial atypia n 1 p −4 Table 1 n Epithelial atypia n n n n No. of cases (%) No. of cases (%) No. of cases (%) No. of cases (%) Without cancer 84 83 220 64 139 62 227 74 With cancer 17 17 122 36 84 38 78 26 DCIS/DCIS-MI 12 12 103 30 58 26 60 20 IDC/NOS – – 16 4.7 17 8 11 3 ILC 1 1 1 0.3 6 3 5 2 TC 4 4 2 1 3 1 2 1 FEA ADH LN DCIS DCIS-MI IDC ILC TC Cancers without epithelial atypia (malignancy alone) There were 821 malignant SB without epithelial atypia [590 micro-invasive carcinomas, 206 infiltrating ductal carcinomas (IDC), and 25 infiltrating lobular carcinomas (ILC)]. Subsequent cancer in patients with an initial diagnosis of epithelial atypia as a single lesion 5 n n n n n n n n n n n n n n n n n n n Fig. 5 dotted line  Discussion Application of the WHO classification: practical considerations 6 46 67 23 57 59 61 40 2 34 61 51 34 61 64 34 52 70 52 Table 2 Terminologies used for intraductal proliferative lesions with low-grade cytologic atypia, so-called atypical columnar cell lesions Spectrum of lesions 1, 3–5 Layers No polarization* With polarization  Occasional mounding  Mounding, arcades  Cribriform spaces and their variants  No or rare arcades and micropapillary formations  Cohesive micropapillary tufts with a broad base  Non-cohesive micropapillary tufts with a tight base 70 Flat epithelial atypia/DIN 1A ADH/DIN 1B ≤ 2 mm; or in two spaces DCIS/DIN 1C 59 Columnar cell hyperplasia with atypia ADH if not extensive DCIS if extensive 61 Columnar cell change (CCC) with cytologic atypia Complex structures with architectural and cytologic atypia  34 Columnar cell lesions + ADH Microscopic focus of DCIS DCIS Institut Bergonié Ex-clinging carcinoma of monomorphic type ADH “mimicking” DCIS ADH corresponding to “mini” DCIS ≤ 2 mm DCIS 64 CCC with cytologic atypia CCH with architectural atypia CCH with architectural atypia DCIS Or And CCH with cytologic atypia Cytologic atypia Regrouping? DIN 1A ADH/DIN 1B ADH/DIN 1C DCIS/DIN 1C Not measured Measured: ≤3 mm >3 mm *Or incomplete polarization Occurrence of epithelial atypia and their association with a concomitant cancer: practical considerations 15 46 46 46 n n 62 n 67 39 64 20 29 5 8 14 18 35 45 53 63 76 45 3 18 33 63 55 48 18 37 32 65 42 Subsequent cancer after an initial diagnostic of epithelial atypia as a single lesion: practical considerations 6 42 67 16 30 56 36 11 54 64