Extent of atypical hyperplasia stratifies breast cancer risk in 2 independent cohorts of women.

BACKGROUND: Women with atypical hyperplasia (AH) on breast biopsy have a substantially increased risk of breast cancer (BC). Here the BC risk for the extent and subtype of AH is reported for 2 separate cohorts. METHODS: All samples containing AH were included from 2 cohorts of women with benign breast disease (Mayo Clinic and Nashville). Histology review quantified the number of foci of atypical ductal hyperplasia (ADH) and atypical lobular hyperplasia (ALH). The BC risk was stratified for the number of AH foci within AH subtypes. RESULTS: The study included 708 Mayo AH subjects and 466 Nashville AH subjects. In the Mayo cohort, an increasing number of foci of AH was associated with a significant increase in the risk of BC both for ADH (relative risks of 2.61, 5.21, and 6.36 for 1, 2, and ≥3 foci, respectively; P for linear trend = .006) and for ALH (relative risks of 2.56, 3.50, and 6.79 for 1, 2, and ≥3 foci, respectively; P for linear trend = .001). In the Nashville cohort, the relative risks of BC for ADH were 2.70, 5.17, and 15.06 for 1, 2, and ≥3 foci, respectively (P for linear trend < .001); for ALH, the relative risks also increased but not significantly (2.61, 3.48, and 4.02, respectively; P = .148). When the Mayo and Nashville samples were combined, the risk increased significantly for 1, 2, and ≥3 foci: the relative risks were 2.65, 5.19, and 8.94, respectively, for ADH (P < .001) and 2.58, 3.49, and 4.97, respectively, for ALH (P = .001). CONCLUSIONS: In 2 independent cohort studies of benign breast disease, the extent of atypia stratified the long-term BC risk for ADH and ALH. Cancer 2016;122:2971-2978. © 2016 American Cancer Society.

Continued observation of the natural history of low-grade ductal carcinoma in situ reaffirms proclivity for local recurrence even after more than 30 years of follow-up.

Opportunities to study the natural history of ductal carcinoma in situ are rare. A few studies of incompletely excised lesions in the premammographic era, retrospectively recognized as ductal carcinoma in situ, have demonstrated a proclivity for local recurrence in the original site. The authors report a follow-up study of 45 women with low-grade ductal carcinoma in situ treated by biopsy only, recognized retrospectively during a larger review of surgical pathology diagnoses and original histological slides for 26 539 consecutive breast biopsies performed at Vanderbilt, Baptist and St Thomas Hospitals in Nashville, TN from 1950 to 1989. Long-term follow-up was previously reported on 28 of these women. Sixteen women (36%) developed invasive breast carcinoma, all in the same breast and quadrant as their incident ductal carcinoma in situ. Eleven invasive breast carcinomas were diagnosed within 10 years of the ductal carcinoma in situ biopsy. Subsequent cases were diagnosed at 12, 23, 25, 29 and 42 years. Seven women, including one who developed invasive breast cancer 29 years after her ductal carcinoma in situ biopsy, developed distant metastasis, resulting in death 1-7 years postdiagnosis of invasive breast carcinoma. The natural history of low-grade ductal carcinoma in situ may extend more than four decades, with invasive breast cancer developing at the same site as the index lesion. This protracted natural history differs markedly from that of patients with high-grade ductal carcinoma in situ or any completely delimited ductal carcinoma in situ excised to negative margins. This study reaffirms the importance of complete margin evaluation in women treated with breast conservation for ductal carcinoma in situ as well as balancing recurrence risk with possible treatment-related morbidity for older women.

Clinicopathologic characteristics of carcinomas that develop after a biopsy containing columnar cell lesions: evidence against a precursor role.

BACKGROUND: Columnar cell lesions are frequently associated with atypical ductal hyperplasia, lobular neoplasia, and tubular carcinoma, and have been suggested as a precursor lesion for low-grade carcinomas. However, in long-term follow-up studies, columnar cell lesions are associated with only a slight increase in later breast cancer development. If columnar cell lesions are precursor lesions, one would expect subsequent cancers to develop at the same site as the biopsy and to be preferentially of low grade. The goal of this article is to review the clinical and pathologic features of carcinomas that develop after a diagnosis of columnar cell lesion to try to establish whether these lesions are precursors to low-grade invasive carcinoma. METHODS: The authors reviewed biopsies containing columnar cell lesions, using the criteria of Schnitt and Vincent-Salomon, from 77 women in the Nashville Breast Cohort who developed subsequent breast carcinoma. Clinicopathologic features including laterality, type, and grade of the subsequent cancer were recorded. RESULTS: Breast cancer developed a median of 11 years after initial biopsy. The median age at diagnosis was 60 years. The majority of invasive carcinomas were of no special type and of intermediate grade. Moreover, the carcinomas were as likely to occur in the contralateral breast as in the breast that was originally diagnosed with columnar cell lesion, regardless of columnar cell lesion subtype (P = .48). CONCLUSIONS: Carcinoma subsequent to columnar cell lesions may occur in either breast and tends to show a similar grade and type distribution as sporadic breast cancer. These findings argue against columnar cell lesions being a true precursor for low-grade invasive carcinoma.

Grading breast cancer on microarray samples: comparison with Nottingham grade, and use of boosting classification.

AIMS: Nottingham breast cancer grade (NG) is a subjective morphological assessment based on evaluation of the entire tumour. The value of many novel immunohistochemical and molecular markers is being assessed on tiny microarray samples of tumour and compared with NG. The aim of this study was to investigate whether tumour morphology in microarray samples would correlate with NG. METHODS AND RESULTS: We examined over 40 morphological features in each of 568 breast tumour samples on a microarray obtained from the US National Cancer Institute. Evaluations were subjective, and features were recorded as being present or absent in each tumour. Subsequently, on the basis of binary results, a boosting classification algorithm was implemented to help assign a 'microarray score' and 'microarray grade' to each tumour. Microarray grade was significantly correlated with NG (P < 0.01). High-grade versus low-grade discrepancies were rare (five of 568 samples). CONCLUSIONS: The strong correlation of microarray grade with NG supports pathologist reproducibility in subjective evaluations.

Heritable variation of ERBB2 and breast cancer risk.

Amplification of the epithelial growth factor receptor gene ERBB2 (HER2, NEU) in breast cancer is associated with a poor clinical prognosis. In mammary gland development, this receptor plays a role in ductal and lobuloalveolar differentiation. We conducted a systematic investigation of the role of genetic variation of the ERBB2 gene in breast cancer risk in a study of 842 histologically confirmed invasive breast cancer cases and 1,108 controls from the Shanghai Breast Cancer Study. We observed that the ERBB2 gene resides within a locus of high linkage disequilibrium, composed of three major ancestral haplotypes in the study population. These haplotypes are marked by simple tandem repeat and single nucleotide polymorphisms, including the missense variants I655V and P1170A. We observed a risk-modifying effect of a highly polymorphic simple tandem repeat within an evolutionarily conserved region, 4.4 kb upstream from the ERBB2 transcription start site. Under a dominant genetic model, the age-adjusted odds ratio was 1.74 (95% confidence interval, 1.27-2.37). Its association with breast cancer, and with breast cancer stratified by histology, by histologic grade, and by stage, remained significant after correction for multiple comparisons. In contrast, we observed no association of ERBB2 single nucleotide polymorphism haplotypes with breast cancer predisposition.

Menstrual and reproductive history, postmenopausal hormone use, and risk of benign proliferative epithelial disorders of the breast: a cohort study.

Menstrual and reproductive history and postmenopausal hormone use are well-established risk factors for breast cancer. However, previous studies that have assessed these factors in association with risk of benign proliferative epithelial disorders (BPED) of the breast, putative precursors of breast cancer, have yielded inconsistent findings. To investigate these associations, we conducted a cohort study among 68,132 postmenopausal women enrolled in the Women's Health Initiative randomized clinical trials. Women were prospectively followed and those reporting an open surgical biopsy or a core needle biopsy had histological sections obtained for centralized pathology review. Over an average of 7.8 years of follow-up, we identified 1,792 women with BPED of the breast. We used Cox proportional hazards models to estimate hazard ratios (HRs) and 95% confidence limits (CLs) for the associations of interest. Menstrual and reproductive histories were not associated with risk of BPED of the breast, overall or by histological subtype. Women who had used postmenopausal hormones for 15 years or more had a two-fold increase in risk of BPED of the breast compared to women who had never used postmenopausal hormones (HR = 2.03 95% CL = 1.73, 2.38) and the increase in risk was observed for both BPED of the breast without atypia and for atypical hyperplasia. Furthermore, the risk of BPED of the breast decreased with time since cessation of use so that there was essentially no increase in risk 5 or more years after ending use (HR for stopping >or=5 years earlier = 0.96, 95%CL = 0.79, 1.16; HR for stopping <5 years earlier = 1.32, 95% CL = 1.08,1.61).

A randomized controlled trial of calcium plus vitamin D supplementation and risk of benign proliferative breast disease.

Experimental evidence provides strong support for anti-carcinogenic effects of calcium and vitamin D with respect to breast cancer. Observational epidemiologic data also provide some support for inverse associations with risk. We tested the effect of calcium plus vitamin D supplementation on risk of benign proliferative breast disease, a condition which is associated with increased risk of breast cancer. We used the Women's Health Initiative randomized controlled trial. The 36,282 participants were randomized either to 500 mg of elemental calcium as calcium carbonate plus 200 IU of vitamin D(3) (GlaxoSmithKline) twice daily (n = 18,176) or to placebo (n = 18,106). Regular mammograms and clinical breast exams were performed. We identified women who had had a biopsy for benign breast disease and subjected histologic sections from the biopsies to standardized review. After an average follow-up period of 6.8 years, 915 incident cases of benign proliferative breast disease had been ascertained, with 450 in the intervention group and 465 in the placebo group. Calcium plus vitamin D supplementation was not associated with altered risk of benign proliferative breast disease overall (hazard ratio = 0.99, 95% confidence interval = 0.86-1.13), or by histologic subtype. Risk varied significantly by levels of age at baseline, but not by levels of other variables. Daily use of 1,000 mg of elemental calcium as calcium carbonate plus 400 IU of vitamin D(3) for almost 7 years by postmenopausal women did not alter the overall risk of benign proliferative breast disease.

Syringomatous adenoma of the nipple occurring within a supernumerary breast: a case report.

Originally described by Rosen in 1983, syringomatous nipple adenoma (SAN) is a tumor of disputed histogenesis, which can be problematic both diagnostically and therapeutically. It is a benign primary tumor of breast epithelium with histology similar to that of the syringoma. In the current case, we describe a 40-year-old female with this lesion occurring within a supernumerary breast. This case represents, to our knowledge, the first such reported case, and represents a significant finding as its presence could lend some confusion as to whether or not this represents a benign primary process of breast or a potentially infiltrative tumor of the skin.

Estrogen plus progestin and risk of benign proliferative breast disease.

Women with benign proliferative breast disease are at increased risk of subsequent breast cancer. Estrogens and progesterone exert proliferative effects on mammary epithelium, and combined hormone replacement therapy has been associated with increased breast cancer risk. We tested the effect of conjugated equine estrogen plus progestin on the risk of benign proliferative breast disease in the Women's Health Initiative (WHI) randomized controlled trial. In the WHI trial of estrogen plus progestin, 16,608 postmenopausal women were randomly assigned either to 0.625 mg/day of conjugated equine estrogen plus 2.5 mg/day of medroxyprogesterone acetate or to placebo. Baseline and annual breast exams and mammograms were required. The trial was terminated early (average follow-up, 5.5 years). We identified women who had had a biopsy for benign breast disease, and subjected histologic sections from the biopsies to standardized review. Overall, 178 incident cases of benign proliferative breast disease were ascertained in the estrogen plus progestin group and 99 in the placebo group. The use of estrogen plus progestin was associated with a 74% increase in the risk of benign proliferative breast disease [hazard ratio, 1.74; 95% confidence interval (CI), 1.35-2.25]. For benign proliferative breast disease without atypia the hazard ratio was 2.00 (95% CI, 1.50-2.66), while for atypical hyperplasia it was 0.76 (95% CI, 0.38-1.52). The risk varied little by levels of baseline characteristics. The results of this study suggest that the use of estrogen plus progestin may increase the risk of benign proliferative breast disease.

Why I became a surgical pathologist with interest in breast disease.

Factors that influenced my decision to become a surgical pathologist are presented below. Early childhood experience shaped my interest in epidemiology and public health. Formal education at Johns Hopkins Medical School cemented my decision to become a surgical pathologist and elective time with pioneers in the field of surgical pathology sealed my career choice. As a mentor, I have tried to teach residents and fellows how to practice pathology, not merely how to make a diagnosis.

Diabetic (lymphocytic) mastopathy with exuberant lymphohistiocytic and granulomatous response: a case report with review of the literature.

We report a case of a 66-year-old woman who presented with multiple painless masses in both breasts. Prior bilateral biopsies were diagnosed as Rosai-Dorfman disease (Sinus Histiocytosis with Massive Lymphadenopathy). A recent lumpectomy specimen revealed a gray-white smooth cut surface with a discrete masslike lesion. The histopathology demonstrated a fibrotic breast parenchyma with foci of dense fibrosis and scattered inconspicuous breast epithelium surrounded by lymphocytes that formed aggregates and follicles with germinal centers. The inflammation was in a periductal, perilobular, and perivascular distribution. In addition, an exuberant inflammatory response with histiocytes and fibroblasts was present. This inflammatory response focally surrounded areas of fat necrosis and formed noncaseating granulomas with rare multinucleated giant cells. This process had infiltrative, ill-defined edges and involved the subcutaneous tissues. The overlying epidermis was normal. The final diagnosis was diabetic mastopathy with an exuberant lymphohistiocytic response. The differential diagnosis included Rosai-Dorfman disease, inflammatory myofibroblastic tumor, granulomatous mastitis, sclerosing lipogranulomatous response/sclerosing lipogranuloma, lupus panniculitis, and rheumatoid nodules. Immunohistochemical studies and flow cytometry confirmed the polyclonal nature of the lymphoid infiltrate. After the histologic evaluation, we inquired if the patient had a history of diabetes mellitus, and learned that she did have type 2 noninsulin-dependent diabetes mellitus. In conclusion, we report a case of diabetic mastopathy that presents with bilateral tumorlike masses and an unusual exuberant lymphohistiocytic response with granuloma formation. The pathologist may not be provided with a history of diabetes mellitus, but the characteristic fibrosis, lymphocytic ductitis/lobulitis, and sclerosing lobulitis with perilobular and perivascular lymphocytic infiltrates should provide clues for an accurate diagnosis, even when an exuberant and an unusual lymphohistiocytic response is present. A timely accurate diagnosis can help limit repeat surgeries in this vulnerable group of patients.

Gray-level grouping (GLG): an automatic method for optimized image contrast enhancement--Part I: the basic method.

Contrast enhancement has an important role in image processing applications. Conventional contrast enhancement techniques either often fail to produce satisfactory results for a broad variety of low-contrast images, or cannot be automatically applied to different images, because their parameters must be specified manually to produce a satisfactory result for a given image. This paper describes a new automatic method for contrast enhancement. The basic procedure is to first group the histogram components of a low-contrast image into a proper number of bins according to a selected criterion, then redistribute these bins uniformly over the grayscale, and finally ungroup the previously grouped gray-levels. Accordingly, this new technique is named gray-level grouping (GLG). GLG not only produces results superior to conventional contrast enhancement techniques, but is also fully automatic in most circumstances, and is applicable to a broad variety of images. An extension of GLG, selective GLG (SGLG), and its variations will be discussed in Part II of this paper. SGLG selectively groups and ungroups histogram components to achieve specific application purposes, such as eliminating background noise, enhancing a specific segment of the histogram, and so on. The extension of GLG to color images will also be discussed in Part II.

Gray-level grouping (GLG): an automatic method for optimized image contrast enhancement--Part II: the variations.

This is Part II of the paper, "Gray-Level Grouping (GLG): an Automatic Method for Optimized Image Contrast Enhancement". Part I of this paper introduced a new automatic contrast enhancement technique: gray-level grouping (GLG). GLG is a general and powerful technique, which can be conveniently applied to a broad variety of low-contrast images and outperforms conventional contrast enhancement techniques. However, the basic GLG method still has limitations and cannot enhance certain classes of low-contrast images well, e.g., images with a noisy background. The basic GLG also cannot fulfill certain special application purposes, e.g., enhancing only part of an image which corresponds to a certain segment of the image histogram. In order to break through these limitations, this paper introduces an extension of the basic GLG algorithm, selective gray-level grouping (SGLG), which groups the histogram components in different segments of the grayscale using different criteria and, hence, is able to enhance different parts of the histogram to various extents. This paper also introduces two new preprocessing methods to eliminate background noise in noisy low-contrast images so that such images can be properly enhanced by the (S)GLG technique. The extension of (S)GLG to color images is also discussed in this paper. SGLG and its variations extend the capability of the basic GLG to a larger variety of low-contrast images, and can fulfill special application requirements. SGLG and its variations not only produce results superior to conventional contrast enhancement techniques, but are also fully automatic under most circumstances, and are applicable to a broad variety of images.

Revision of the American Joint Committee on Cancer staging system for breast cancer.

PURPOSE: To revise the American Joint Committee on Cancer staging system for breast carcinoma. MATERIALS AND METHODS: A Breast Task Force submitted recommended changes and additions to the existing staging system that were (1) evidence-based and/or consistent with widespread clinical consensus about appropriate diagnostic and treatment standards and (2) useful for the uniform accrual of outcome information in national databases. RESULTS: Major changes included the following: size-based discrimination between micrometastases and isolated tumor cells; identifiers to indicate usage of innovative technical approaches; classification of lymph node status by number of involved axillary lymph nodes; and new classifications for metastasis to the infraclavicular, internal mammary, and supraclavicular lymph nodes. CONCLUSION: This revised staging system will be officially adopted for use in tumor registries in January 2003.

Adenomyoepithelioma: clinical, histologic, and immunohistologic evaluation of a series of related lesions.

Adenomyoepithelioma, strictly defined, is a proliferation of both epithelial and myoepithelial elements. The broad range of lesions that may fall under this umbrella, however, may be quite diverse. The diagnostic confusion surrounding this entity and its prognostic implications have led to a diagnosis by default as malignant and to overtreatment of some patients. We evaluated available material from a series of 35 women whose slides were seen in consultation and who were diagnosed with adenomyoepithelioma or a closely related lesion. This comprehensive review of the varied histology of adenomyoepithelioma and similar lesions and their immunohistochemical properties will assist general pathologists in evaluating these sometimes difficult lesions. Follow-up and treatment information demonstrates their benignity. Architecture and histologic features should be combined with immunohistochemistry when determining categorization.

Immunohistochemical expression of estrogen receptor in enlarged lobular units with columnar alteration in benign breast biopsies: a nested case-control study.

The prognostic and therapeutic implications of estrogen receptor (ER) status in breast cancer are well known. Whether ER status plays a role in benign breast lesions and the progression to malignancy has not been proven. Enlarged lobular units with columnar alteration (ELUCA), also known as unfolded lobular units, have been associated with mild elevations in subsequent breast cancer risk. We examined the association of ERalpha expression in ELUCA with invasive breast cancer risk. A nested case-control study was performed of women with ELUCA who had undergone benign breast surgery. Eighty-two women who developed invasive breast cancer on follow-up were matched by age and year of biopsy with 166 women who did not develop invasive breast cancer. Entry biopsies were stained for ERalpha (clone 6F11) without knowledge of subsequent cancer outcome. ELUCA lesions were scored as positive if greater than 10% of epithelial nuclei stained with ERalpha and at least one ELUCA contained >50% of cells staining for ERalpha. Relative risks of breast cancer were estimated by odds ratios derived from conditional logistic regression analyses. Thirty-nine percent of cases and 56% of controls had positive ERalpha staining in ELUCA. The relative risk of invasive breast cancer in women with ERalpha-negative ELUCA was 1.85 times that of women with ERalpha-positive lesions (95% confidence interval, 1.0-3.4, P=0.04). The presence of ERalpha staining in women with ELUCA is associated with a lower risk of developing subsequent invasive carcinoma. These findings have implications for risk assessment in benign breast biopsies and are of particular interest given the controversy currently surrounding hormone replacement therapy.

Serum IGFBP-2 and Risk of Atypical Hyperplasia of the Breast.

Atypical hyperplasia of the breast (AH) is associated with increased risk of subsequent invasive breast cancer, yet little is known about the etiology of AH. Insulin-like growth factor binding protein 2 (IGFBP-2) may contribute to the development of AH due to its proliferative effects on mammary tissue. We conducted a nested case-control study of postmenopausal women enrolled in Women's Health Initiative-Clinical Trial. Cases were 275 women who developed incident AH during follow-up, individually (1 : 1) matched to controls. Levels of IGFBP-2 were determined from fasting serum collected at baseline. Multivariable conditional logistic regression models were used to estimate odds ratios for the association of IGFBP-2 with risk of AH. Serum IGFBP-2 was associated with a nonsignificant decrease in risk for AH, when comparing the highest quartile to lowest quartile (OR = 0.65; 95% CI = 0.32-1.31). This decrease in risk was most evident when analyses were restricted to nondiabetic, nonusers of hormone therapy (OR = 0.33, 95% CI = 0.13-0.86, p trend = 0.06) and nondiabetic women who were overweight or obese (OR = 0.43, 95% CI = 0.18-1.03, p trend = 0.05). Results from this study provide some support for an inverse association between serum IGFBP2 levels and risk of AH, particularly in nondiabetic women who are overweight or obese. Further studies are required to confirm these results.

Special types of invasive breast cancer, with clinical implications.

Analysis of invasive breast cancers by histopathology has generally revolved around the analysis of individual characteristics such as nuclear pleomorphism and/or gland formation (grading) or categorization by specific histologic patterns (special type carcinomas). Recognition of special types of invasive breast carcinoma allows for the identification of women with an extremely good prognosis often approaching or equaling that of the general population of the same age (with unique problems for medullary). The distinguishing features of special types of breast cancer are present in complete or classic form in the pure examples of these cases, and the features recognizing these types may be present in lesser degrees in many carcinomas of no special type or not otherwise specified in which they often have important clinical correlates other than strong indicators of survival, such as recognizing the diffusely invasive feature of lobular and lobular-like cancers. Many other features have been noted as contenders for types of breast cancer (such as squamous, cystic, clear cell, histiocytoid, and lipid rich). These terms are not discussed here because they have no special clinical correlates relevant to diagnosis or prognosis. It should be reemphasized here that combined histologic grade (Nottingham system favored) with nuclear, glandular, and mitotic evaluation is expected by most authorities for clarification of diagnosis in all but rare, pure medullary cancers. (5, 13, 14, 27) The Nottingham grade 11 interacts with nodal status to produce a very strong prognostic index. The mitotic count may be the most important part of the grading system, certainly for prognosis within 5 years of diagnosis. (22, 26)

Ductal carcinoma in situ of the breast: impact of pathology on therapeutic decisions.

The therapy of ductal carcinoma in situ (DCIS) is controversial but is being increasingly decided by pathologic evidence. Studies of the natural history of DCIS demonstrate that DCIS is very heterogeneous in its clinical behavior. As detailed in several reviews, studies that followed patients after biopsy alone indicate a great difference between the small noncomedo examples of DCIS and the larger comedo DCIS lesions. The currently available evidence from cases that have been treated by planned surgical excision without radiation therapy would indicate that noncomedo examples of DCIS have a low incidence of recurrence and may be adequately treated by this technique. In contrast, comedo DCIS lesions have a high propensity for recurrence despite excision and radiotherapy. This presentation will review the histopathology of DCIS and highlight the idea that we are currently in a state of transition in our understanding of DCIS. Studies supporting the stratification of DCIS by histologic pattern plus cytology and size will be contrasted with the rapidly disappearing classic posture that all DCIS is biologically similar and treatment options need not be stratified by the different subtypes or varieties of DCIS.

Microglandular adenosis with transition into adenoid cystic carcinoma of the breast.

Microglandular adenosis (MGA) is a well-recognized, if rare and incompletely characterized, entity in which carcinoma is rarely thought to develop. We report 17 cases in which patterns of adenoid cystic carcinoma (ACC) coexisted with MGA. Immunocharacterization with beta-catenin, E-cadherin, cytokeratins (AE1/AE3), epithelial membrane antigen, S-100 protein, smooth muscle actin, and vimentin was also performed. Most cases had areas of invasive ACC characterized by its defining dual-lumen types. Some cases of ACC appeared to have expanded glands intermingled within the MGA, whereas in other cases ACC formed a transition with the characteristic small, gland-like spaces of MGA. MGA and "atypical MGA" stained irregularly and similarly to that seen in myoepithelium with the three markers of myoepithelial cells in breast: S-100 protein, smooth muscle actin, and vimentin. These markers were also positive in the more solid elements of the ACC. Our study suggests that ACC may develop in a background of and in continuity with MGA. Altered myoepithelial cells appear to be the major neoplastic element in both ACC and "atypical MGA." "Atypical MGA" with transition to ACC may show histologic patterns and an immunohistochemical profile similar to that of ACC. These lesions might be best interpreted as ACC in situ. Both MGA and ACC of the breast grow in an expansile and diffusely infiltrative pattern without having significant metastatic capacity. Their unusual interaction with the surrounding stroma may play a role in this benign biologic behavior.