Radial sclerosing lesions found on core needle biopsy: excision can be safely avoided.

AIMS: Radial sclerosing lesions (RSLs) are benign breast lesions composed of glandular and epithelial proliferations with stellate architecture and fibro-elastotic stroma, which can mimic invasive carcinoma on imaging. Surgical management following a core biopsy diagnosis of RSLs remains controversial. METHODS AND RESULTS: We retrospectively identified core biopsies with RSLs without atypia who underwent subsequent surgical excision between 2015 and 2021. All core biopsy slides were reviewed to confirm the diagnosis. Imaging was reviewed to determine radiological-pathological concordance. An upgrade was defined as invasive carcinoma or ductal carcinoma in situ (DCIS) in the excision. The final cohort consisted of 130 core biopsies from 124 women (median age = 52 years, range = 27-76). The imaging modality was mammogram in 52 (40%) cases, MRI in 52 (40%) and ultrasound in 26 (20%). One hundred and seven (82%) core biopsies were vacuum-assisted and 23 (18%) were ultrasound-guided without vacuum assistance. The median lesion size on imaging was 9 mm (range = 2-41). Overall, two (1%) cases were upgraded at excision, including one microinvasive lobular carcinoma and one 2 mm focus of invasive mammary carcinoma with associated DCIS. In both cases, the upgraded foci of carcinoma were not closely associated with the biopsy site and were considered incidental upgrades. CONCLUSIONS: This study adds to the body of literature supporting observation, rather than routine excision of radial sclerosing lesions without atypia.

Integrated digital pathology at scale: A solution for clinical diagnostics and cancer research at a large academic medical center.

OBJECTIVE: Broad adoption of digital pathology (DP) is still lacking, and examples for DP connecting diagnostic, research, and educational use cases are missing. We blueprint a holistic DP solution at a large academic medical center ubiquitously integrated into clinical workflows; researchapplications including molecular, genetic, and tissue databases; and educational processes. MATERIALS AND METHODS: We built a vendor-agnostic, integrated viewer for reviewing, annotating, sharing, and quality assurance of digital slides in a clinical or research context. It is the first homegrown viewer cleared by New York State provisional approval in 2020 for primary diagnosis and remote sign-out during the COVID-19 (coronavirus disease 2019) pandemic. We further introduce an interconnected Honest Broker for BioInformatics Technology (HoBBIT) to systematically compile and share large-scale DP research datasets including anonymized images, redacted pathology reports, and clinical data of patients with consent. RESULTS: The solution has been operationally used over 3 years by 926 pathologists and researchers evaluating 288 903 digital slides. A total of 51% of these were reviewed within 1 month after scanning. Seamless integration of the viewer into 4 hospital systems clearly increases the adoption of DP. HoBBIT directly impacts the translation of knowledge in pathology into effective new health measures, including artificial intelligence-driven detection models for prostate cancer, basal cell carcinoma, and breast cancer metastases, developed and validated on thousands of cases. CONCLUSIONS: We highlight major challenges and lessons learned when going digital to provide orientation for other pathologists. Building interconnected solutions will not only increase adoption of DP, but also facilitate next-generation computational pathology at scale for enhanced cancer research.

Multi-magnification-based machine learning as an ancillary tool for the pathologic assessment of shaved margins for breast carcinoma lumpectomy specimens.

The surgical margin status of breast lumpectomy specimens for invasive carcinoma and ductal carcinoma in situ (DCIS) guides clinical decisions, as positive margins are associated with higher rates of local recurrence. The "cavity shave" method of margin assessment has the benefits of allowing the surgeon to orient shaved margins intraoperatively and the pathologist to assess one inked margin per specimen. We studied whether a deep convolutional neural network, a deep multi-magnification network (DMMN), could accurately segment carcinoma from benign tissue in whole slide images (WSIs) of shave margin slides, and therefore serve as a potential screening tool to improve the efficiency of microscopic evaluation of these specimens. Applying the pretrained DMMN model, or the initial model, to a validation set of 408 WSIs (348 benign, 60 with carcinoma) achieved an area under the curve (AUC) of 0.941. After additional manual annotations and fine-tuning of the model, the updated model achieved an AUC of 0.968 with sensitivity set at 100% and corresponding specificity of 78%. We applied the initial model and updated model to a testing set of 427 WSIs (374 benign, 53 with carcinoma) which showed AUC values of 0.900 and 0.927, respectively. Using the pixel classification threshold selected from the validation set, the model achieved a sensitivity of 92% and specificity of 78%. The four false-negative classifications resulted from two small foci of DCIS (1 mm, 0.5 mm) and two foci of well-differentiated invasive carcinoma (3 mm, 1.5 mm). This proof-of-principle study demonstrates that a DMMN machine learning model can segment invasive carcinoma and DCIS in surgical margin specimens with high accuracy and has the potential to be used as a screening tool for pathologic assessment of these specimens.

Morphologic and immunohistochemical features of carcinoma involving microglandular adenosis of the breast following neoadjuvant chemotherapy.

Microglandular adenosis (MGA)-related lesions, including atypical MGA (AMGA) and carcinoma involving MGA (C-MGA), are characterized by epithelial atypia, negative hormone receptors, and HER2 status, and can mimic invasive triple negative breast cancer (TNBC) in core needle biopsies (CNB) resulting in selection for treatment with neoadjuvant chemotherapy (NAC). We identified 12 cases of AMGA and/or C-MGA in post-NAC excision specimens (EXC) and analyzed their morphologic and immunohistochemical (IHC) features. All CNBs were initially diagnosed as containing TNBC. Upon re-review, TNBC was confirmed in nine cases. In three CNBs AMGA and/or C-MGA had been interpreted as TNBC. AMGA was initially recognized in only one case but AMGA and/or C-MGA were present in an additional nine CNBs. At EXC, no residual TNBC was present in 5 of 9 EXCs and all 12 cases showed residual AMGA and/or C-MGA. Similar to conventional MGA, AMGA, and C-MGA were positive for S-100, laminin and collagen IV and negative for calponin and p63. Following NAC, these lesions retained their typical staining pattern despite acquiring treatment-related morphologic alterations, most notably of which were areas of single cell growth pattern seen in eight EXCs. This study is the first to report the effects of NAC on AMGA and C-MGA. Our data showed no response of the AMGA and/or C-MGA following NAC in contrast to the high response rate of conventional TNBC. In particular, the infiltrative single cell pattern of post-NAC MGA-related lesions closely mimicked residual TNBC. The persistence of AMGA and C-MGA following NAC supports the notion that these lesions are distinct from conventional TNBC. Our findings also highlight the challenges in recognizing AMGA and C-MGA in CNBs which may lead to unwarranted treatment with NAC in the absence of conventional TNBC.

Deep Multi-Magnification Networks for multi-class breast cancer image segmentation.

Pathologic analysis of surgical excision specimens for breast carcinoma is important to evaluate the completeness of surgical excision and has implications for future treatment. This analysis is performed manually by pathologists reviewing histologic slides prepared from formalin-fixed tissue. In this paper, we present Deep Multi-Magnification Network trained by partial annotation for automated multi-class tissue segmentation by a set of patches from multiple magnifications in digitized whole slide images. Our proposed architecture with multi-encoder, multi-decoder, and multi-concatenation outperforms other single and multi-magnification-based architectures by achieving the highest mean intersection-over-union, and can be used to facilitate pathologists' assessments of breast cancer.

E-cadherin immunohistochemical expression in invasive lobular carcinoma of the breast: correlation with morphology and CDH1 somatic alterations.

E-cadherin (ECAD) immunohistochemical (IHC) expression is lost in ∼90% of invasive lobular carcinomas (ILCs) owing to genomic alterations of CDH1. We examined morphologic features and ECAD IHC expression in invasive breast carcinomas (BCs) with known CDH1 alterations. Between January 2014 and May 2018, 202 cases of BC with a CDH1 somatic alteration were identified. ECAD expression was lost in 77% (155/202) of cases and was retained in 23% (47/202) cases. Most (90%, 139/155) ECAD-negative cases were morphologically classified as ILC, while the remaining (10%, 16/155) were invasive mammary carcinoma with mixed ductal and lobular features (IMC). Of 47 cases with ECAD staining, 62% (29/47) were classified as ILC, 23% (11/47) were classified as IMC, and 15% (7/47) were classified as invasive ductal carcinoma (IDC). Of note, 51% (24/47) of ECAD-positive cases were initially diagnosed as IDC or IMC based on ECAD expression alone. For ECAD-negative BCs, 98% (152/155) of CDH1 alterations were truncating, and 2% (3/155) were variants of unknown significance (VUS). Truncating CDH1 alterations were identified in the majority of ECAD-positive BCs (72%, 34/47); however, VUS-type CDH1 alterations were more prevalent (28%, 13/47) in ECAD-positive BCs than in ECAD-negative BCs. Although 90% of ECAD-negative tumors were compatible with ILC in this study, 17% (29/168) of ILC cases were ECAD positive. In addition, CDH1 truncating alterations were seen in ECAD-positive ILC, supporting the notion of aberrant ECAD staining. Therefore, ECAD IHC expression must be interpreted in conjunction with morphology, and BC with classic histologic features of ILC should not be reclassified as IDC/IMC based solely on the status of ECAD IHC expression.

Validation of a digital pathology system including remote review during the COVID-19 pandemic.

Remote digital pathology allows healthcare systems to maintain pathology operations during public health emergencies. Existing Clinical Laboratory Improvement Amendments regulations require pathologists to electronically verify patient reports from a certified facility. During the 2019 pandemic of COVID-19 disease, caused by the SAR-CoV-2 virus, this requirement potentially exposes pathologists, their colleagues, and household members to the risk of becoming infected. Relaxation of government enforcement of this regulation allows pathologists to review and report pathology specimens from a remote, non-CLIA certified facility. The availability of digital pathology systems can facilitate remote microscopic diagnosis, although formal comprehensive (case-based) validation of remote digital diagnosis has not been reported. All glass slides representing routine clinical signout workload in surgical pathology subspecialties at Memorial Sloan Kettering Cancer Center were scanned on an Aperio GT450 at ×40 equivalent resolution (0.26 µm/pixel). Twelve pathologists from nine surgical pathology subspecialties remotely reviewed and reported complete pathology cases using a digital pathology system from a non-CLIA certified facility through a secure connection. Whole slide images were integrated to and launched within the laboratory information system to a custom vendor-agnostic, whole slide image viewer. Remote signouts utilized consumer-grade computers and monitors (monitor size, 13.3-42 in.; resolution, 1280 × 800-3840 × 2160 pixels) connecting to an institution clinical workstation via secure virtual private network. Pathologists subsequently reviewed all corresponding glass slides using a light microscope within the CLIA-certified department. Intraobserver concordance metrics included reporting elements of top-line diagnosis, margin status, lymphovascular and/or perineural invasion, pathology stage, and ancillary testing. The median whole slide image file size was 1.3 GB; scan time/slide averaged 90 s; and scanned tissue area averaged 612 mm2. Signout sessions included a total of 108 cases, comprised of 254 individual parts and 1196 slides. Major diagnostic equivalency was 100% between digital and glass slide diagnoses; and overall concordance was 98.8% (251/254). This study reports validation of primary diagnostic review and reporting of complete pathology cases from a remote site during a public health emergency. Our experience shows high (100%) intraobserver digital to glass slide major diagnostic concordance when reporting from a remote site. This randomized, prospective study successfully validated remote use of a digital pathology system including operational feasibility supporting remote review and reporting of pathology specimens, and evaluation of remote access performance and usability for remote signout.

Whole slide imaging equivalency and efficiency study: experience at a large academic center.

Whole slide imaging is Food and Drug Administration-approved for primary diagnosis in the United States of America; however, relatively few pathology departments in the country have fully implemented an enterprise wide digital pathology system enabled for primary diagnosis. Digital pathology has significant potential to transform pathology practice with several published studies documenting some level of diagnostic equivalence between digital and conventional systems. However, whole slide imaging also has significant potential to disrupt pathology practice, due to the differences in efficiency of manipulating digital images vis-à-vis glass slides, and studies on the efficiency of actual digital pathology workload are lacking. Our randomized, equivalency and efficiency study aimed to replicate clinical workflow, comparing conventional microscopy to a complete digital pathology signout using whole slide images, evaluating the equivalency and efficiency of glass slide to whole slide image reporting, reflective of true pathology practice workloads in the clinical setting. All glass slides representing an entire day's routine clinical signout workload for six different anatomic pathology subspecialties at Memorial Sloan Kettering Cancer Center were scanned on Leica Aperio AT2 at ×40 (0.25 µm/pixel). Integration of whole slide images for each accessioned case is through an interface between the Leica eSlide manager database and the laboratory information system, Cerner CoPathPlus. Pathologists utilized a standard institution computer workstation and viewed whole slide images through an internally developed, vendor agnostic whole slide image viewer, named the "MSK Slide Viewer". Subspecialized pathologists first reported on glass slides from surgical pathology cases using routine clinical workflow. Glass slides were de-identified, scanned, and re-accessioned in the laboratory information system test environment. After a washout period of 13 weeks, pathologists reported the same clinical workload using whole slide image integrated within the laboratory information system. Intraobserver equivalency metrics included top-line diagnosis, margin status, lymphovascular and/or perineural invasion, pathology stage, and the need to order ancillary testing (i.e., recuts, immunohistochemistry). Turnaround time (efficiency) evaluation was defined by the start of each case when opened in the laboratory information system and when the case was completed for that day (i.e., case sent to signout queue or pending ancillary studies). Eight pathologists participated from the following subspecialties: bone and soft tissue, genitourinary, gastrointestinal, breast, gynecologic, and dermatopathology. Glass slides signouts comprised of 204 cases, encompassing 2091 glass slides; and digital signouts comprised of 199 cases, encompassing 2073 whole slide images. The median whole slide image file size was 1.54 GB; scan time/slide, 6 min 24 s; and scan area 32.1 × 18.52 mm. Overall diagnostic equivalency (e.g., top-line diagnosis) was 99.3% between digital and glass slide signout; however, signout using whole slide images showed a median overall 19% decrease in efficiency per case. No significant difference by reader, subspecialty, or specimen type was identified. Our experience is the most comprehensive study to date and shows high intraobserver whole slide image to glass slide equivalence in reporting of true clinical workflows and workloads. Efficiency needs to improve for digital pathology to gain more traction among pathologists.

Sentinel lymph node B cells can predict disease-free survival in breast cancer patients.

Tumor invasion into draining lymph nodes, especially sentinel lymph nodes (SLNs), is a key determinant of prognosis and treatment in breast cancer as part of the TNM staging system. Using multicolor histology and quantitative image analysis, we quantified immune cells within SLNs from a discovery cohort of 76 breast cancer patients. We found statistically more in situ CD3+ T cells in tumor negative vs. tumor positive nodes (mean of 8878 vs. 6704, respectively, p = 0.006), but no statistical difference in CD20+ B cells or CD1a+ dendritic cells. In univariate analysis, a reduced hazard was seen with a unit increase in log CD3 with HR 0.49 (95% CI 0.30-0.80) and log CD20 with HR 0.37 (95% CI 0.22-0.62). In multivariate analysis, log CD20 remained significant with HR 0.42 (95% CI 0.25-0.69). When restricted to SLN tumor negative patients, increased log CD20 was still associated with improved DFS (HR = 0.26, 95% CI 0.08-0.90). The CD20 results were validated in a separate cohort of 21 patients (n = 11 good outcome, n = 10 poor outcome) with SLN negative triple-negative breast cancer (TNBC) ("good" mean of 7011 vs. "poor" mean of 4656, p = 0.002). Our study demonstrates that analysis of immune cells within SLNs, regardless of tumor invasion status, may provide additional prognostic information, and highlights B cells within SLNs as important in preventing future recurrence.

Acinic cell carcinoma of breast: morphologic and immunohistochemical review of a rare breast cancer subtype.

Acinic cell carcinoma of breast is a rare subtype of triple-negative breast carcinoma and demonstrates extensive morphologic overlap with acinic cell carcinoma of the salivary gland. In this study, we perform a detailed morphologic and immunohistochemical description of 2 cases of this rare entity and undertake a comprehensive review of all reported cases of breast acinic cell carcinoma in the English language literature to date. One-third of reported cases of breast acinic cell carcinoma have been associated with the presence of a ductal carcinoma not otherwise specified component, which is frequently poorly differentiated. Breast acinic cell carcinoma can demonstrate focal morphologic features similar to microglandular adenosis; these areas are frequently negative for collagen IV and laminin on immunohistochemistry. The true relationship between these 2 entities remains unclear, but we advocate that microglandular adenosis-like areas at the periphery of a breast acinic cell carcinoma should be considered part of the carcinomatous process and re-excised if this process extends to the initial surgical margins.

Breast Carcinoma in Young Women: No Evidence of Increasing Rates of Metastatic Breast Carcinoma in a Single Tertiary Center Review.

Breast carcinoma in young women aged less than 40 years attracts a high level of mainstream media coverage, and there is a gap between societal perceptions of the disease as a growing problem and epidemiological trends. Several population studies have reported that the overall incidence of breast carcinoma in young women is stable, while one recent article suggested that the relative proportion of breast carcinoma in young women that is metastatic at diagnosis is growing. We sought to establish whether these trends were apparent at our institution. In this study, the clinical database at a breast carcinoma tertiary center was reviewed in terms of clinicopathologic data on patient age, diagnosis, clinical and pathologic stage, hormone receptor status, and HER-2 overexpression status for the period 2000-2011. Over the study period, young patients represented a decreasing proportion of all breast carcinoma cases (10.8% [2000-2003] to 8.7% [2008-2011]; p < 0.0001) treated at our institution. Young patients were more likely than patients aged 40 years or older to present with metastatic (M1) disease (5.4% versus 4.4%; p = 0.009), to be triple negative (21.6% versus 13%; p < 0.001), or to be HER-2 positive (24.3% versus 14.8%; p < 0.01). Young patients with HER-2-positive cancers were significantly more likely to present with metastatic disease (8.3% versus 4.8%; p = 0.004). This study showed no demonstrable increase in the relative proportion of breast cancer occurring in patients aged <40 years over the 12-year period 2000-2011 and no increase in the proportion of young patients presenting with metastatic disease.

Is There a Role for Oncotype Dx Testing in Invasive Lobular Carcinoma?

Oncotype Dx Breast Cancer Assay is a 21-gene assay used in estrogen receptor (ER)-positive breast cancer to predict benefit from chemotherapy (CT). Tumors are placed into one of three risk categories based on their recurrence score (RS). This paper explores the impact of tumor histopathologic features and Oncotype Dx RS on the treatment plan for invasive lobular carcinoma (ILC). Invasive lobular carcinoma cases submitted for Oncotype Dx testing were identified from a clinical data base. The histopathologic and immunohistochemical features and RS subcategory of each tumor, and treatment regimen and medical oncologic assessments of each patient were reviewed. A total of 135 cases of ILC had RS testing, which represented 15% of all ILC diagnosed at the institution over the time period. 80% of ILC was of the classical subtype and all tumors were ER positive and human epidermal growth factor receptor 2 (HER-2) negative by immunohistochemistry. Sixty three percent of cases were low risk (LR), 35.5% were intermediate risk (IR) and 1.5% were high risk (HR). Both HR cases were pleomorphic ILC. Sixty eight percent of classical ILC had a LR score, while 70% of pleomorphic ILC had an IR score. Patients in the IR category were significantly more likely to undergo CT than patients in the LR category (54% versus 18%; p < 0.0001). In the LR category, those undergoing CT were significantly younger and more likely to have positive lymph nodes (p < 0.05). Qualitative analysis of medical oncologic assessments showed that RS played a role in decision-making on CT in 74% of cases overall. At our institution, Oncotype Dx RS currently plays a role in the management of a proportion of ILC and impacts on treatment decisions.

Mucinous micropapillary carcinoma of the breast: an aggressive counterpart to conventional pure mucinous tumors.

Mucinous micropapillary carcinoma of the breast, also described as "pure mucinous carcinoma with micropapillary pattern," has recently come to attention as an unusual form of invasive breast cancer exhibiting dual mucinous and micropapillary differentiation. Despite increasing awareness of this morphologic variant, its clinical significance has not yet been elucidated. Here, we present 15 additional examples of these rare tumors to highlight some important differences between mucinous micropapillary carcinoma of the breast and ordinary pure mucinous carcinomas. The key features of mucinous micropapillary carcinoma of the breast included (a) largely or entirely mucinous appearance (>90% mucinous morphology), (b) distinctive micropapillary arrangement of the neoplastic cells, (c) intermediate to high nuclear grade, (d) "hobnail" cells, and (e) frequent psammomatous calcifications. In contrast to ordinary pure mucinous carcinomas, 20% of mucinous micropapillary carcinomas of the breast were characterized by human epidermal growth factor receptor 2 positivity, and 23% were p53 positive. More than half of mucinous micropapillary carcinomas of the breast (60%) demonstrated lymphovascular invasion, sometimes extensive. Synchronous axillary lymph node metastases were detected in 33% of patients and, on 2 occasions, involved more than 10 nodes. With a median follow-up of 4.5 years, we identified 1 patient (7%) with chest wall recurrence of mucinous micropapillary carcinoma of the breast after mastectomy. We conclude that mucinous micropapillary carcinomas of the breast constitute a clinically aggressive subset of mucin-producing breast carcinomas characterized by an increased capacity for lymphatic invasion and regional lymph node metastasis, reflective of their dual phenotype. Recognition of the morphologic and biologic heterogeneity within breast cancer subtypes should allow for a more accurate classification of the individual tumors and better patient stratification for treatment.

Non-mammary metastases to the breast and axilla: a study of 85 cases.

Non-mammary metastases to the breast and axilla are rare occurrences. However, they are important diagnostic considerations as their treatment and prognosis differ significantly from primary breast cancer. Between 1990 and 2010, we identified a total of 85 patients, 72 women and 13 men, with non-mammary malignancies involving the breast, axilla, or both. The tumor types consisted of carcinoma (58%), melanoma (22%) and sarcoma (20%). Ovary was the most common site of origin for carcinoma, and metastatic high-grade ovarian serous carcinoma was most frequently misdiagnosed as a primary breast carcinoma. Melanoma was the single most common non-carcinomatous tumor type to involve the breast and/or axilla, and uterine leiomyosarcoma was the most common type of sarcoma. Most patients (77%) had other metastases at the time of diagnosis of the tumor, but in 11% the breast or axillary lesion was the first presentation. Without a clinical history, non-mammary metastases were difficult to diagnose because the majority of cases presented with a solitary nodule and lacked pathognomonic pathologic features. There were, however, certain recurrent histological findings identified, such as the often relatively well-circumscribed growth pattern of the metastatic lesion surrounded by a fibrous pseudocapsule, and the absence of an in situ carcinoma. Overall, these patients had poor survival; 96% of patients with follow-up available are dead of disease, with a median survival of 15 months after the diagnosis of the breast or axillary lesion. This finding emphasizes the need to accurately identify these tumors as metastases in order to avoid unnecessary procedures and treatments in these patients.

Phase 2 trial of a novel capecitabine dosing schedule in combination with bevacizumab for patients with metastatic breast cancer.

BACKGROUND: Capecitabine has antitumor activity in metastatic breast cancer (MBC); however, its optimal dose and schedule remain unclear. Mathematical modeling predicts that a capecitabine schedule 7 days of treatment followed by 7 days of rest (7-7) will improve efficacy and minimize toxicity. Bevacizumab has demonstrated the ability to improve outcomes when it is added to chemotherapy, including capecitabine, in the first-line and second-line settings. METHODS: Patients with measurable MBC received oral capecitabine (2000 mg twice daily; 7-7), and intravenous bevacizumab (10 mg/kg every 2 weeks). The primary endpoint was the response rate. Secondary endpoints included toxicity, the clinical benefit rate, and progression-free survival (PFS). RESULTS: Forty-one patients were treated. After a median of 7 cycles (range, 1-32 cycles), partial responses were observed in 20% of patients, and stable disease for ≥6 months was noted in 35% patients. The median PFS was 8 months. The most common treatment-related toxicities were hand-foot syndrome (49% grade 2, 20% grade 3/4) hypertension (12% grade 2, 10% grade 3/4), and fatigue (12% grade 2, 2% grade 3/4). Diarrhea (5% grade 2, 0% grade 3/4), nausea (0% grade 2-4), and vomiting (0% grade 2-4) were rare. CONCLUSIONS: Capecitabine administered for 7 days followed by a 7-day rest in combination with bevacizumab had modest efficacy and an acceptable toxicity profile in patients with MBC. Gastrointestinal toxicity with this schedule was minimal.

Is pleomorphic lobular carcinoma really a distinct clinical entity?

BACKGROUND: Attempts to define the clinical behavior of pleomorphic lobular carcinoma (PLC) have been limited to small series, and clinical management strategies have yet to be established. We describe our experience with PLC as compared to classic ILC and invasive ductal carcinoma (IDC). METHODS: From 9/1996 to 5/2003, clinical and histopathologic data for 5,635 patients undergoing primary surgical treatment and sentinel lymph node biopsy for breast cancer were collected. Four hundred eighty one (8.5%) patients were diagnosed with ILC; 3,978 (70.6%) with IDC. Of those with ILC, 356 (74%) patients had material available for pathologic re-review and comprise our study population: 52 were classified as PLC; 298 were classified as classic ILC; and 6 cases were reclassified as IDC. We compared clinical, pathologic, and treatment factors for patients with PLC, ILC, and IDC using the Wilcoxon rank sum and Fisher's exact tests. RESULTS: PLC were larger than ILC and IDC (20 vs. 15 vs. 13, P < 0.001), had more positive nodes (median 1 vs. 0 vs. 0, P < 0.05) and more frequently required mastectomy (63.5% vs. 38.7% vs. 28.8%, P < 0.001). In addition, more patients with PLC had developed metastatic disease compared to patients with ILC (11.5% vs. 3.7%, P < 0.05). CONCLUSIONS: These findings suggest that PLC is a distinct clinical entity that presents at a more advanced stage and may require more aggressive surgical and adjuvant treatment.