18F]FACBC PET/CT as a novel imaging alternative in invasive lobular breast carcinoma

Background: Invasive ductal carcinoma (IDC) and invasive lobular carcinoma (ILC) are the two most common histologic subtypes of breast cancer (BC), with IDC accounting for approximately 70-80% of cases and ILC comprising 10-15%. Improved screening and advances in breast cancer treatment have increased survivorship but also heightened the risk of second primary malignancies (SPMs) among long-term survivors. Our study presents a comparative analysis of the types of SPMs among ILC and IDC survivors. Methods: Using the National Cancer Institute’s Surveillance, Epidemiology, and End Results(SEER) 8 database registry, we identified a cohort of female patients diagnosed with ILC and IDC between 1975 and 2022. We analyzed the incidence of SPM using the SEER*Stat MultiplePrimary-Standardized Incidence Ratio (MP-SIR) session for the overall cohort of females with ILC and IDC. The cohort was further stratified by age (< 40 years and >/= 40 years). Standardized incidence ratios (SIRs) were calculated to compare the observed rates with the expected rates between ILC/IDC and the general population. Results: Both ILC and IDC had an increased risk of all types of SPMs, with SIRs of 1.09 (95%CI: 1.06-1.11) for ILC and 1.14 (95% CI: 1.14-1.15) for IDC. Some sites emerged as commonsites of SPMs for both ILC and IDC, and those with highest SIRs include soft tissue (ILC: SIR =1.72; IDC: SIR = 1.55, p < 0.05), thyroid (ILC: SIR = 1.42; IDC: SIR = 1.22, p < 0.05), skinexcluding basal and squamous (ILC: SIR = 1.19; IDC: SIR = 1.06, p < 0.05), and acute myeloidleukemia (ILC: SIR = 1.90; IDC: SIR = 1.78, p < 0.05). Sites for SPM unique to ILC include mainly the stomach (SIR = 1.58, CI 1.33 - 1.86) and melanoma of the skin (SIR = 1.18, CI 1.05 -1.32). Sites for SPM unique to IDC with p < 0.05 include salivary gland, esophagus, pleura, lung, bones and joints, corpus uteri, and acute lymphocytic leukemia (Table). Greater incidence of SPMs was observed for patients age< 40 for both ILC and IDC (ILC: age < 40 SIR = 1.84 vs age >/= 40 SIR = 1.08; IDC: age < 40SIR = 1.98 vs age >/= 40 SIR = 1.11). For ILC, SPM with involvement of the stomach is seen in the cohort of both age categories, but did not reach statistical significance for age < 40 (age < 40:SIR = 4.22, CI 0.87-12.33; age >/= 40: SIR = 1.56, CI 1.31-1.84). Conclusion: Our study provides a detailed understanding of the variation in the pattern of SPMs for both ILC and IDC. Patients with ILC exhibited a higher propensity to develop SPM at specific sites, such as the stomach and skin melanoma, highlighting the need for further investigation into the underlying biological mechanisms. Conversely, IDC has a broader distribution of unique SPMs involving the lung, pleura, salivary gland, esophagus, uterus, and bones. Younger patients exhibited a higher risk of SPMs in both ILC and IDC cohorts. These findings underscore the importance of a tailored surveillance strategy based on histological subtype. Citation Format: S. Singh, S. Sangam, B. Zengin, S. Modi, H. Jain, A. MariamRoy. Comparative analysis of second primary malignancies in invasive lobular versus ductal breast carcinoma: A SEER-based study [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS5-04-10.

Breast cancer heterogeneity in histology and molecular subtype influences metabolic and proliferative activity and hence the acid load on cancer cells. We hypothesized that acid-base transporters and intracellular pH (pHi) dynamics contribute inter-individual variability in breast cancer aggressiveness and prognosis. We show that Na+,HCO3– cotransport and Na+/H+ exchange dominate cellular net acid extrusion in human breast carcinomas. Na+/H+ exchange elevates pHi preferentially in estrogen receptor-negative breast carcinomas, whereas Na+,HCO3– cotransport raises pHi more in invasive lobular than ductal breast carcinomas and in higher malignancy grade breast cancer. HER2-positive breast carcinomas have elevated protein expression of Na+/H+ exchanger NHE1/SLC9A1 and Na+,HCO3– cotransporter NBCn1/SLC4A7. Increased dependency on Na+,HCO3– cotransport associates with severe breast cancer: enlarged CO2/HCO3–-dependent rises in pHi predict accelerated cell proliferation, whereas enhanced CO2/HCO3–-dependent net acid extrusion, elevated NBCn1 protein expression, and reduced NHE1 protein expression predict lymph node metastasis. Accordingly, we observe reduced survival for patients suffering from luminal A or basal-like/triple-negative breast cancer with high SLC4A7 and/or low SLC9A1 mRNA expression. We conclude that the molecular mechanisms of acid-base regulation depend on clinicopathological characteristics of breast cancer patients. NBCn1 expression and dependency on Na+,HCO3– cotransport for pHi regulation, measured in biopsies of human primary breast carcinomas, independently predict proliferative activity, lymph node metastasis, and patient survival.

The roles of breast conservation and surgical evaluation of the contralateral breast in the treatment of lobular carcinoma of the breast remain unclear. The aim of this study was to compare local recurrence, 5-year survival, and incidence of contralateral breast cancer in women with lobular carcinoma to that in women with infiltrating ductal carcinoma. Women with infiltrating ductal carcinoma (IDC) and invasive lobular breast carcinoma (ILC) diagnosed during the years 1984 to 1994 were identified through a statewide tumor registry. The women were divided into groups based on their histology and treatment (breast conservation or modified radical mastectomy). The incidences of contralateral breast cancer, local recurrence, and 5-year survival were compared within each histologic group and treatment category. During the period 1984 to 1994, 4886 women were diagnosed with invasive lobular or ductal breast carcinoma. Of these, 316 (6.5%) had infiltrating lobular cancer. The 5-year survival rates were 68% and 71% for ILC and IDC, respectively (p = 0.5). The local recurrence rates were 2.8% and 4.3% for ILC treated with lumpectomy and axillary nodal dissection (LAND) and modified radical mastectomy (MRM), respectively, which were not significantly different from that obtained with IDC (LAND = 2.5%, MRM = 2.1%). The incidence of contralateral breast cancer during the period was 6.6% and 6.5% for ILC and IDC, respectively. Invasive lobular carcinoma can be safely treated with breast conservation with no difference in local recurrence or survival. In the absence of a suspicious finding on clinical or radiologic examination, routine contralateral breast intervention is not recommended.

Concomitant endometrial and gallbladder metastasis in advanced multiple metastatic invasive lobular carcinoma of the breast: A rare case report

Introduction: Invasive lobular carcinoma (ILC) is common and accounts for 5-15% of all breast cancers. ILC has distinct clinical and histological features that separate it from invasive ductal carcinoma (IDC) with regards to its breast imaging characteristics, patterns of recurrence and sensitivity to systemic therapy. ILC presents challenges to the physician in many aspects of local-regional and systemic therapy choices. We surveyed breast cancer physicians on their beliefs and practice patterns on issues around the management of ILC. Methods: A questionnaire was developed and circulated electronically using a modified Dillman technique to surgical, radiation, and medical oncologists across Canada and Ireland. Results: The questionnaire was completed by 91 of 429 physicians (21% response rate). Response rate by specialty was 25/69 (36%), 21/54 (41%) and 45/306 (13%) for surgical, radiation and medical oncologists respectively. Most surgeon responders (77%) would feel "uncomfortable making treatment decisions for ILC with a mammogram alone" and 100% would be "more comfortable with an MRI". Although 55% reported treating ILC as they would IDC, 22% of surgeons will purposefully obtain larger gross margins intra-operatively. Some radiation oncologists believe ILC is an independent risk factor for local-regional recurrence after breast conserving surgery (49%), and after mastectomy (29%). 33% of radiation oncologists would offer radiation therapy after mastectomy specifically because of the ILC subtype even in the absence of usual indications for radiotherapy. Most medical oncologists treat ILC comparably to IDC with the factors having the largest influence on systemic treatment decisions being tumour stage, hormone receptor status and HER-2 status. 51% of medical oncologists treat ILC with adjuvant chemotherapy as they do for IDC at least ‘most of the time’, while 40% use neoadjuvant chemotherapy as frequently as they do for IDC at least ‘most of the time’. 75% of medical oncologists manage the hormonal treatment of ILC as they do IDC ‘most of the time’ or ‘always’. Conclusions: There remains significant clinical equipoise in the local-regional and systemic management of ILC. This survey has demonstrated wide variations in both beliefs and practices of management for ILC. Clinical guidance, developed on clinical trials specifically assessing the management of ILC, is required. Citation Format: Carmel Jacobs, Mark Clemons, Mohamed FK Ibrahim, Christina Addison, Jean-Michel Caudrelier, Ian D Graham, Brain Hutton, Angel Arnaout. Oncologist treatment choices in patients with early stage invasive lobular breast carcinoma - a survey [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-21-09.

We compared the breast cancer patients with invasive lobular carcinoma (ILC), invasive ductal carcinoma (IDC) and mixed invasive ductal and lobular carcinoma (IDLC) in terms of clinicopathological and treatment features, metastatic patterns and long-term survival. In a 10 years patient cohort, 3412 patients with unilateral breast carcinoma were enrolled in the study. Tumors were classified histologically according to criteria described by World Health Organization classification. The highest rate of T3 tumors were found in IDLC patients, the lowest in IDC patients, and the difference between groups was significant only in comparison of IDC vs IDLC. Axillary positivity rate was highest in IDLC, lowest in ILC; differences were significant in comparisons of IDLC vs ILC and IDLC vs IDC. There was no significant difference between the patient groups in terms of surgical treatment, mastectomy and breast conserving surgery. Rate of bone metastasis was highest in IDLC, lowest in IDC, with significant difference between IDLC and IDC. Locoregional recurrence-free survival (LRFS) rate was 90.9% in ILC patients, 92.5% in IDC patients, 92.9% in IDLC patients, with no significant difference between the groups; in multivariate Cox analysis, histological type had no prognostic significance (p=0.599). Distant metastasis-free survival (DMFS) rate was 66.2% in ILC patients, 66.7% in IDC patients, 57.1% in IDLC patients; in multivariate Cox analysis, histological type had no prognostic significance (p=0.392). Although these results suggest that IDLC may have a worse prognosis than IDC and ILC, in multivariate analysis LRFS and DMFS were not significantly different among the histological type groups.

The leading problem for women with estrogen receptor α (ER) breast cancer – approximately 70% of all breast cancers – is that while the majority of tumors initially respond to anti-estrogen treatment including Tamoxifen and aromatase inhibitors, they will develop endocrine resistance. A histological subset of breast cancer called invasive lobular carcinoma (ILC) accounts for 15% of all breast cancer cases and approximately 90% of ILC are ER-positive. Despite clinical markers suggesting a better prognosis for ILC compared to the more common invasive ductal carcinoma (IDC), recent data suggests women with ILC develop Tamoxifen resistance at a higher rate than IDC. New strategies for treating hormone refractory tumors are needed that not only overcome anti-estrogen resistance, but also present a more tolerable side effect profile and have fewer dose-limiting toxicities than adjuvant chemotherapy. The majority of ER-positive breast cancers also express androgen receptor (AR) and while AR presence in breast cancer is controversial, increased expression of AR leads to Tamoxifen resistance in IDC cell lines. Since ILC has more AR expression than IDC, the role of AR in ILC was studied by treating cells with the non-aromatizable androgen dihydrotestosterone (DHT) and the synthetic androgen R1881 as well as the anti-androgen Enzalutamide. We have previously begun to characterize a Tamoxifen resistant variant of the ILC cell line Sum44PE termed LCCTam cells. In both Sum44PE and LCCTam cells, AR protein expression increases in response to androgens, an action that is blocked by Enzalutamide. Interestingly, androgens do not promote growth in ILC cells, but Enzalutamide is still able to inhibit growth. These results suggest Enzalutamide may be an effective alternative therapy for women with ILC. Future studies will focus on whether regulation of nuclear receptor co-factors influences differences in AR signaling in ILC. Citation Format: Hillary Stires, Rebecca B. Riggins. The role of androgen receptor in invasive lobular breast carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3605. doi:10.1158/1538-7445.AM2017-3605

Background: Young women in the US are experiencing rising rates of breast cancer, necessitating effective treatment strategies. Neoadjuvant chemotherapy (NACT) is widely used in clinical practice, yet research on post-NACT outcomes among young women is limited, especially for those with less common histology, e.g., invasive lobular carcinoma (ILC) or invasive ductal and lobular carcinoma (IDLC). In this study, we examined differential pathologic complete response (pCR) and overall survival (OS) between invasive ductal carcinoma (IDC), ILC, or IDLC among female patients (pts) aged ≤40 years with stage I-III disease who received NACT, stratified by molecular subtype. Methods: This hospital-based, retrospective study analyzed data from the 2010-2020 National Cancer Database. We assessed 4 molecular subtypes: HR+/HER2–, HR+/HER2+, HR–/HER2+, and TNBC. pCR, defined as ypT0/Tis ypN0, was modeled using logistic regression, and adjusted odds ratios (aOR) were calculated. OS was event or censored at the time of death from any cause or last known contact, with 5-/10-year OS rates estimated using the Kaplan-Meier method and compared using log-rank tests. We performed Cox regression to generate adjusted hazard ratios (aHR). All models were stratified by molecular subtype, adjusting for clinical T/N stage, tumor grade, PR status (HR+ only), race/ethnicity, year of diagnosis, and comorbidity score. Results: Of 26,480 young women (median follow-up 52.9 [IQR 32.9, 79.2] months), 95.8% had IDC, 2.1% ILC, and 2.1% IDLC. In the HR+/HER2– cohort, 14.9% of IDC pts achieved pCR compared to 3.3% of ILC and 4.1% of IDLC pts (p<.001). After covariate adjustment, pts with ILC (aOR 0.45, 95% CI: 0.23-0.86) or IDLC (aOR 0.49, 95% CI: 0.27-0.89) had lower odds of pCR than IDC pts. In the HR+/HER2+ cohort, a higher pCR rate was observed among IDC pts (33.8%) than ILC (24.3%) or IDLC (27.5%) pts (p=.033). However, the odds of pCR were not significantly different between IDC and ILC (aOR 0.90, 95% CI: 0.56-1.43) or IDLC (aOR 0.77, 95% CI: 0.52-1.13). In the HR–/HER2+ cohort, pCR rates were similar across histologic types (IDLC: 70.6%; ILC: 58.8%; IDC: 49.9%; p=.180). On multivariable regression, pts with ILC (aOR 1.68, 95% CI: 0.60-4.70) or IDLC (aOR 1.96, 95% CI: 0.67-5.73) had similar odds of pCR as those with IDC. In the TNBC cohort, IDC pts achieved a higher rate of pCR than ILC or IDLC pts (36.0%, 21.3% vs. 18.0%; p=.003). Compared to IDC pts, IDLC pts had lower odds of pCR (aOR 0.40, 95% CI: 0.18-0.9) while ILC pts had similar odds of pCR (aOR 0.70, 95% CI: 0.34-1.42). IDC was associated with significantly longer 5-/10-year OS rates (compared to ILC or IDLC) among pts with HR+/HER2– (p=.012) or TNBC (p<.001). In the adjusted models, pts with HR+/HER2– ILC (aHR 1.55, 95% CI: 1.15-2.10) or IDLC (aHR 1.52, 95% CI: 1.15-2.03) had a greater mortality risk than IDC pts. HR–/HER2+ ILC pts also had a higher risk of death than IDC pts (aHR 3.51, 95% CI: 1.10-11.15). In the TNBC cohort, ILC was associated with an increased mortality risk compared to IDC (aHR 2.28, 95% CI: 1.41-3.69). Conclusions: In this US national registry of young women with early-stage breast cancer who received NACT, pts with ILC or IDLC consistently exhibited lower pCR rates and poorer OS than those with IDC across molecular subtypes. These disparities underscore the need for tailored treatment interventions that account for histologic type, particularly for ILC, among young women. Our findings not only highlight the importance of informed NACT counseling, but also underscore the necessity for oncology programs to optimize therapeutic approaches and continue multidisciplinary efforts to reduce disparities specifically for young women with ILC. Citation Format: Jincong Freeman, Jared H. Hara, Olasubomi J. Omoleye, Ted O. Akhiwu, Shreyas Kalantri, Heather J. Hoffman. Early Invasive Lobular or Ductal Carcinoma with Differential Clinical Outcomes Across Molecular Subtypes among Young Women Who Received Neoadjuvant Chemotherapy [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P5-10-14.

Background: Invasive lobular carcinoma (ILC) is the second most common histological subtype of breast cancer after invasive ductal carcinoma (IDC). While specific clinical and pathological features differ between ILC and IDC, both histologies are treated the same, due to a lack of knowledge of targetable pathways underlying the observed differences. To identify potential genetic drivers of ILC progression, we set out to identify genes with copy number (CN) alterations, comparing tumors with good outcome to those with poor outcome. Method: We designed probes for a total of 67 genes known to be frequently altered in breast cancer and used sensitive nanoString technology to comprehensively investigate CN alterations of these genes in 70 well-curated primary ILCs. ILC cell lines MDA-MB-134-VI, SUM44PE, and BCK4 were used for functional studies including proliferation, apoptosis, colony formation, and analysis of gene expression. Results: Our studies reveal that ESR1 is frequently amplified in primary ILC (14% gains and 10% amplification), and that tumors with amplified ESR1 are more likely to recur compared to those with normal CN. Our analysis also identified a subset of ILCs with HER2 amplification (19%) despite a negative clinical IHC score, and these tumors expressed high HER2 mRNA, protein, and demonstrated enrichment of a molecular HER2 signature. The other most frequently amplified genes included CCND1 (33%), MDM4 (17%), and MYC (17%), and most frequently lost genes were NCOR2 (7%), FGFR4 (6%) and TP53 (6%). MDM4, a negative regulator of p53, has previously been reported to play a role in breast cancer, though little is known about its role in ILC. We demonstrate that decreasing MDM4 levels in p53 wild type ILC cell lines results in increased apoptosis, decreased proliferation associated with cell cycle arrest, and activation of p53 target genes. Intriguingly, a similar induction of G0/G1 cell cycle arrest and increase in apoptosis was observed in p53 mutant ILC cells after MDM4 downregulation, suggesting a p53-independent function of MDM4. Conclusion: Sensitive detection of CN changes identified amplifications of ESR1 and MDM4 as potential drivers of ILC. Functional studies demonstrate that MDM4 has both p53 dependent and independent functions that warrant further study. Citation Format: Atkinson JM, Cao L, Basudan A, Sikora MJ, Bahreini A, Tasdemir N, Jankowitz RC, McAuliffe PF, Dabbs D, Haupt S, Haupt Y, Peter Lucas PC, Lee AV, Oesterreich S. Copy number analysis identifies ESR1 and MDM4 as drivers of progression in invasive lobular breast carcinoma [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-06-03.

A Comparative Analysis of Lobular and Ductal Carcinoma of the Breast: Presentation, Treatment, and Outcomes

Cancer-associated fibroblasts (CAFs) can promote breast cancer progression and therapy response through paracrine and juxtacrine signaling pathways. CAFs are abundantly observed in especially the invasive lobular carcinoma (ILC) subtype of breast cancer. Surprisingly, there is nothing known about the function of these CAFs in ILC. Given the fact that human ILC is generally resistant to standard chemotherapy, we believe that the cause of this therapy resistance lies into the active crosstalk between CAFs and tumor cells leading to a more progressive and resistant phenotype. Therefore, we aim to discover the exact function of CAFs in ILC as well as their drugable targets. In order to reliably study the role of CAFs in ILC, we generated a genetically engineered mouse model for ILC based on the tissue-specific loss of E-cadherin and PI3K signaling pathway activation, two hallmarks of human ILC. This clinically relevant mouse model spontaneously developed tumors that grow as typically invasive ‘indian files’ into a rigid extracellular matrix (ECM) that is very rich of CAFs. Uniquely, these tumors closely resemble human classical ILC, including its growth rate. Based on a mammary gland transplantation setting using fluorescent labeled hosts, we have shown that fibroblasts are recruited by ILC, both in vivo and in vitro, suggesting ILC-secreted mediators. Interestingly, in vitro inhibition of PDGFRβ (expressed by fibroblasts) can at least in part inhibit this recruitment. We are currently investigating whether inhibition of PDGFRβ in vivo can prevent the recruitment of fibroblasts and subsequently tumor development or progression. In parallel, we have isolated fibroblasts and epithelial cells from primary mouse ILCs and wild-type mammary glands to screen for other important players in the tumor-stroma crosstalk using RNA-seq and secretome proteomics. We will subsequently assess their functional significance during ILC development, progression and therapy response. Citation Format: Mirjam C. Boelens, Ellen Wientjens, Eva Schut, Sjoerd Klarenbeek, Karin E. de Visser, Jos Jonkers. Cancer-associated fibroblasts in invasive lobular breast carcinoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2394. doi:10.1158/1538-7445.AM2015-2394

Invasive lobular carcinoma with tubular elements (ILC-TE) is a recently identified variant of invasive lobular breast carcinoma (ILC). The histology of ILC-TE is defined by noncohesive carcinoma cells mixed with cohesive tubular elements and complete loss of epithelial (E)-cadherin. Cell-cell adhesion is partially restored by switching from an E-cadherin-deficient to a placental (P)-cadherin-proficient status (EPS). The prevalence of ILC-TE remains unknown. Here, we report data from the central pathology review of >4500 hormone receptor-positive/HER2-negative breast cancer (BC) cases recruited to the West German Study Group (WSG) ADAPTcycle trial (NCT04055493). The central pathology review included prospective assessment of BC types, variants, and E-cadherin expression. Cases classified as ILC-TE were analyzed for their molecular features and clinicopathological characteristics. Pure ILC with complete loss of E-cadherin accounted for 630 of 4619 (13.6%) BC cases. ILC-TE accounted for 47 of 630 (7.5%) lobular carcinomas, making it more than twice as prevalent as mixed BC (NST/ILC). ILC-TE harbored deleterious CDH1/E-cadherin mutations in 27 of 35 (77%) cases tested. EPS was detected in 43 of 47 (91%) ILC-TE cases. EPS was significantly more common in ILC-TE than in classic ILC or other ILC variants (P < .001). Clinically, ILC-TE was associated with cT1 stage (P = .023), cN0 status (P = .024), lower histologic grade (P = .004), and lower Ki67 (P = .012). In contrast, solid ILC was associated with higher Ki67 (P = .006). Following preoperative endocrine therapy, higher post-preoperative endocrine therapy Ki67 levels were observed in trabecular ILC, solid-papillary ILC, and pleomorphic ILC (P < .001, P = .006, and P = .021, respectively). In summary, ILC-TE is a quite common ILC variant that is associated with EPS, less-aggressive clinical features, and slow growth.

Cancer-associated fibroblasts (CAFs) can promote breast cancer growth and metastasis, but the underlying mechanisms are still largely unknown. In order to study the role of CAFs in breast cancer development in an in vivo setting, we make use of a spontaneous mammary tumor mouse model for invasive lobular carcinoma (ILC). ILC is the second most common type of breast cancer, and is characterized by loss of E-cadherin, a rich stromal compartment, infiltrative growth, and treatment resistance. Often, the PI3K pathway is observed activated in the ILC subtype. Based on these characteristics, we have generated mouse models with mammary epithelial-specific conditional knockout of E-cadherin and PTEN that develop classical ILCs with a strong resemblance to human ILC. Using PDGFRβ as a fibroblast marker, we have observed an abundant presence of stromal fibroblasts in these ILCs, similar to human ILC. Based on a mammary gland transplantation setting using fluorescent labeled recipient mice, we have shown that fibroblasts get recruited from the host during tumor development. In vitro, we have also shown that tumor cells recruit fibroblasts. Inhibition of PDGFRβ in this in vitro model can at least in part inhibit this recruitment. We are currently investigating whether inhibition of PDGFRβ in vivo can prevent the recruitment of fibroblasts and subsequently tumor development or progression. In parallel, we have isolated fibroblasts from primary mouse ILCs and wild-type mammary glands to screen for other important players in the tumor-stroma crosstalk using RNA-seq and cytokine profiling. We will subsequently assess their functional significance during ILC development and progression. This research is supported by a fellowship of the Dutch Cancer Society Citation Format: Mirjam C. Boelens, Ellen Wientjens, Eva Schut, Sjoerd Klarenbeek, Karin E. de Visser, Jos Jonkers. The role of fibroblasts in invasive lobular breast carcinoma. [abstract]. In: Abstracts: AACR Special Conference on Cellular Heterogeneity in the Tumor Microenvironment; 2014 Feb 26-Mar 1; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2015;75(1 Suppl):Abstract nr B77. doi:10.1158/1538-7445.CHTME14-B77

Background: Invasive lobular carcinoma (ILC) has distinctive clinical and genomic features compared to invasive ductal carcinoma (IDC); however, for patients (pts) with ILC treatment is selected according to the same guidelines as IDC. Better characterization of ILC and development of specific approaches for ILC pts is an unmet need. Liquid biopsy (LB) is a useful tool to achieve this goal. Our group showed that, compared to IDC, ILC has specific circulating tumor DNA (ctDNA) alterations and higher CTC count. Another study reported higher detection of CTC clusters (CTC-CL), considered the main seed of metastasis, in ILC. This finding is paradoxical since cell-cell adhesion (a key feature of CTC-CL) is impaired in ILC. Interestingly, an association between CTC-CL counts and ctDNA CDH1 alterations (a hallmark of ILC) was reported, regardless of histology. To investigate whether different mechanisms are responsible for CTC clustering in ILC and IDC, in this study we characterized CTC-CL according to the breast cancer (BC) histotype. Methods: Blood samples were collected from 351 pts with stage IV BC before starting a new line of therapy at Northwestern University (Chicago, IL) between 2016 and 2021 (NU16B06 trial). Blood samples were processed with the CellSearch system for CTC and CTC-CL enumeration by a single expert operator. CTC-CL were defined as groups of ≥2 CTCs , or ≥1 CTC clustered with ≥2 white blood cells (WBCs). The number and size of CTC-CL, and the presence of WBCs in CTC-CL (heterotypic) were compared between ILC and IDC. Also, the association between CTC-CL and overall survival (OS) was tested. To further explore mechanisms of CTC-CL formations, we assessed potential differences in the association between CTC-CL presence and ctDNA alterations in ILC vs IDC. For ctDNA analysis, matched plasma samples were analyzed using Guardant360 and tested for the 10 most altered genes and CDH1. Results: Of the 351 pts included, 255 (73%) had IDC while 45 (13%) had ILC. Overall, CTC-CL were identified in 45 (13%) pts and only in those with ≥ 5 CTCs. The presence of CTC-CL was significantly higher among ILC pts (27% vs 11% in IDC, p=0.004) but the total number of clustered CTCs was significantly lower in ILC than IDC (median 4.5 vs 9.5, p=0.039), suggesting a smaller size of CTC-CL in ILC. Indeed, the median and maximum number of CTCs per CTC-CL was numerically lower in ILC than IDC. Heterotypic CTC-CL were identified in 6 (50%) and 10 (36%) of ILC and IDC pts, respectively. Among these pts, there was a trend for a higher median (0 vs 2, p=0.37) and maximum number of WBCs (2.5 vs 3.5, p=0.26) in CTC-CL in ILC than IDC. Overall, the presence of &amp;gt;3 CTC-CL was associated with shorter OS (6 vs 21 months, p=0.001). A matched plasma sample for ctDNA analysis was available for 129 IDC and 19 ILC pts. CDH1 alterations were detected in 2 IDC and 1 ILC pts and associated with CTC-CL (p=0.015) only in IDC. No significant association between ctDNA alterations and CTC-CL was observed in ILC possibly due to the small sample size; further analysis is ongoing. Conclusion: ILC is characterized by a higher number of CTC-CL than IDC. CTC-CL in ILC appear to be different from IDC, being smaller but more frequently associated with WBCs. This suggests a possible different biology for CTC-CL formation in ILC related to the impaired cell-cell adhesion and a specific role played by the CTC-CL microenvironment. Indeed, the interaction with immune cells in ILC may promote the survival in the bloodstream of smaller CTC-CL thus enhancing metastatic efficacy. Further studies including a larger number of pts are needed to validate and better elucidate these findings. The study of CTC-CL could shed light on the distinct pattern of metastatic spread of ILC, potentially offering therapeutic opportunities, and serving as a useful prognostic factor. Citation Format: Eleonora Nicolò, Elisabetta Molteni, Lorenzo Foffano, Lorenzo Gerratana, Mara S. Serafini, Letizia Pontolillo, Caterina Gianni, Laura Munoz-Arcos, Nadia Bayou, Kaylan Strickland, Hunter Gaudio, Brenno Pastò, Maroua Manai, Youbin Zhang, Paolo D’Amico, Andrew A. Davis, Jeannine Donahue, Huiping Liu, William J. Gradishar, Giuseppe Curigliano, Carolina Reduzzi, Massimo Cristofanilli. Investigating differences in the composition of circulating tumor cells (CTCs) clusters in invasive lobular and ductal carcinoma to decipher lobular breast cancer metastasis [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P3-09-23.

We aimed to compare the expression of DNA methylation-related proteins in invasive lobular carcinoma (ILC) of breast with those of invasive ductal carcinoma (IDC) of breast and to assess its potential clinical application. Immunohistochemical staining of DNS methylation-related proteins (5-meC, DNMT1, DNMT3B and ISL-1) was applied to tissue microarrays generated from 108 ILCs and 203 IDCs. Protein expression and its correlation with clinicopatholgic variables were statistically analyzed. ISL-1 and DNMT3B were highly expressed in ILC (p<0.001) and tumoral 5-meC was highly expressed in IDC (p=0.006). DNMT1 (p<0.001) showed higher expression rate in luminal A type ILC. ISL-1 and DNMT3B showed higher expression rate in both luminal A type and luminal B type of ILC (p<0.05). In IDC, tumoral 5-meC commonly showed high positive (p=0.039). On univariate analysis, shorter disease free survival of ILC was associated with DNMT1 high positivity (p=0.001) and ISL-1 positivity (p=0.018). DNA methylation-related proteins are differentially expressed in ILC and IDC, and DNMT1, DNMT3B and ISL-1 show high expression rate in ILC.