Esophageal neuroendocrine carcinoma (ENEC) is a rare and aggressive malignancy, characterized by its high-grade, poorly differentiated nature. Despite advancements in understanding neuroendocrine tumors (NETs), ENEC remains an under-researched and poorly understood entity, presenting significant challenges in both diagnosis and treatment. Molecular studies have revealed frequent mutations in TP53 and RB1, along with genetic profiles paralleling small cell lung cancer (SCLC), suggesting shared oncogenic pathways that may guide targeted therapeutic approaches. However, the rarity of ENEC has limited the development of standardized treatment regimens, with most strategies borrowed from other malignancies such as SCLC. Current treatments, including platinum-based chemotherapy, show moderate success, yet response rates remain variable, particularly in advanced stages. The role of immunotherapy and targeted therapies is under investigation, with promising case reports but limited clinical evidence. Emerging diagnostic technologies, such as molecular profiling and liquid biopsy, offer enhanced precision in early detection and personalized treatment plans. Prognostic factors, such as Ki-67 proliferation index, serum biomarkers, and tumor stage, have shown significant associations with survival outcomes, though they remain under-explored in prospective clinical trials. Given the unique molecular features of ENEC, including high tumor mutational burden and distinct immune profiles, further research is essential to develop effective biomarkers, refine prognostic models, and create personalized, targeted therapies. This review aims to consolidate current knowledge on ENEC's molecular biology, diagnosis, and treatment, while identifying gaps in research and suggesting directions for future investigation to improve patient outcomes.

Comprehensive next generation sequencing of middle ear neuroendocrine tumors.

Mammary mucinous cystadenocarcinoma (MCA) is an exceedingly rare subtype of breast carcinoma with fewer than 50 cases reported worldwide and limited molecular characterization. We investigated three mammary MCA cases using integrated histopathological assessment, tumor mutation burden (TMB) analysis and targeted next‑generation sequencing of 275 cancer‑associated genes. All tumors demonstrated characteristic multiloculated cystic architecture with papillary proliferations and abundant extracellular mucin. TMB values were uniformly low, ranging from 1.461 to 4.129 mutations/Mb. Representative pivotal alterations included TP53 (truncating and missense) and RB1 (truncating) mutations in all three cases, activating PIK3CA mutations (p.Q546K, p.H419Y) and oncogenic AKT1 mutations (p.E49K, p.E17K) in two cases, whereas KRAS and TERT alterations were each identified in one case. The expanded targeted sequencing results further revealed additional pathogenic and likely pathogenic variants within TP53, PIK3CA and KRAS, particularly in case #1. Of note, the case #1 PIK3CA p.Q546K alteration was concordantly identified in our prior study. Overall, recurrent TP53 and RB1 involvement, together with frequent PI3K-AKT pathway abnormalities and low TMB, supports a distinctive molecular profile for mammary MCA by comparison to conventional mucinous breast carcinoma. These findings may assist in differential diagnosis when interpreted in conjunction with histomorphology and immunophenotype.

A working group of cIMPACT-NOW evaluated the literature on IDH-mutant gliomas for opportunities to improve pathology-based risk stratification and grading, especially at the interface of CNS WHO grade 2 and 3, since therapeutic decisions depend on risk assessment for clinical management. The group also evaluated newly described IDH-mutant glioma subgroups and considered multidisciplinary aspects of therapeutic decision-making. PDGFRA amplification was associated with highly aggressive clinical behavior of IDH-mutant astrocytomas, consistent with CNS WHO grade 4. Other genetic alterations associated with intermediate risk and most consistent with CNS WHO grade 3 included PIK3 mutations, EGFR amplification, MYCN amplification and specific RB pathway alterations, including CDK4, CDK6 and CCND2 amplification, RB1 homozygous deletion or mutation, and mutation of CDKN2A. DNA methylation signatures of G-CIMP-low or A_IDH_HG were consistent with CNS WHO grade 3 or 4, depending upon other grading criteria. We suggest a mitotic count of ≥ 3 per 2.4 mm2 could be considered as a CNS WHO grade 3 criterion. Our review did not uncover features to improve grading of oligodendroglioma, IDH-mutant and 1p/19q-codeleted, although CNS WHO grade 3 tumors with elevated mitotic rates, yet lacking necrosis, microvascular proliferation, CDKN2A/B homozygous deletion and MRI contrast enhancement, may be associated with extended survival. Implementing evidence-based criteria for risk stratification and grading will improve guidance for clinical decision-making.

High frequency of RB1 mutations p16 positive early stage oropharyngeal carcinoma: A genomic and transcriptomic analysis.

The Bowenoid subtype of cutaneous squamous cell carcinoma (SCC) has been distinguished from conventional SCC for over a century based on its different histopathologic appearance, clinical features, and biologic potential. While there has been extensive molecular analysis of conventional SCC, there has been comparatively little investigation of Bowenoid SCC. Here we show that loss of RB1 protein expression is a defining feature of Bowenoid SCC and reflects biallelic genetic inactivation which most commonly proceeds through mutation of one RB1 allele and copy number loss of the other allele. Neither RB1 protein loss nor RB1 mutations were seen in conventional SCC. A third, but much less common, form of cutaneous SCC is caused by human papillomavirus (HPV). This subtype showed similar morphologic features to Bowenoid SCC and also showed loss of RB1 protein expression. However, these tumors lacked RB1 mutation and likely inactivate RB1 through HPV's known capacity to promote post-translational degradation of RB1 protein. These data suggest that the two most common subtypes of cutaneous SCC proceed through distinct pathways of molecular pathogenesis and highlight an unexpected relationship between Bowenoid and HPV-associated cutaneous SCC.

Background: Mutations in the retinoblastoma gene (RB1) are associated with risks of both retinoblastoma and other cancers. Parents of children with retinoblastoma can be categorised according to their likelihood of carrying a germline RB1 mutation. Some categories of such parents may have an increased risk of cancer. Methods: A cohort of 1180 parents of children with retinoblastoma were categorised according to the likelihood that they carried an RB1 mutation and followed up for cancer through national records. We calculated Standardised Incidence Ratios (SIRs) for all non-ocular cancers combined and for individual diagnostic groups, with expected numbers derived from national cancer registration rates. Finally, we pooled the all-cancers results from the present study with those from an earlier study of largely the same cohort with non-overlapping follow-up. Results: In total, 183 non-ocular cancers were identified among the parents. Parents who themselves had retinoblastoma had a significantly higher risk of non-ocular cancer than the general population: for fathers, the SIR was 3.56 (95% confidence interval (CI) 1.84-6.22); for mothers, it was 3.25 (1.49-6.18). For the very small group of parents known to be carrying a germline RB1 mutation but not affected by retinoblastoma, there was a lower and non-significant increase in risk (SIR = 1.9). Parents categorised as either possible carriers or probable non-carriers had similar observed risks to the general population. When the all-cancers results were pooled with those from an earlier study of very largely the same cohort with non-overlapping follow-up, the estimated lifetime SIR for non-ocular cancer among mutation-carrier parents after the birth of their affected child was 4.32 (95% CI 3.06-5.93). Conclusions: Our results confirm that parents who themselves had retinoblastoma have an increased risk of subsequent cancers, and parents who are not mutation carriers have a risk similar to the general population.

Summary Metastatic luminal breast cancer remains an incurable disease; however, survival outcomes have improved substantially in recent years due to the development of novel endocrine-based and targeted therapeutic strategies aimed at overcoming endocrine resistance. Nevertheless, therapeutic decision-making after disease progression on CDK4/6 inhibition represents a major clinical challenge. Treatment selection after progression is guided by multiple factors, including prior treatment duration, tumor biology, metastatic pattern, molecular alterations, and patient preferences. The clinical definitions of primary and secondary endocrine resistance remain useful in daily practice and are increasingly complemented by biomarker-driven concepts, such as the detection of acquired ESR1 mutations. Alongside ESR1 mutations, alterations in the PI3K/AKT/mTOR pathway, germline BRCA 1/2 or PALB2 mutations, changes in estrogen receptor expression, emergence of RB1 mutations, and intra-tumoral heterogeneity contribute to endocrine and CDK4/6 inhibitor resistance and inform subsequent treatment strategies. Recent clinical trials have expanded therapeutic options across different molecular subgroups, including oral selective estrogen receptor degraders, PI3K/AKT inhibitors, PARP inhibitors, and antibody–drug conjugates. Combination strategies and biomarker-guided treatment adaptations are emerging as promising approaches to improve disease control, particularly in patients with endocrine-resistant disease. However, in a substantial proportion of patients, no actionable molecular alteration can be identified, highlighting the continued need for individualized treatment strategies. This review summarizes current evidence and provides a clinically oriented overview of treatment sequencing in metastatic luminal breast cancer, highlighting the importance of integrating clinical characteristics, molecular profiling, and patient preferences in an increasingly complex therapeutic landscape.

MYCN amplification occurs in a subset of retinoblastoma cases, both with and without RB1 inactivation. It has been suggested that retinoblastomas with MYCN amplification represent a distinct entity with more aggressive clinical behavior. We examined the incidence of MYCN gain/amplification and RB1 inactivation in 192 unilateral retinoblastoma samples from children enucleated between 2011 and 2018 at the German reference center. MYCN copy number was assessed using quantitative PCRand confirmed by single nucleotide polymorphism microarray analysis. Clinical characteristics, RB1 mutation status, and histopathological features were compared between MYCN-amplified and nonamplified retinoblastomas. MYCN gain/amplification was found in 10 of 139 retinoblastomas included in the final analysis (7.2%). All 10 tumors exhibited alterations in at least one RB1 allele (RB1-/- or RB1+/-). TheRB1mutation spectrum and overall genomic copy number changes were similar between MYCN-amplified and MYCN-nonamplified retinoblastomas. Age at diagnosis did not differ significantly between the two groups (p=0.21); however, secondary glaucoma, massive choroidal, and scleral invasion occurred more often in MYCN-amplified retinoblastomas (p=0.038, p=0.03, and p=0.04, respectively). No cases of extraocular retinoblastoma or distant metastasis were observed during a median follow-up of 50 months. MYCN gain/amplification was identified in 7.2% of enucleated unilateral retinoblastomas, all of which showedRB1inactivation. MYCN amplification was associated with more advanced disease and more aggressive clinical and histopathological features.

Uterine leiomyosarcoma (uLMS) is a rare but aggressive malignant mesenchymal tumor, accounting for 2-5% of uterine malignancies. Because its symptoms and imaging features often resemble those of benign uterine leiomyoma (LM), accurate preoperative diagnosis remain difficult. This review summarizes recent advances in liquid biopsy for uLMS and explores its potential for early detection, molecular characterization, and treatment monitoring. Liquid biopsy enables minimally invasive detection of tumor-derived components such as circulating tumor DNA (ctDNA), circulating tumor cells (CTCs), non-coding RNAs, and extracellular vesicles (EVs). Recurrent mutations in TP53, RB1, and ATRX have been identified through ctDNA analysis, while CTCs, ncRNAs, and EVs provide complementary information for monitoring tumor dynamics and therapeutic response. Emerging technologies including CRISPR-Cas systems, nanotechnology, electrochemical biosensors, and multi-omics integration enhance detection sensitivity and specificity. Liquid biopsy holds promise for improving uLMS diagnosis and management. However, standardization and biomarker validation remain essential to achieve reliable clinical translation and enable earlier, more precise treatment strategies.

PURPOSEAdvanced esophageal squamous cell carcinoma (ESCC) has a poor prognosis, and current treatments provide limited survival benefits. This study aimed to identify prognostic biomarkers and therapeutic targets by genomic profiling of advanced ESCC using circulating tumor DNA (ctDNA).METHODSThe SCRUM-MONSTAR GOZILA study is a nationwide, plasma-based molecular profiling project using Guardant360, involving 31 core cancer institutions in Japan. We evaluated the genomic landscape of advanced ESCC and investigated associations between specific alterations and overall survival (OS). The correlation between blood tumor mutation burden (bTMB) and clinical outcomes in patients with PD-1 inhibitors was also assessed using multiple cutoff values (2, 4, 6, 8, and 10 mutations/Mb).RESULTSAmong 313 patients, alterations predominantly consisted of single nucleotide variants (SNVs, 68.9%) and copy number alterations (20.7%). TP53 was the most frequent alterations (88.5%), followed by PIK3CA (36.4%), NFE2L2 (24.3%), CCND1 (22.4%), and EGFR (20.1%). Amplifications in PIK3CA, FGFR1, CCND1, and EGFR, as well as mutations in NFE2L2, FGFR1, and RB1 were associated with worse OS (any P < .05). Multivariable analysis confirmed mutations in NFE2L2 or FGFR1 and amplifications in EGFR or PIK3CA as independent poor prognostic factors (all P < .05). In 142 patients treated with PD-1 inhibitors, no significant differences in objective response rate or progression-free survival were observed at different bTMB cutoff values (all P > .1).CONCLUSIONctDNA analysis identified key genomic alterations linked to poor outcomes in advanced ESCC, revealing potential prognostic biomarkers and therapeutic targets. In contrast, bTMB did not show predictive value for the efficacy of PD-1 inhibitors in this study.

The RB1 gene is a tumor suppressor whose primary function is to regulate the cell cycle through the retinoblastoma protein (pRB). RB1 mutations are the primary cause of retinoblastoma. Beyond retinoblastoma, RB1 inactivation is associated with many other human cancers, including glioblastoma, multiple myeloma, osteosarcoma, melanoma, small cell lung cancer, bladder, prostate, breast, and endometrial cancers. Mutations in RB1 disrupt the G1 to S phase checkpoint, promoting uncontrolled cell proliferation and genomic instability. Advanced studies highlight the role of RB1 in immune evasion and resistance to therapy, particularly in prostate and breast cancers. RB1 gene loss in small cell lung cancer and triple-negative breast cancer represents a key molecular feature that influences treatment resistance and prognosis. RB1 status serves as a predictive biomarker in clinical oncology and guides the development of targeted therapies, especially cyclin-dependent kinase 4/6 inhibitors. Understanding RB1 is crucial for advancing personalized cancer treatment and improving patient outcomes across different malignancies.

BackgroundProstate cancer (PC) is the most common cancer in men in the United States with a 5-year relative survival rate for people with distant metastases of 36%. We conducted a single institution, retrospective cohort study of patients with metastatic PC to investigate whether certain gene mutations can be used as predictive biomarkers.Methods200 patients with metastatic hormone sensitive (mHSPC) and castration resistant (mCRPC) prostate cancer who had a FoundationOne report and were treated from June 2007 to October 2024 were included in the study. Disease progression was evaluated according to RECIST criteria, PCWG3 criteria, and PSA values. Assessed gene mutations included SPOP, p53, Rb, PTEN, and HRR genes. Overall survival (OS) and progression-free survival (PFS) were calculated for mHSPC and mCRPC.ResultsAmong 200 patients, there were 182 patients with mHSPC and 174 patients with mCRPC. Average age at diagnosis of metastatic disease was 71.5, ECOG was 0.76, and median PSA was 75.6 ng/mL. 152 patients had high-volume disease. 102 patients passed away. Patients with a p53 mutation (n=99) had lower OS in mHSPC (50.7 months vs 86.2 months, p<0.01). Patients with a PTEN mutation (n=50) had a lower OS in mHSPC (50.6 months vs 65.8 months, p=0.03) and mCRPC (29.5 months vs 46.0 months, p=0.04). Patients with HRR mutations (n=43) had lower OS in mHSPC (41.4 months vs 64.6 months, p<0.01) and mCRPC (18.4 months vs 42.8 months, p=0.04). p53 and PTEN mutations were associated with shorter PFS in mCRPC (13.8 months vs 23.2 months, p=0.03; 12.2 months vs 22.6 months, p<0.01, respectively). Mutations in SPOP (n=13 mHSPC, n=9 mCRPC) and RB1 (n=12 mHSPC, n=10 mCRPC) were not associated with statistically significant differences in OS and PFS.ConclusionsPTEN, and HRR mutations were associated with shorter OS in both mHSPC and mCRPC. p53 mutations are associated with shorter OS only in mHSPC. p53 and PTEN mutations were associated with shorter PFS only in mCRPC.

PURPOSE Next-generation sequencing (NGS) provides an opportunity to leverage genomic data to identify novel factors associated with treatment outcomes. We examined outcomes of systemic therapy in patients with soft tissue (nonuterine) leiomyosarcoma (LMS) who had NGS at our center to identify associations of those NGS findings with radiologic outcomes. METHODS We reviewed 168 patients with soft tissue LMS who received systemic therapy between 2009 and 2024 for metastatic LMS and also had Memorial Sloan-Kettering Integrated Mutation Profiling of Actionable Cancer Targets profiling. We conducted univariate and multivariate Cox regression analyses of clinicopathologic factors and genomic alterations for progression-free survival (PFS) for first- and second-line systemic therapy. RESULTS There were 110 female and 58 male patients in this cohort. Patients received 1-12 lines of systemic therapy for metastatic disease. Treatment regimens were categorized as (1) doxorubicin-based, (2) pegylated liposomal doxorubicin (PLD)–based, (3) gemcitabine-based, (4) pazopanib, and (5) other. On univariate analysis, inferior PFS for first-line chemotherapy was associated with high mitotic activity, necrosis, and TP53 or RB1 mutation. On multivariate analysis, only the therapeutic class was associated with PFS, with statistically inferior outcomes for PLD-containing combinations across the full cohort. Tumor size &gt;10 cm, high mitotic rate, and TP53 alterations were associated with inferior PFS in the PLD-based treatment subgroup. High mitotic rate was associated with inferior PFS in the gemcitabine-based treatment subgroup. CONCLUSION Both doxorubicin- and gemcitabine-based therapy showed significantly greater activity than PLD-based therapy. NGS results were not predictive for outcomes with first- or second-line treatment for metastatic soft tissue LMS. These data may merit re-examination when data are accumulated regarding the increasing use of doxorubicin-trabectedin in this population.

294P Evaluating malignancy risk in soft tissue FNAs: Insights from the WHO reporting framework

293P Impact of diagnostic delay in rare cancers: Experiences and perspectives of patients and healthcare professionals

Genomic alterations are common in metastatic castration-resistant prostate cancer (mCRPC), but limited data exist on the response to 177Lu-PSMA-617 (LuPSMA) in patients with these aberrations. We aimed to characterize oncologic outcomes of patients with mCRPC and germline or somatic aberrations after treatment with LuPSMA. Methods: The medical record was surveyed for all patients with mCRPC treated with LuPSMA between October 2022 and October 2024. All patients who had received at least 1 cycle of LuPSMA and underwent either germline or somatic testing were included. Results: Seventy-two patients were included. Patients with TP53/PTEN/RB1 mutations demonstrated inferior overall survival, even after adjustment for age and race. TP53/PTEN/RB1, BRCA1/2, and CHEK2/PALB2/ATM were not associated with inferior progression-free survival. No individual mutation was significantly associated with changes in the percentage decline in prostate-specific antigen levels from baseline. Conclusion: TP53, PTEN, and RB1 mutations were linked to inferior overall survival in LuPSMA-treated patients and may serve as prognostic biomarkers. Prospective validation is required to establish their predictive value.

Abstract Background: The tumor-suppressor gene RB1 acts as a master regulator of the cell cycle by controlling the critical G1-S transition and also modulates hormone-receptor signaling and tumor immunogenicity. However, whether the loss of RB1 uniformly influences patient prognosis across different metastatic breast cancer subtypes has been unclear. Therefore, this study aimed to contrast the clinical impact of RB1 mutations in metastatic hormone-receptor-positive/HER2-negative (HR+/HER2-) and triple-negative breast cancer (TNBC). Methods: This observational study analyzed data from the Flatiron Health-Foundation Medicine clinicogenomics database (∼280 US cancer clinics). Adults ≥18 years with metastatic HR+/HER2- and TNBC breast cancer who underwent genomic profiling between 90 days before metastatic diagnosis and 90 days after first-line therapy initiation (January 2015-September 2022) were included. Patients (pts) were excluded if they had participated in a clinical trial or had a follow-up period of less than six months. Demographic and clinical characteristics were summarized using descriptive statistics, while overall survival (OS) and time-to-treatment-discontinuation (TTD), were evaluated using the Kaplan-Meier method and Cox regression analyses. Results: We analyzed cohorts of 286 TNBC pts and 1,713 HR+/HER2- pts, stratified by RB1 mutation status. In the TNBC cohort, RB1-mutated pts comprised 16% (n = 46) of the cohort. The median age at metastatic diagnosis was younger for pts with RB1-mutant TNBC at 54 years compared to 61 years for those with wild-type tumors. Additionally, all RB1-mutant tumors exhibited TP53 co-mutation. Median OS in the TNBC cohort was similar for RB1-mutant versus wild-type disease (18.9 vs 12.9 mo; log-rank = 0.7, adjusted HR for wild-type vs mutant(aHR) = 1.1, 95% CI: 0.7-1.7). In patients treated in the first -line (1L) setting with chemotherapy, the median OS (14.2 vs 11.5 mo; log-rank= 0.7, aHR 1.2, 95% CI 0.6-2.3) and TTD (4.6 vs 3.6 mo; log-rank = 0.4, a HR 1.2, 95% CI 0.7-2.1) were also similar. Patients with RB1 mutations who received anti-PD-1/PD-L1 immunotherapy in the first-line setting had comparable median OS (12.3 vs 12.6 months; log-rank p = 0.6, aHR ≈ 1.1, 95% CI: 0.4-2.2) and a trend toward worse TTD (2.8 vs 3.6 months; log-rank = 0.1, aHR ≈ 1.3, 95% CI: 0.6-2.8). In HR+/HER2- patients, RB1 mutations occurred in 5% of the pts (n=83). RB1-mutated patients were slightly older with a median age of 62 years versus 61 years for wild-type tumors. RB1-mutant tumors exhibited greater enrichment for endocrine-resistance drivers including PTEN, TP53, and ESR1. Median OS was significantly shorter in RB1-mutant pts (21.4 vs 40.1 mo; log-rank p &amp;lt; 0.01, aHR for wild-type vs mutant = 0.8, 95% CI: 0.5-1.2). RB1 loss also shortened TTD (4.3 vs 8.3 mo; log-rank &amp;lt;0.01; aHR = 0.8, 95% CI: 0.6-1.1). In the CDK4/6 inhibitor treatment cohort, RB1 deficiency was associated with significantly shorter median TTD (10.9 vs 13.7 mo; log-rank = 0.03, aHR = 0.9, 95% CI: 0.61-1.26), suggesting potential therapeutic resistance. Conclusions: This real-world analysis suggests that the clinical impact of RB1 mutations may be subtype dependent. In TNBC, where RB1 mutations were more common (16% vs 5%), RB1 loss had no meaningful impact on chemotherapy or immunotherapy effectiveness or overall survival. In HR+/HER2- breast cancer, RB1 mutations represent a marker of poor prognosis, with shorter OS overall and TTD with CDK4/6 inhibitors, where RB1 loss may contribute to therapeutic resistance. The co-occurrence with other resistance drivers including PTEN and ESR1 further compounds treatment resistance. These findings potentially have implications for precision medicine approaches, with RB1 mutation status potentially guiding therapeutic strategies in HR+/HER2- disease. Citation Format: P. Farrokhi, D. Stenehjem. Impact of RB1 Mutations on Clinical Outcomes in Metastatic Breast Cancer: A Real-World Subtype-Dependent Survival Analysis [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 PS2-06-25.

Abstract Background: Patients (pts) with ER+/HER2- MBC often develop ESR1m during or after 1L treatment with ET with or without a CDK4/6i. Reported ESR1m prevalence rates vary, ranging from 20% to 50% after ≥1 line of therapy in the metastatic setting. Here, we explore the real-world prevalence of ESR1m and co-occurring mutations after 1L ET and/or CDK4/6i among pts with MBC in the GuardantINFORM database. Methods: This was a retrospective cohort study of adults in the United States with ER+/HER2- MBC and a Guardant360 comprehensive genomic profiling blood test of cancer-related gene mutations in circulating tumor DNA (ctDNA) between Jan 1, 2014 and Aug 31, 2024. Eligible pts had received any of the following 1L treatments within 6 months of diagnosis of metastatic disease: ET monotherapy (aromatase inhibitor [AI], selective ER modulator [SERM], or selective ER degrader [SERD]), CDK4/6i monotherapy, or ET (AI/SERM and/or SERD) + CDK4/6i combination therapy. Pts must have subsequently discontinued 1L therapy and received second-line (2L) treatment with follow-up &amp;gt;3 months. Pts were considered ESR1 evaluable if they had an ESR1 test ≤90 days before initiating 2L therapy (to rule out false negatives) or had a positive ESR1m result at any time before starting 2L therapy. ESR1m positivity was defined as missense mutations in codons 310-547. The proportion of ESR1-evaluable pts who were ESR1m positive was stratified by the type and duration of 1L treatment. The incidence of co-occurring AKT1 E17K, PIK3CA, PTEN, CCND1, ERBB2, FGFR1, KRAS, MYC, NF1, and RB1 mutations was analyzed. Results: Of 8335 adults with MBC treated with 1L ET and/or a CDK4/6i who had a Guardant 360 test, 4790 went on to receive 2L treatment with sufficient follow-up, and 1511 were ESR1 evaluable. Of these, 686 pts (45%) were ESR1m-positive, with the highest rates of ESR1m positivity among pts who received 1L SERD monotherapy or SERD + AI/SERM, and those who received 1L treatment for 12 to &amp;lt;24 months (Table). Among pts with ESR1m, 68% (466/686) had ≥1 other co-occurring mutation at start of 2L therapy, most commonly in genes involved in the PI3K/AKT signaling pathway (AKT1 E17K, PIK3CA, or PTEN; 50% [341/686]). Conclusions: In this real-world analysis of the GuardantINFORM database, 45% of pts with ER+/HER2- MBC were ESR1m-positive following 1L treatment with ET and/or CDK4/6i. The ESR1m positivity rate remained &amp;gt;40% in most subgroups analyzed according to 1L therapy type and 1L treatment duration. Other mutations co-occurred in &amp;gt;65% of pts with ESR1m. Citation Format: D. Chandiwana, D. Benjumea, K. Greco, C. Grace Rose, S. Stergiopoulos, J. Liao, M. Edwards. Real-world prevalence of ESR1 mutations (ESR1m) among patients with estrogen receptor (ER)-positive/human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer (MBC) after first-line (1L) treatment with endocrine therapy (ET) and/or a cyclin dependent kinase 4/6 inhibitor (CDK4/6i) [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 PS1-11-09.

This study investigated autoimmune diseases (ADs) clinical characteristics, their association with B-cell lymphoma (BCL), and related frequent genetic mutations. A retrospective review of 2,040 BCL patients at Henan Provincial People’s Hospital (2016–2022) identified 98 with concurrent ADs. Hashimoto’s thyroiditis was the most common AD identified, followed by Sjögren’s syndrome, rheumatoid arthritis, and autoimmune hemolytic anemia. The median interval from initial AD diagnosis to BCL development was 9 years, varying significantly across AD subtypes (P = 0.038). Marginal zone lymphoma (MZL) were more frequent in patients with Sjögren’s syndrome (P < 0.001) and inflammatory bowel disease (P = 0.03). BCL with concurrent ADs occurred more frequently in female patients (P = 0.009). These patients also exhibited higher levels of β2-microglobulin ≥ 2.5 mg/L (P = 0.001), demonstrated more frequent bone marrow infiltration (P = 0.017), and demonstrated a higher prevalence of gastrointestinal tract involvement (P = 0.033). However, the presence of concurrent ADs did not significantly affect overall BCL patient prognosis (P = 0.188). The next-generation sequencing (NGS) results indicate that the top four mutated genes identified in the AD-associated DLBCL cohort are B2M, BIRC3, RB1, and PLCG2. In the DLBCL cohort, the predominant mutated genes are KMT2C, EGR1, BCL2, and DDX3X. AD-associated BCL predominantly affects females, with more common bone marrow involvement. Hashimoto’s thyroiditis is the most prevalent AD type, with a 9-year median interval from AD diagnosis to BCL development. Concurrent ADs do not significantly impact BCL survival. Mutations in BIRC3, RB1, B2M, PLCG2, and EP300 are present in patients with AD-associated DLBCL.