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569 Background: Several studies established the prognostic role of both the amount and locations of tumor-infiltrating lymphocytes (TILs) in TNBC. Three distinct immunotypes were described based on the amount and locations of TILs: immune enriched (IN), immune excluded, and immune desert. Using single-cell spatial transcriptomic analysis in the Mayo Clinic TNBC cohort, our previous studies showed the central role of interferon (IFN) signaling in IN phenotype. Herein, we evaluated the association between IFN and outcomes in TNBC in 3 independent datasets. Methods: NanoString IO360 was performed in 114 samples from FinXX (NCT00114816) to generate 22-gene IFNα and 33-gene IFNγ signatures. RNA sequencing was performed in 388 samples from CALGB 40603 (NCT00861705). 3038 TNBC samples were tested by WTS (NovaSeq; Caris Life Sciences, Phoenix, AZ). Median values were used as cut-offs for high vs low IFNγ RNA expression and 18-gene IFNγ signature scores. Caris Life Science CODEai was used to evaluate real-world overall survival (OS) obtained from insurance claims and calculated from tissue collection to last contact using Kaplan-Meier estimates. Chi-square, Mann-Whitney U, ANOVA, and Cox regression were used. Results: A high 22-gene IFNα signature score was associated with significantly improved recurrence-free survival (RFS) in FinXX (hazard ratio [HR] 0.32, 95% confidence interval [CI] 0.14-0.74, p 0.007) and OS (HR 0.28, 95%CI 0.12-0.66, p 0.003). Similar findings were observed with 33-gene IFNγ signature with significant improvement in RFS (HR 0.21, 95%CI 0.09-0.51, p < 0.001) and OS (HR 0.18, 95%CI 0.08-0.44, p < 0.001). Furthermore, in CALGB 40603, both IFNα and IFNγ scores were positively associated with pathologic complete response (pCR: IFNα p 0.019 and IFNγ p 0.007) and residual cancer burden (RCB: IFNα p 0.044 and IFNγ p 0.013). Using the Caris data platform to further validate, we identified 2899 TNBC patients (pts) with genomic and clinical outcome data. High IFNγ expression was associated with significant improvement in OS (25.95 vs 17.43 months; HR 0.65, 95% CI 0.59 – 0.72, p < 0.001). Similarly, pts with high IFNγ signature scores had significant improvement in median OS (25.79 vs 16.22 months; HR 0.66, 95% CI 0.6 – 0.73, p < 0.001). Conclusions: This study underscores the pivotal role of IFN signaling in TNBC. High IFNα and IFNγ signatures were consistently associated with improved RFS, OS, higher pCR rates, and lower RCB across clinical trial cohorts and real-world data. These findings signify IFN signaling as a potential key biomarker and therapeutic target in TNBC. Support: U10CA180821, U10CA180882, U24CA196171; Breast Cancer Research Foundation, Mayo Clinic Breast Cancer SPORE (P50CA116201-17), Bankhead Coley, W81XWH-15-1-0292, P50CA015083, R35CA253187; https://acknowledgments.alliancefound.org . Genentech. Clinical trial information: NCT00114816 and NCT00861705 .

561 Background: Obesity and other lifestyle factors are associated with breast cancer (BC) risk and outcomes. These relationships appear to vary in pre- vs postmenopausal women. Here we explore associations between pathological features, diet quality and physical activity (PA) in patients (pts) with BC enrolled in the BWEL (Alliance for Clinical Trials in Oncology A011401) trial, stratifying for menopausal status. Methods: BWEL is a phase III trial evaluating the impact of a weight loss intervention on disease outcomes in 3180 pts with stage II-III HER2– BC and body mass index ≥27 kg/m 2 . The first 542 BWEL pts underwent assessment of PA (7-Day PA Recall) and diet (24-hour Diet Recall) at enrollment. Healthy Eating Index 2020 (HEI) score (0-100, higher value = healthier diet) and minutes/week (min/wk) of moderate/vigorous PA (MVPA) were calculated. Estrogen receptor (ER) and progesterone receptor (PR) expression level (%) were abstracted from pathology reports. Analyses of links between HEI score, MVPA min/wk, % ER/PR, stratified by menopausal status, were conducted using t-tests; differential associations by menopausal status used linear regression models with interaction term. Results: In 523 pts with available pathology data, 76.5% had ER/PR+ BC, 56.2% were postmenopausal. Median HEI score was 50.1 (range 18.1-96.4), median MVPA min/wk was 0 (range 0-630), median time from diagnosis to enrollment was 10.0 months (range 2.4-13.1). In postmenopausal pts, higher diet quality was linked to increased % ER and PR (HEI above vs below median, mean % ER 78.0 vs 66.1, p=0.012; mean % PR 57.7 vs 43.1, p=0.004). The relationship between HEI score and % ER/PR differed significantly by menopausal status (interaction term of menopausal status and % ER: p=0.006; and % PR: p=0.012); there were no significant associations between HEI scores and % ER/PR in premenopausal pts. There were no significant associations between PA and % ER/PR. Conclusions: Healthier diet was associated with higher % ER and PR in postmenopausal pts with BC, but no relationship was seen in premenopausal pts. These data suggest that lifestyle factors may influence BC pathologic features related to outcomes in older women. Support: U10CA180821, U10CA180882, UG1CA189823, U24CA196171, https://acknowledgments.alliancefound.org . Clinical trial information: NCT02750826 . % ER mean (SD) p value % PR mean (SD) p value Post-Menopausal HEI* score≤50.1 [n=134]>50.1 [n=156] 66.1 (43.3)78.0 (36.8) 0.012 43.1 (41.8)57.7 (41.0) 0.004 MVPA † min/wk0 [n=147]≥1 [n=147] 74.5 (39.3)70.3 (41.2) 0.37 50.0 (41.8)53.0 (42.0) 0.41 Pre-Menopausal HEI* score≤50.1 [n=124]>50.1 [n=103] 70.5 (38.5)65.5 (43.0) 0.15 53.5 (41.1)49.1 (43.0) 0.43 MVPA † min/wk0 [n=112]≥1 [n=113] 66.3 (41.0)66.3 (41.2) 0.99 49.5 (41.4)52.6 (42.7) 0.59 *Healthy Eating Index. † Moderate/Vigorous Physical Activity.

e12562 Background: Previous research has suggested that breast cancer (BC) diagnosed during pregnancy or the postpartum period exhibits more aggressive clinical features, possibly due to pregnancy-related hormonal changes, immune modulation, and alterations in the mammary microenvironment compared to nulligravid or women many years out from pregnancy. Emerging data have demonstrated potential worse outcomes for patients (pts) diagnosed in post-partum. This study evaluated the influence of timing of diagnosis in relation to pregnancy history and recency of childbirth on long-term outcomes in a modern cohort of young pts with early-stage BC. Methods: Pts with stage I–III BC age < 40y in the prospective Young Women’s Breast Cancer Study (YWBCS) were categorized at diagnosis as Nulligravid (no prior pregnancy), Nulliparous (prior pregnancy without live birth), Currently Pregnant, or Parous (subdivided by time since last childbirth: within 5 years [≤ 5y PP], and 5–10 years postpartum [5–10y PP]). Baseline characteristics and outcomes were compared across BC subtypes (estrogen receptor+/HER2- [ER+/HER2-], HER2+, and triple-negative BC [TNBC]). Prognostic analyses for distant recurrence-free survival (DRFS) used Cox models. Results: 859 pts were included, 257 (30%) nulligravid, 50 (6%) nulliparous, 37 (4%) pregnant, and 515 (60%) parous (348 [68%] were ≤5y PP, and 167 [32%] were 5–10y PP). Median follow-up was 133 (6–209) months. Median age at diagnosis was lower in nulligravid pts (33 vs. 35–38 years); T1 tumors were most common in pts >5y PP (62%), and N0 status was highest in nulligravid pts (64%) (all p < 0.0001). Pregnant pts had higher rates of TNBC (27%), and T3 tumors (14%), N1 status, N1 (41%), and Grade 3 (81%) tumors compared to other groups (all p < 0.0001). Overall, only tumor size (T2-T4 vs T1, HR 2.03; 95% CI 1.37–3.03) and nodal status (N1 vs N0, HR 2.22; 1.48–3.34; N2/3 vs N0, HR 3.23; 1.96-5.33) were independently associated with worse DRFS; pregnancy history was not: nulligravid + nulliparous pts had no significant differences in DRFS compared to Pregnant (HR 1.46; 0.75–2.86), Parous <5y PP (HR 0.86; 0.58–1.26), and Parous 5–10y PP (HR 0.89; 0.54–1.48). Subgroup analyses by BC subtype (Table) and sensitivity analyses (excluding currently pregnant pts or separating nulligravid and nulliparous pts) demonstrated no significant associations between pregnancy history and DRFS. Conclusions: In this cohort study of very young patients with early breast cancer with contemporary treatment including management of breast cancer during pregnancy, pregnancy history and recency of pregnancy were not independent predictors of long-term disease outcomes. DRFS results by subgroup. Pregnancy Group N (%) HR (95% CI) ER+/HER2- Nulligravid + Nulliparous 173 (38.1) Ref Pregnant 16 (3.5) 0.93 (0.32–2.70) Parous ≤5y PP 177 (39.0) 0.77 (0.45–1.29) Parous 5–10y PP 88 (19.4) 0.78 (0.39–1.53) HER2+ Nulligravid + Nulliparous 77 (30.8) Ref Pregnant 11 (4.4) 3.27 (0.80–13.38) Parous ≤5y PP 113 (45.2) 1.50 (0.60–3.75) Parous 5–10y PP 49 (19.6) 1.45 (0.45–4.66) TNBC Nulligravid + Nulliparous 56 (36.8) Ref Pregnant 10 (6.6) 1.92 (0.61–6.09) Parous ≤5y PP 57 (37.5) 0.68 (0.28–1.63) Parous 5–10y PP 29 (19.1) 1.16 (0.43–3.14)

501 Background: EA1181 (NCT04266249) is a single-arm trial of neoadjuvant THP for patients with clinical anatomic stage II/III HER2+ breast cancer; patients with cT4 or cN3 disease were excluded . Assessing the primary endpoint, 3-year recurrence-free survival in patients with a pCR (ypT0/Tis, ypN0), requires longer follow-up. Here, we present results for the secondary objective of pCR rate and its relation to clinicopathologic factors and the HER2DX pCR likelihood score (Reveal Genomics) derived from gene expression and clinical features. Methods: Patients received 4 cycles of trastuzumab and pertuzumab (HP) with weekly paclitaxel (12 weeks) or docetaxel (q3w x 4), followed by surgery. Clinicopathologic features were assessed for all patients and HER2DX pCR score (stratified by ER status) was determined using the diagnostic biopsy in a representative subset of study participants. Results: 2175 patients were enrolled. Median age was 55 years (range 22-88 years); 58% had clinical stage IIA, 33% stage IIB, and 9% stage III. 45% had nodal involvement (mostly cN1). 781 tumors were HER2+/ER- and 1394 HER2+/ER+ (locally tested ). 2141 patients started THP, for whom the pCR rate was 44% overall, 63.7% in HER2+/ER- and 32.4% in HER2+/ER+ tumors. Disease progressed during THP in 16 patients (0.7%). The pCR rate varied inversely to the proportion of cells staining for ER among patients with HER2+/ER+ breast cancer: 1-10%+, 62.5%; 11-69%, 51.6%; ≥70%, 22.5% (p <0.001). The pCR rate was significantly associated with higher grade, especially in HER2+/ER+ disease. T and N stage did not significantly affect pCR rate. Among 569 patients assessed for the HER2DX pCR score, the pCR rate was significantly greater for patients with a higher vs lower score, regardless of ER status (Table). Further correlations and interactions will be presented. Conclusions: Neoadjuvant THP resulted in pCR in nearly two-thirds of pts with clinical stage II/III HER2+/ER- and in one-third with HER2+/ER+ breast cancer. There was no association with clinical stage. Lower ER expression and higher grade were associated with higher pCR rates. The HER2DX pCR score was a significant predictor of pCR, regardless of ER status. Clinical trial information: NCT04266249 . All Participants HER2+/ER- HER2+/ER+ # patients enrolled pCR rate (95% CI) # patients enrolled pCR rate (95% CI) # patients enrolled pCR rate (95% CI) Evaluable Cohort 2141 44%(42-46%) 774 64%(60-67%) 1367 32%(30-35%) HER2DX Cohort 569 48%(44- 52%) 230 69% (63-75%) 339 34%(29-39%) HER2DX pCR-high score 182 (32%) 68%(60-74%) 147 (64%) 70% (62-77%) 36 (11%) 58%(41-74%) HER2DX pCR-medium score 161 (28%) 67% (59- 74%) 70 (30%) 74%(62- 84%) 89 (26%) 61%(50-71%) HER2DX pCR-low score 226 (40%) 19%(14- 25%) 13 (6%) 31%(9-61%) 214 (63%) 18%(13-24%) P-value HER2DX score <0.001 0.010 <0.001

PURPOSE To investigate the clinical behavior of breast cancer in young BRCA carriers according to the specific BRCA gene ( BRCA1 v BRCA2 ) and the association of the timing of genetic testing (before v at diagnosis) with prognosis. METHODS This was an international, multicenter, hospital-based, retrospective cohort study that included 4,752 patients harboring germline pathogenic/likely pathogenic variants (PVs) in BRCA1 or BRCA2 , who were diagnosed with stage I-III invasive breast cancer at 40 years or younger between January 2000 and December 2020 in 78 centers worldwide (ClinicalTrials.gov identifier: NCT03673306 ). RESULTS Compared with BRCA2 carriers (n = 1,683), BRCA1 carriers (n = 3,069) had more frequently hormone receptor–negative (74.4% v 15.5%) and high-grade (77.5% v 49.1%) tumors. Similar outcomes were observed in BRCA1 and BRCA2 carriers but with a different pattern and risk of disease-free survival events over time. Compared with patients tested for BRCA at diagnosis (ie, between 2 months before and up to 6 months after diagnosis; n = 1,671), those tested before diagnosis (ie, any time up to 2 months before diagnosis; n = 411) had smaller tumors (T1: 61.3% v 32.4%), less nodal involvement (N0: 65.9% v 50.8%), less frequently received chemotherapy (84.4% v 92.9%), and axillary dissection (37.5% v 47.4%). Patients tested before diagnosis had better overall survival (OS; unadjusted hazard ratio [HR], 0.61 [95% CI, 0.40 to 0.92]); however, this result lost statistical significance after adjustment for potential confounders including tumor stage (adjusted HR, 0.74 [95% CI, 0.47 to 1.15]). CONCLUSION This global study provides evidence on the different clinical behavior of breast cancer in young BRCA1 and BRCA2 carriers. Identifying a BRCA PV in healthy individuals was associated with earlier-stage breast cancer diagnosis and lower treatment burden, as well as better unadjusted OS.

The rising incidence of cancer among young women highlights the importance of understanding treatment‐related gonadotoxicity, which can lead to infertility and long‐term health complications from premature estrogen decline. Although anti‐Müllerian hormone is considered the most reliable indicator of ovarian reserve, its interpretation among oncology patients is complex because it is dependent on the treatment modality, necessitating further research to fully understand the impact of specific treatments on anti‐Müllerian hormone and to identify alternative markers to reliably assess the ovarian toxicity of newer cancer therapies.

PURPOSE In light of evolving evidence that some patients with node-positive estrogen receptor–positive (ER+) disease may receive less benefit from chemotherapy, this study reports 12-year outcomes of the C9741 trial overall, and by the sensitivity to endocrine therapy (SET2,3) test index, a biomarker measuring endocrine transcriptional activity, to identify patients most likely to benefit from dose-dense chemotherapy. METHODS In all, 1,973 patients were randomly assigned to dose-dense versus conventional chemotherapy. Hazard ratios (HRs) for prognosis and for predictive interaction with chemotherapy schedule were estimated from Cox models of long-term disease-free survival (DFS) and overall survival (OS). SET2,3 was tested on the 682 banked RNA samples from ER+ cancers. RESULTS Dose-dense chemotherapy improved DFS in the overall study population by 23% (HR, 0.77 [95% CI, 0.66 to 0.90]) and OS by 20% (HR, 0.80 [95% CI, 0.67 to 0.95]); the benefits of dose-dense therapy were seen for ER+ and ER-negative subsets, without significant interaction between treatment arm and ER status. Low SET2,3 status was highly prognostic, but also predicted improved outcomes from dose-dense chemotherapy (interaction P = .0998 for DFS; 0.027 for OS), independent of menopausal status. Specifically, low endocrine transcriptional activity predicted benefit from dose-dense chemotherapy, whereas tumor burden and proliferation-driven signatures for molecular subtype classification did not. CONCLUSION At 12-year follow-up, C9741 confirmed the sustained long-term benefit of adjuvant dose-dense chemotherapy for node-positive breast cancer. SET2,3 identified patients with ER+ breast cancer who benefited from dose-dense chemotherapy, and specifically, this benefit was predicted by low endocrine activity in the cancer, rather than tumor burden, molecular subtype, or menopausal status.

IMPORTANCEYoung adults aged 18 to 39 years represent the minority of breast cancer diagnoses but are particularly vulnerable to financial hardship. Factors contributing to sustained financial hardship are unknown.OBJECTIVESTo identify financial hardship patterns over time and characterize factors associated with discrete trajectories; it was hypothesized that treatment-related arm morbidity, a key source of expense, would be associated with long-term financial difficulty.DESIGN, SETTING, AND PARTICIPANTSThis cohort study included US young adults aged 40 years or younger treated between 2006 and 2016. Eligible patients were treated for stage 0 to stage III breast cancer at institutions participating in the Young Women’s Breast Cancer Study, which included a specialized cancer institute and 12 other academic and community hospitals. Patients who responded at baseline and returned a 1-year survey were included in analysis. Data were analyzed in March 2024.MAIN OUTCOMES AND MEASURESTrajectory modeling classified patterns of financial difficulty from baseline through 10 years postdiagnosis using the Cancer Rehabilitation Evaluation System (CARES) scale. Multinomial regression examined characteristics, including treatment-related arm morbidity, associated with each trajectory.RESULTSA total 1008 patients were included (median [IQR] age at diagnosis, 36 [33-39] years; 60 Asian [6.0%], 35 Black [3.5%], 47 Hispanic [4.7%], 884 White [87.7%]); 840 patients were college graduates (83.3%), 764 were partnered at baseline (75.8%), 649 were nulliparous (64.4%), and 908 were without comorbidities at enrollment (90.1%). Patients’ tumors were primarily stage I-II (778 [77.2%]), estrogen receptor/progesterone receptor–positive (754 [74.8%]), and ERBB2-negative (formerly HER2) (686 [68.1%]). Patients were more frequently treated with mastectomy than breast conservation (771 [76.5%] vs 297 [29.5%]; P &amp;lt; .001). A majority of patients received radiation therapy (627 [62.2%]), chemotherapy (760 [75.4%]), and endocrine therapy (610 [60.6%]). A total of 727 patients (72.1%) reported arm symptoms within 2 years of surgery. Three distinct trajectories of experiences with finances emerged: 551 patients (54.7%) had low financial difficulty (trajectory 1), 293 (29.1%) had mild difficulty that improved (trajectory 2), and 164 (16.3%) had moderate to severe difficulty peaking several years after diagnosis before improving (trajectory 3). Hispanic ethnicity (OR, 3.71; 95% CI, 1.47-9.36), unemployment at baseline and 1 year (OR, 2.66; 95% CI, 1.63-4.33), and arm symptoms (OR, 1.77; 95% CI, 1.06-2.96) were associated with increased odds of experiencing trajectory 3. Having a college degree (OR, 0.20; 95% CI, 0.12-0.34) or being partnered (OR, 0.24; 95% CI, 0.15-0.38) were associated with increased odds of experiencing trajectory 1.CONCLUSIONIn this cohort study of young adults with breast cancer, we identified a subset of patients who experienced a high degree of financial difficulty persisting into early survivorship. Targeted interventions to mitigate financial toxicity—modifiable factors that include support for the employability or return to work support for those experiencing arm symptoms after treatment—are needed.