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PURPOSE Erb-B2 receptor tyrosine kinase 2 ( ERBB2)–positive breast cancer (BC) is particularly common in young women. Genomic features of ERBB2-positive tumors before and after chemotherapy and trastuzumab (chemo + H) have not been described in young women and are important for guiding study of therapeutic resistance in this population. METHODS From a large prospective cohort of women age 40 years or younger with BC, we identified patients with ERBB2-positive BC and tumor tissue available before and after chemo + H. Whole-exome sequencing (WES) was performed on each tumor and on germline DNA from blood. Tumor-normal pairs were analyzed for mutations and copy number (CN) changes. RESULTS Twenty-two women had successful WES on samples from at least one time point; 12 of these had paired sequencing results from before and after chemo + H and 10 had successful sequencing from either time point. TP53 was the only significantly recurrently mutated gene in both pre- and post-treatment samples. MYC gene amplification was observed in four post-treatment tumors. Seven of 12 patients with paired samples showed acquired and/or clonally enriched alterations in cancer-related genes. One patient had an increased clonality putative activating mutation in ERBB2. Another patient acquired a clonal hotspot mutation in TP53. Other genomic changes acquired in post-treatment specimens included alterations in NOTCH2, STIL, PIK3CA, and GATA3. There was no significant change in median ERBB2 CN (20.3 v 22.6; Wilcoxon P = .79) between paired samples. CONCLUSION ERBB2-positive BCs in young women displayed substantial genomic evolution after treatment with chemo + H. Approximately half of patients with paired samples demonstrated acquired and/or clonally enriched genomic changes in cancer genes. ERBB2 CN changes were uncommon. We identified several genes warranting exploration as potential mechanisms of resistance to therapy in this population.

10503 Background: Prior data suggests that the risk of second primary breast cancer (SPBC) is higher among women diagnosed with primary breast cancer (BC) before age 40. There is evidence that risk differs by primary tumor characteristics, cancer treatments, and genetic predisposition. However, studies investigating risk of SPBC by clinical risk factors are often limited by low numbers of young women and lack of complete patient data, particularly regarding germline genetic risk. Methods: Using the Young Women’s BC Study (YWS), a prospective cohort of 1,302 women ≤40 years diagnosed with BC from 2006-2016, we sought to compare clinical and treatment characteristics among women who developed an SPBC vs. those who did not. The analytic cohort included women diagnosed with Stage 0-III BC who did not undergo bilateral mastectomy (N = 685). Demographics, genetic testing and results, clinical events, and treatment data were collected through self-report on baseline and follow-up surveys (every 6 months for 3 years, then annually). Detailed tumor and treatment information, confirmation of genetic testing results, and new and recurrent cancer events were obtained through regular and triggered medical record review. Univariate and multivariate Cox proportional hazards models were used to examine associations of age, race, genetic testing results, hormone receptor status, stage, and grade of primary tumor, and cancer treatments with SPBC. Participants were censored at the time of local recurrence, metastasis, death, or loss of follow-up. Results: At a median follow-up time of 9.9 years [inter-quartile range (IQR) = 6.6-12.0 y], 17 patients developed an SPBC (2.5%), in whom 3 occurred in the ipsilateral breast after lumpectomy as primary BC surgery. Median time from primary BC diagnosis to SPBC diagnosis was 4.2 years (IQR = 3.3-5.6 y). Compared to those who did not develop an SPBC, a greater percentage of women who developed an SPBC had in situ disease as their primary BC (24% v. 6%) and were of Ashkenazi descent (12% v. 5%). In the 577 women who had had genetic testing, 544 (94%) were non-carriers of pathogenic variants. Risk of SPBC was 2.2% (N = 12) in non-carriers and 9.1% (N = 3) in the 33 carriers. In Cox univariate analyses, carriers had a 4.1 times higher risk of SPBC compared to non-carriers (HR = 4.14, 95% confidence interval (CI) = 1.67-14.66, p = 0.03), and primary diagnosis with in situ BC was associated with a 5-fold increased risk of SPBC (HR = 5.02, 95% CI = 1.63-15.41, p = 0.005). Both factors remained strongly associated with SPBC when tested together in a multivariate model. Conclusions: This prospective study demonstrated a low risk of SPBC in the first 10 years after diagnosis of early BC in young women who are not germline pathogenic variant carriers. The finding that risk of SPBC was higher after in situ primary BC at a young age warrants further investigation. These findings have important implications for the treatment and follow-up care of young women with BC.

505 Background: Increased TILs are associated with better prognosis in triple-negative BC, including in patients (pts) age < 40. However, the role of TILs remains unclear in HR+/HER2- BC and little data exist in young pts, in whom immune microenvironment could be altered by age-related host/tumor differences. We assessed the extent and composition of immune infiltration in HR+ tumors of young women and correlated with clinicopathologic features and survival outcomes. Methods: From a prospective cohort study of women with BC diagnosed age ≤40, we identified those with stage I-III HR+/HER2- BC and available pre-treatment (tx) tumor tissue. Multiplexed immunofluorescence was used to quantify cytotoxic T (CD8+), T helper (Th, CD3+CD8-), T regulatory (Tregs, FOXP3+CD3+) and exhausted T (PD1+CD8+) cells in stroma and tumor. Univariate analyses tested associations between clinicopathologic variables and immune markers by high or low expression, divided based on median. High vs. low TILs were evaluated in Cox regression analyses for invasive breast cancer-free survival (iBCFS), distant disease-free survival (DDFS) and overall survival (OS). Results: In 390 pts, median age was 37 years (21-40), most had grade 2 (51%), T1 (65%), N0 (63%) tumors and 67% received adjuvant chemotherapy. Black pts (n = 17) had higher expression of stromal CD8+ ( P= .010), FOXP3+CD3+ ( P= .027) and PD1+CD8+ TILs ( P= .043); intratumoral TILs did not differ by race. Older age (36-40) was associated with high expression of CD8+ ( P= .033) and PD1+CD8+ TILs ( P= .031) within stroma and CD3+CD8- TILs within tumor ( P= .046). Grade 3 tumors had higher stromal and intratumoral expression of CD3+CD8- ( P= .002; P< .001) and FOXP3+CD3+ TILs ( P= .020; P< .001). No differences in TILs were seen according to recency of pregnancy, BRCA1/2 status or T/N stage. Over a median follow up of 8 years, 85 iBCFS events, 64 DDFS events and 37 deaths occurred. High stromal expression of CD3+CD8- TILs was associated with better iBCFS (HR 0.49, P= .002) and DDFS (HR 0.57, P= .046), which remained significant when adjusted for T/N stage, grade and chemotherapy (iBCFS HR 0.41, P< .001; DDFS HR 0.45, P= .008). High stromal expression of CD3+CD8- and FOXP3+CD3+ TILs was associated with better OS (HR 0.47, P= .038) and iBCFS (HR 0.58, P= .018), respectively, on adjusted analyses only. High intratumoral expression of CD3+CD8- and FOXP3+CD3+ TILs was associated with better iBCFS (HR 0.59, P= .025; HR 0.63, P= .043) after adjustment only. Conclusions: The distribution of TIL subtypes in young women’s HR+ BC varied according to race, age and grade. High stromal and intratumoral expression of Th and Tregs was associated with improved BC outcomes. Characterization of immune cell subsets could help refine the prognostic value of TILs in HR+ BC, particularly in young pts who may benefit from individualized escalated/de-escalated tx strategies.

AbstractBackgroundObesity and inactivity are poor prognostic factors in breast cancer, but less is known regarding physical activity (PA) and weight patterns in young breast cancer survivors.MethodsThe Young and Strong Study was a cluster‐randomized trial evaluating education and support interventions for young women (age <45 years) with newly diagnosed breast cancer. Sites were randomized 1:1 to a Young Women's Intervention (YWI) or a contact‐time control physical activity intervention (PAI). Changes in PA and weight were compared between groups using general estimating equations to evaluate clustered binary and Gaussian data.ResultsA total of 467 patients enrolled between July 2012 and December 2013 across 54 sites. Median age at diagnosis was 40 years (range, 22–45). At baseline, median body mass index (BMI) was 25.4 kg/m2 (range, 16.1–61.1), and participants reported a median of 0 minutes (range, 0–2190) of moderate/vigorous PA/week. PA increased significantly over time in both groups (p < .001), with no difference between groups at any time point. BMI increased modestly but significantly (p < .001) over time in both groups. Provider attention to PA was observed in 74% of participants on PAI and 61% on YWI (p = .145) and correlated with PA at 12 months (median 100 min/week of PA in participants with provider attention to PA vs. 60 min/week in those without, p = .016).ConclusionsIn a cohort of young women with breast cancer, rates of obesity and inactivity were high. PA and BMI increased over time and were not impacted by an educational PA intervention. Findings provide important information for developing lifestyle interventions for young breast cancer survivors.

Abstract Purpose: Young women’s breast cancers (YBCs), defined herein as breast cancer (BC) diagnosed at age ≤ 40 years, are etiologically and biologically distinct from postmenopausal BCs, demonstrating a higher proportion of estrogen receptor (ER) negative subtypes and a worse prognosis. Studies have not assessed whether risk factors such as recent parity and body mass index (BMI) are linked to protein expression profiles in YBCs. Accordingly, we examined protein expression assessed with GeoMx® Digital Spatial Profiling (DSP) in 640 YBCs prepared as tissue microarrays (TMAs) in the Young Women’s BC Study (YWBCS). Methods: The YWBCS is a multi-institutional cohort of 1,297 women with incident BC diagnosed at age &lt;40 years identified through medical record review, with risk factor data collected via questionnaires. TMAs including 1,505 cores of 640 YBCs that were collected prior to therapy and were suitable for molecular analysis were evaluated for 72 markers with NanoString GeoMx DSP. Associations of log-transformed biomarker expression with BMI and time from last live birth to BC diagnosis were carried out using linear mixed modeling approaches, accounting for multiple tumor tissue cores per individual. Results: Mean age at BC was 35.5 years (range 17-40). Tumor subtypes based on clinical immunostaining were the following: luminal B 46%; luminal A 36%; triple negative 12% and HER2+ 6%. Of 72 biomarkers tested, 53 (74%) were evaluable after QC and normalization. GeoMx ER and PR protein levels were higher in luminal A and luminal B YBCs than in HER2+ and TNBC (p=2x10-94 and 2x10-52, respectively), supporting the accuracy of the GeoMx assay. Compared to nulliparas or those with most recent birth &gt; 3 years before BC, those who gave birth ≤ 3 years prior to BC had tumors with higher expression of PR (8x10-6) and ER (p=0.006), and lower expression of BCL-2 (p=0.006). In contrast, women with last birth ≥ 6 years prior to BC had higher expression of CD8 (p=0.006) and PD-1 (p=0.001) than nulliparas and those with more recent births. Compared to women with BMI between 18.5 and 29.9, those with higher or lower BMIs had higher expression of CD34 (p=0.0003), CD4 (p=0.0009) and CD14 (p=0.001). BCL-2 expression was higher among women with BMI &lt; 18.5 compared to those with higher values (p=0.005). Women with BMI of 30+ had higher β2-microglobulin levels than those with lower BMIs (p=0.003). Inverse dose-response associations (with higher BMI related to lower levels) were observed for expression of ER (p=0.01), pan cytokeratin (p=0.004) and four proteins in the p13K/AKT signaling pathway (p ≤ 0.01 for each). Conclusions: Preliminarily, we identify differences in hormonal and immune markers in YBCs in relation to timing of births and BMI, suggesting that these factors may contribute to etiologic heterogeneity. Multivariate modeling is ongoing to incorporate molecular subtype and other risk factors. Citation Format: Robert A. Vierkant, Laura M. Pacheco-Spann, Stacey J. Winham, Jennifer M. Kachergus, Ji Shi, Craig Snow, Fergus J. Couch, Janet E. Olson, Chen Wang, Derek C. Radisky, Amy C. Degnim, E. Aubrey Thompson, Laura C. Collins, Kathryn J. Ruddy, Ann H. Partridge, Mark E. Sherman. Associations of parity and body mass index with NanoString digital spatial protein profiling in early onset breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1000.

PURPOSE Breast-conserving therapy (BCT) is the preferred treatment for unifocal breast cancer (BC). The oncologic safety of BCT for multiple ipsilateral breast cancer (MIBC) has not been demonstrated in a prospective study. ACOSOG Z11102 (Alliance) is a phase II, single-arm, prospective trial designed to evaluate oncologic outcomes in patients undergoing BCT for MIBC. PATIENTS AND METHODS Women age 40 years and older with two to three foci of biopsy-proven cN0-1 BC were eligible. Patients underwent lumpectomies with negative margins followed by whole breast radiation with boost to all lumpectomy beds. The primary end point was cumulative incidence of local recurrence (LR) at 5 years with an a priori rate of clinical acceptability of <8%. RESULTS Among 270 women enrolled between November 2012 and August 2016, there were 204 eligible patients who underwent protocol-directed BCT. The median age was 61 years (range, 40-87 years). At a median follow-up of 66.4 months (range, 1.3-90.6 months), six patients developed LR for an estimated 5-year cumulative incidence of LR of 3.1% (95% CI, 1.3 to 6.4). Patient age, number of sites of preoperative biopsy–proven BC, estrogen receptor status and human epidermal growth factor receptor 2 status, and pathologic T and N categories were not associated with LR risk. Exploratory analysis showed that the 5-year LR rate in patients without preoperative magnetic resonance imaging (MRI; n = 15) was 22.6% compared with 1.7% in patients with a preoperative MRI (n = 189; P = .002). CONCLUSION The Z11102 clinical trial demonstrates that breast-conserving surgery with adjuvant radiation that includes lumpectomy site boosts yields an acceptably low 5-year LR rate for MIBC. This evidence supports BCT as a reasonable surgical option for women with two to three ipsilateral foci, particularly among patients with disease evaluated with preoperative breast MRI.

AbstractBackgroundRisk‐stratified follow‐up guidelines that account for the absolute risk and timing of recurrence may improve the quality and efficiency of breast cancer follow‐up. The objective of this study was to assess the relationship of anatomic stage and receptor status with timing of the first recurrence for patients with local‐regional breast cancer and generate risk‐stratified follow‐up recommendations.MethodsThe authors conducted a secondary analysis of 8007 patients with stage I–III breast cancer who enrolled in nine Alliance legacy clinical trials from 1997 to 2013 (ClinicalTrials.gov identifier NCT02171078). Patients who received standard‐of‐care therapy were included. Patients who were missing stage or receptor status were excluded. The primary outcome was days from the earliest treatment start date to the date of first recurrence. The primary explanatory variable was anatomic stage. The analysis was stratified by receptor type. Cox proportional‐hazards regression models produced cumulative probabilities of recurrence. A dynamic programming algorithm approach was used to optimize the timing of follow‐up intervals based on the timing of recurrence events.ResultsThe time to first recurrence varied significantly between receptor types (p < .0001). Within each receptor type, stage influenced the time to recurrence (p < .0001). The risk of recurrence was highest and occurred earliest for estrogen receptor (ER)‐negative/progesterone receptor (PR)‐negative/Her2neu‐negative tumors (stage III; 5‐year probability of recurrence, 45.5%). The risk of recurrence was lower for ER‐positive/PR‐positive/Her2neu‐positive tumors (stage III; 5‐year probability of recurrence, 15.3%), with recurrences distributed over time. Model‐generated follow‐up recommendations by stage and receptor type were created.ConclusionsThis study supports considering both anatomic stage and receptor status in follow‐up recommendations. The implementation of risk‐stratified guidelines based on these data has the potential to improve the quality and efficiency of follow‐up.

Abstract Background: The ATEMPT trial primary analysis found that one year of adjuvant trastuzumab emtansine (T-DM1) achieved a 3-year iDFS of 97.8% for patients with stage I HER2+ breast cancer, but was not associated with fewer clinically relevant toxicities (CRTs) compared with paclitaxel and trastuzumab (TH). In this end-of-study analysis, we report 5-year survival outcomes and correlative analyses from the trial. Methods: Patients with stage I centrally confirmed HER2+ breast cancer were randomly assigned 3:1 to adjuvant T-DM1 for one year or TH and received T-DM1 3.6 mg/kg IV every 3 weeks for 17 cycles or paclitaxel 80 mg/m2 IV with weekly trastuzumab IV followed by trastuzumab for 9 months. The co-primary objectives were to compare the incidence of CRTs between the 2 arms and to evaluate iDFS in patients receiving T-DM1. To investigate proteomic correlates of recurrence, spatial proteomic analyses were performed on samples from 13 patients experiencing iDFS events (cases) and 24 matched controls using the NanoString GeoMx Digital Spatial Profiler. The impact of HER2 heterogeneity on outcomes was investigated among 17 cases and 51 matched controls by fluorescence in-situ hybridization (FISH). HER2 genetic heterogeneity was assessed by scrutinizing the whole tumor area and defined as the occurrence of HER2 gene amplification in &gt;5% but &lt; 50% invasive tumor cells. The risk of recurrence was evaluated centrally with the HER2DX genomic assay from 225 primary tumor samples. Germline whole genome sequencing (WGS) was conducted among 55 patients experiencing T-DM1-induced thrombocytopenia and/or bleeding and 55 matched controls to identify genomic correlates for this side effect. Results: A total of 497 patients who initiated protocol therapy were included in this analysis (383 T-DM1 and 114 TH). After a median follow up 5.8 years, among patients receiving T-DM1 there were a total of 11 iDFS events, with 3 distant recurrences. The 5-year iDFS for T-DM1 was 97.0% (95% CI, 95.3-98.8%), the 5-year recurrence-free interval (RFI) was 98.6% (95% CI: 97.4-99.8%) and the 5-year overall survival (OS) for T-DM1 was 97.8 % (95% CI, 96.3-99.3%). Although the study was not powered to evaluate the efficacy of TH, among the 114 patients receiving TH, a total of 9 iDFS events were observed, including 2 distant events; the 5-year iDFS with TH was 91.3% (95% CI: 86.0-96.9%), 5-year RFI was 93.3% (95% CI: 88.6-98.2%) and 5-year OS was 97.9% (95% CI: 95.2-100%). A total of 56 samples were evaluable for heterogeneity analyses, among which 14% (n=8) harbored HER2 genetic heterogeneity. Spatial proteomic analyses found that NF1 (adjusted p=0.72 × 10-6) and CTLA-4 (adjusted p=0.15 × 10-3) were significantly upregulated in primary samples from cases, while cleaved caspase 9, CD25, GITR, ICOS, p53 and PD-L2 were significantly upregulated in controls (all with adjusted p&lt; 0.05). Germline WGS found that the top gene associations with thrombocytopenia and thrombocytopenia or bleeding were ALMS1 (p=0,19 × 10-3) and APBA3 (p=0,23 × 10-3), respectively, although none reaching the threshold for genome wide significance. rs62143195 and rs114169776 were the top single nucleotide polymorphisms associated with thrombocytopenia and thrombocytopenia or bleeding, respectively. Data on the impact of HER2 heterogeneity and of HER2DX score on survival outcomes will be presented. Conclusion: With longer follow-up, adjuvant T-DM1 confirmed outstanding long-term outcomes among patients with stage I HER2+ breast cancer, demonstrating a 5-year RFI of 98.6%. Spatial proteomic analyses identified a potential association between NF1 and CTLA-4 expression with recurrence. Details on the impact of HER2 heterogeneity and HER2DX assay on prognosis will be presented. Citation Format: Paolo Tarantino, Nabihah Tayob, Chau T Dang, Denise Yardley, Steven J. Isakoff, Vicente Valero, Meredith Faggen, Therese Mulvey, Ron Bose, Douglas Weckstein, Antonio C. Wolff, Katherine Reeder-Hayes, Hope Rugo, Bhuvaneswari Ramaswamy, Dan Zuckerman, Lowell Hart, Vijayakrishna K. Gadi, Michael Constantine, Kit Cheng, Audrey Merrill Garrett, Paul K. Marcom, Kathy S. Albain, Patricia DeFusco, Nadine Tung, Blair Ardman, Rita Nanda, Rachel C. Jankowitz, Mothaffar Rimawi, Vandana Abramson, Paula R. Pohlmann, Catherine Van Poznak, Andres Forero-Torres, Minetta C. Liu, Kathryn Ruddy, Yue Zheng, Romualdo Barroso-Sousa, Adrienne Waks, Michelle K. DeMeo, Molly K. DiLullo, Giuseppe Curigliano, Harold Burstein, Ann Partridge, Eric Winer, Giuseppe Viale, Winnie Hui, Elizabeth A. Mittendorf, Bryan P. Schneider, Aleix Prat, Ian Krop, Sara Tolaney. Adjuvant Trastuzumab Emtansine Versus Paclitaxel plus Trastuzumab for Stage I HER2+ Breast Cancer: 5-year results and correlative analyses from ATEMPT (TBCRC033) [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD18-01.

Abstract Background: Breast cancer (BC) is the most common malignancy among women of reproductive age. Given limited data describing the conception and pregnancy experience of young BC survivors, we sought to explore these outcomes to inform counseling of women interested in future childbearing. Methods: Participants with stage 0-III BC in the Young Women’s Breast Cancer Study (NCT01468246), a multi-center, prospective cohort of women diagnosed at age ≤ 40 from 2006-2016 who reported ≥ 1 live birth from a pregnancy after diagnosis were sent a survey with investigator-developed questions focused on their first post-diagnosis live birth. Women who had been diagnosed with BC during pregnancy were excluded from this analysis. The survey assessed conception, use of assisted reproductive technology (ART), pre-implantation genetic testing (PGT), endocrine therapy (ET), and peripartum complications. Summary statistics are presented. Results: 92/119 eligible women completed the survey (response rate: 77%). Median age at diagnosis was 32 (range:17-40) years and at delivery was 37 (range: 29-47) years. Median time from diagnosis to delivery was 58 months (range: 11-154). Most women had stage 2 BC (43%, 40/92); 68% received chemotherapy (63/92); about half (51%, 47/92) were nulligravida at diagnosis. Overall, 61% of pregnancies were conceived naturally (56/92) and 39% with ART (36/92): 32% by in-vitro fertilization (IVF, 29/92), 7% with fertility medications only (6/92), and 1 with intrauterine insemination. 38% of IVF pregnancies were conceived using products from fertility preservation prior to BC treatment (11/29). Among women who used ART, 74% attempted to conceive naturally (25/36) for a median of 9 (range: 2-48) months prior to pursuing ART. The most common reasons for pursuing ART include infertility following BC treatment (33%, 12/36) and expediting conception to resume treatment (17%, 6/36). 11% of those with known inherited pathologic variant mutations underwent PGT (2/19). Reasons for not pursuing PGT included belief in more effective cancer risk reduction in the future (29%, 5/17), not being offered PGT (24%, 4/17), high cost (12%, 2/17), no interest in IVF (12%, 2/17), acceptable odds for inheriting the mutation (24%, 4/17), and belief in other risk reduction strategies (18%, 3/17); 1 woman reported ethical concerns. Of 57 women who took ET pre-pregnancy (63%), nearly all (96%, 55/57) discontinued ET &gt; 3 months prior to attempting to conceive; 1 discontinued after awareness of pregnancy. Of those who had received prior ET, 60% resumed ET (34/57) a median of 3 (range: 1-50) months after pregnancy. Among 23 women who did not resume, 13 (23%) had completed the recommended duration; the remaining 10 reported one or more of the following reasons: felt better while off (28%, 6/23), desire for another child (22%, 5/23), and desire to breastfeed (17%, 4/23). Median time to delivery was 39 (range: 28-42) weeks with 12% delivering preterm &lt; 37 weeks (11/92). 47% had a Caesarean section (43/92), with prolonged labor the most common indication (33%, 14/43). Hypertensive disorders of pregnancy (HDP, 20%, 18/92), gestational diabetes (7%, 6/92), small for gestational age (7%, 6/92), and postpartum hemorrhage (5%, 5/92) were the most common obstetrical complications. 9% of women had newborns requiring NICU admission (8/92) and 9% had low birth weight (8/92). Conclusion: Among young BC survivors with a live birth following diagnosis, most conceived naturally, with the majority who used ART first attempting natural conception. There was limited use of PGT among mutation carriers with ¼ not having been offered testing. Patient reported peripartum complications appear consistent with population norms, though the relatively higher rate of HDP bears further research. Among those yet to complete their ET, a notable proportion did not resume following delivery. This novel data may help to inform the care of young breast cancer survivors pursuing pregnancy. Citation Format: Kimia Sorouri, Tal Sella, Shoshana Rosenberg, Margaret Loucks, Kathryn Ruddy, Shari I. Gelber, Rulla M. Tamimi, Jeffrey M. Peppercorn, Lidia Schapira, Virginia F. Borges, Steven E. Come, Ellen Warner, Ann Partridge. Conception and Pregnancy Among Young Breast Cancer Survivors [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-08-05.

Abstract Background: Breast cancer (BC) is the most common malignancy in women of reproductive age, and the incidence of the disease is rising in this population. Many of these women are interested in childbearing after their BC treatment and a substantial minority will go on to have a live birth. Some will also want to breastfeed. However, there is only limited information available regarding the experience of young BC survivors breastfeeding following treatment. Methods: Participants in the Young Women’s Breast Cancer Study (YWS), a multi-center, prospective cohort study of women diagnosed with BC at age ≤ 40 years between 2006-2016, who reported at least one live birth following their diagnosis with stage 0-III BC were sent an additional survey including investigator-developed questions focused on breastfeeding after breast cancer treatment. Women who had been diagnosed with BC during pregnancy were excluded from this analysis. The survey assessed whether they breastfed, reasons for attempting and stopping breastfeeding, breastfeeding with the treated breast and untreated breast, and supports. Summary statistics, including medians and proportions, are presented. Results: Of 118 eligible women sent a survey, 92 completed the survey (78% response rate). Median age at diagnosis of BC was 32 (range: 17-40) years and at delivery was 37 (range: 29-47) years. 54% of women had attempted to breastfeed (50/92). Among those who had not, 93% noted a history of bilateral mastectomies (39/42). Additional reasons for not attempting to breastfeed included no interest regardless of BC history (5%, 2/42) and 1 woman underwent a unilateral mastectomy and did not think her supply would be sufficient. Among the women who did attempt breastfeeding, 68% had undergone lumpectomy and radiotherapy (34/50) with 85% of those women reporting that the treated breast did not produce milk (29/34). The 5 women who produced milk from the treated breast noted that the supply was substantially less than the untreated breast. To assist with breastfeeding, 76% used a pump only on the untreated breast (38/50) and 14% on both breasts (7/50). Women breastfed for a median of 5.5 (range:&lt; 1-60) months and 64% were “somewhat”/“very much” satisfied with their ability to breastfeed (32/50). The most common reasons cited for stopping breastfeeding included having completed the planned duration (36%, 18/50), to start/resume endocrine therapy (22%, 11/50), and to resume breast imaging (8%, 4/50). Among patients who had not undergone a double mastectomy, 47% recalled receiving specific information about breastfeeding after a history of breast cancer (25/53), most commonly from the oncology team (56%, 14/25), lactation consultant (48%, 12/25), or online resources (44%, 11/25). Conclusion: In the largest series to date detailing the breastfeeding experiences of young BC survivors, approximately half of young BC survivors with a successful pregnancy attempted to breastfeed. Among those who had undergone prior lumpectomy and radiotherapy, women reported no milk production or only limited supply from the treated breast. Despite these limitations, most women who attempted to breastfeed were satisfied with their ability to do so. Specific resources to support the experience of breastfeeding in BC survivors are needed. Citation Format: Tal Sella, Kimia Sorouri, Shoshana Rosenberg, Margaret Loucks, Kathryn Ruddy, Shari I. Gelber, Rulla M. Tamimi, Jeffrey M. Peppercorn, Lidia Schapira, Virginia F. Borges, Steven E. Come, Ellen Warner, Ann Partridge. Breastfeeding in Survivors of Early Breast Cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-08-06.