Abstract Background: Patients treated for early stage breast cancer (BC) have a 30% lifetime risk of developing metastatic disease. Numerous studies have demonstrated that dormant bone marrow disseminated tumor cells (DTCs) are independently associated with risk of recurrence and death, yet interventions targeting these cells are lacking. The PENN-SURMOUNT (Surveillance Markers of Utility for Recurrence after (Neo)adjuvant Therapy) Screening Study was launched in 2016 to screen high risk BC survivors for DTCs using bone marrow aspirate (BMA) and identify eligible DTC positive patients for clinical trials. Given the novelty of this approach, we concurrently developed and pilot tested a PRO measurement strategy to study how the screening method of BMA and disclosure of DTC results impacts early-stage BC patients. Methods: PENN-SURMOUNT is a single center prospective, longitudinal cohort study examining BM and blood biomarkers of MRD among patients within 5 years of BC diagnosis who have high risk criteria (positive axillary nodes, triple negative biology, ER+ with Oncotype Dx ≥ 25 and/or high risk Mammaprint, or pathologic residual disease after neoadjuvant chemotherapy). From May 2019 – August 2021, we recruited patients on SURMOUNT to complete PRO surveys at baseline (T0), after BMA (T1), and after disclosure of DTC results (T2). Surveys were administered in paper form initially, then electronic form starting Feb 2021. PRO survey instruments were selected through literature review, followed by consensus among multidisciplinary clinical and research experts and patient advocates. PRO measures assess recurrence distress (Quality of Life in Adult Cancer Survivors, QLACS), illness intrusiveness (Illness Intrusiveness Ratings Scale, IIRS), and decision making (Decision Regret Scale). Additional survey items assess tolerability of the BMA and patients’ risk perception and cognitive understanding after DTC results disclosure. Descriptive statistics summarize PRO survey compliance and responses at T0, T1, and T2 in the total population and the population who reported longitudinal data for T2. Results: 61 of 66 eligible patients on the SURMOUNT trial enrolled in the PRO pilot study and completed a baseline survey, of which 47 (77%) tested negative for DTCs. Mean completion rates were 0.92 at T0, 0.85 at T1, and 0.56 at T2. After electronic survey implementation, completion rates increased to 0.94 (T0), 0.97 (T1) and 0.81 (T2). At T0, 36 (59%) patients reported a high risk perception of developing BC recurrence at 5 years and 42 (69%) during their lifetime. Mean T0 recurrence distress using the QLACS subscale was 14.6 (SD 6.3) out of possible score 4-28, compared to an expected mean of 11.42 (SD 5.48) in a general survivorship population. Mean T0 illness intrusiveness was 27.3 (SD 13.9) out of possible score 13-91. At T1, approximately 85% of patients agreed that they correctly understood the purpose of the bone marrow procedure and what the procedure would entail. 44 (72%) of patients reported a maximum pain score <= 4 in the week post-procedure and 42 (69%) reported the BMA was same or better than expected tolerability. Exploratory subset analysis of patients with complete longitudinal data at T2 (n = 34) showed average scores of 13.4 (SD 6.0), 30.1 (SD 14.0), and 2.8 (SD 6.2) for recurrence distress, illness intrusive, and decision regret scores (scale 0-100), respectively. At T2, 26 (76%) of patients reported no decision regret for undergoing testing for DTCs; 27 (79%) reported feeling less anxious after DTC results disclosure. Conclusions: Participants of PENN-SURMOUNT perceived risk of recurrence as high. The BMA procedure was well-tolerated and better than expected among the majority of this cohort, and most did not regret having undergone BMA after DTC status disclosure. Longitudinal completion rates were low, limiting assessment of PROs at later time points, a major focus of future work in this setting. Citation Format: Tara Kaufmann, Patrick Chang, Shoshana Rosenberg, Elizabeth Frank, Brian Hobbs, Lauren J. Bayne, Isoris Nivar, Brooke L. Goodspeed, Killian M. Rohn, Emily M. Kugler, Kevin Fox, Susan Domchek, Angela Bradbury, Payal Shah, Hayley Knollman, Rachel C. Jankowitz, Igor Makhlin, Amy S. Clark, Lewis A. Chodosh, Angela DeMichele, Katherine Goodfellow. Pilot study of a patient-reported outcome (PRO) measurement strategy to determine impact of screening for minimal residual disease (MRD) in high-risk 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 P5-08-01.