Abstract Background: While many clinical and non-clinical factors inform metastatic breast cancer (mBC) treatment recommendations; patient preferences, values, and care goals also influence how treatment options are weighed, deliberated, and decided upon. Additionally, in the mBC care setting, clinical meaningfulness and clinically meaningful outcomes (e.g., survival, quality-of-life) are important factors to consider along the care continuum. However, the extent to which patients and their oncology care team align on the interpretation and use of these concepts during treatment decision-making has not been well elucidated. This study examined perspectives on the concepts of (i) clinical meaningfulness and (ii) clinically meaningful outcomes associated with mBC treatment among five key stakeholder groups: patients, caregivers, oncologists, advanced practice providers (APPs), and oncology nurses. Methods: Qualitative semi-structured in-person and web-based focus groups were conducted between March and June 2023 among: (i) people living with mBC; (ii) unpaid or informal caregivers to people with mBC; and providers involved in the care of patients with mBC, including (iii) oncologists, (iv) APPs (i.e., physician assistants and nurse practitioners), and (v) oncology nurses. Discussion guides were developed to elicit stakeholder perspectives on clinical meaningfulness and clinically meaningful outcomes to help identify areas of convergence and divergence between and within stakeholder groups. All discussions were audio recorded and transcribed. The constant comparative approach was used to identify key themes. Results: Twenty-two focus groups were conducted with 50 patients, 24 caregivers, 8 oncologists, 13 APPs, and 17 oncology nurses. Overall, patients and caregivers were unfamiliar with the concepts of clinical meaningfulness and clinically meaningful outcomes, underscoring the critical need for using accessible and patient-friendly terminology in treatment decision-making discussions. While some providers were familiar with these concepts, they reported not using these terms when discussing treatment recommendations with patients. Although some provider participants thought clinical meaningfulness described quantitative endpoints (e.g., additional months of survival), participants across all stakeholder groups described patients’ abilities to achieve life goals (e.g., participation in milestone events and social activities, ability to travel) as paramount. Recommendations for improving treatment decision-making discussions included taking a patient-centered approach, with inclusive, dynamic patient-provider discussions and continuous evaluation of patient priorities across the care continuum, not just at diagnosis. Meaningful outcomes beyond overall survival need to be considered, including quality of life, progression-free survival, minimal or manageable side effects, and improvement in symptom burden and functioning. Across all stakeholders, participants stressed that outcomes considered meaningful are highly individualized and dynamic, evolving over time as patients move through the treatment journey and life stages. Conclusions: Clinical meaningfulness is poorly understood and not often considered in the clinical setting when making mBC treatment decisions, highlighting a need for accessible and patient-friendly terminology in mBC treatment decision-making conversations. While participants valued overall survival, the importance of meaningful outcomes that support patients’ quality of life was emphasized. Study findings can be used to inform both practice and clinical research to better capture meaningful outcomes for patients. Citation Format: Stephanie Graff, Emily Freeman, Meaghan Roach, Rozanne Wilson, Ricki Fairley, Mary Gullatte, Jeanne Stemland, Becky Chan, Paulina Wochal, Julie Katz, Suepattra May-Slater. Understanding clinical meaningfulness in metastatic breast cancer treatment decision-making: experiences and perspectives of patients, caregivers, and clinicians [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-10-06.