Abstract Introduction: Many people with metastatic breast cancer (MBC) experience life-limiting side effects from MBC treatments, with most patients remaining on treatment indefinitely. While supportive medications are often used to mitigate toxicities, they may introduce additional side effects and financial cost. Individualized dosing, particularly dose reductions and frequency adjustments, is a strategy to improve tolerability with the goal of maintaining treatment efficacy. The objective of this study was to elicit MBC patient experiences with treatment toxicity, including dosing strategies and side effect management. Methods: As a follow up to a 2020 survey to characterize real-world experiences with dosing strategies (Loeser et al. 2024), the Patient-Centered Dosing Initiative (PCDI) conducted a new cross-sectional survey to elicit patient experiences and preferences related to toxicity management. The survey, distributed electronically to a convenience sample of people living with MBC in the United States, combined closed and open-ended questions. Recruitment occurred through support organization e-mail lists and newsletters, as well as closed social media groups. The survey included items associated with side effect management, knowledge and experiences of dose modifications, and supportive medication use among other topics. The survey opened April 14th 2025 and data were pulled on June 22nd for analysis, although data collection is still ongoing. Descriptive statistics, including frequency counts and percentages, were used to summarize participant responses. Associations between knowledge of dose modifications and care primarily by a breast specialist were evaluated using chi-squared tests. Results: As of the abstract submission, 340 responses were available for analysis. Surveyed patients reported all treatment drugs taken in the metastatic setting, including cytotoxic chemotherapy (35%), CDK 4/6 inhibitors (62%), HER2-targeted antibodies (24%), antibody-drug conjugates (23%), and PI3K pathway inhibitors (11%). Of the 26% who started an MBC drug at a lower dose, 69% reported that the oncologist initiated the conversation and 16% said they initiated it themselves. There was no statistically significant difference in who initiated the conversation based on whether the participant was primarily treated by a breast specialist or not (p=0.43). Overall, 57% of the respondents had their dose reduced due to side effects, whether they started at full or reduced dose, and 72% felt better on the lower dose. In addition, 27% of patients reported that their oncologist reduced the frequency of drug administration; 67% of these patients felt better with the reduced frequency. 88% of surveyed patients were aware that an MBC drug with significant side effects may have an approved dose reduction schedule. Further, 59% believed that the highest dose of a drug is not always more effective than a lower dose. Conclusions: Dose modifications are common among people with MBC, often driven by treatment toxicity and side effect management. Experiences with dose reductions did not significantly differ based on receiving care from a breast cancer specialist, suggesting broader awareness and acceptance of dose adjustments in patient and clinical communities. Notably, 83% of respondents reported starting on a lower dose, having the dose reduced, or both, indicating a need for further study into the efficacy of lower doses and for clinical trial designs in MBC that include multiple doses tested in later phases. Given that patients rarely initiated these conversations, further research is needed to understand barriers to discussing dose adjustments and to support more patient-clinician dialogue. Citation Format: A. Beumer, M. Carlson, J. Maues, J. Cowden, S. Walker, A. Veach, J. Collins, K. Shanahan, M. E. Burkard, K. Kalinsky, A. Bardia, H. Rugo, M. Lustberg, E. L. Papautsky, A. Arcuri, J. Brown, M. Roberson. Dose reductions in metastatic breast cancer patients: Frequency, strategies, and patient experience 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 PS2-03-17.
Beumer et al. (Tue,) studied this question.