Introduction The Blood and Marrow Transplant Clinical Trials Network 1102 study (BMT CTN 1102, NCT02016781) was a multicenter, biologic assignment trial, where patients aged 50-75 with intermediate-2 or high risk de novo myelodysplastic syndrome (MDS) were assigned to reduced-intensity allogeneic hematopoietic cell transplantation (alloHCT) or best supportive care (non-alloHCT) based on donor availability. A parallel economic evaluation was performed alongside BMT CTN 1102. Here, we report on the financial impact of the treatment protocol on patients using a prospective cost diary study conducted within the trial. Methods BMT CTN 1102 enrollees were offered the option to participate in the parallel cost diary study at the same time of primary study enrollment. The cost diary survey instrument, comprised of 19 questions, covered the domains of out-of-pocket (OOP) costs of healthcare and financial hardship indicators. Surveys were distributed at 1- (1M), 7- (7M), and 19-months (19M) after enrollment. Study participants were grouped by actual treatment received, rather than group assignment. Responses were analyzed using univariate Fisher's exact and Wilcoxon rank sum tests for association. Results Out of 216 enrollee invitations, 138 patients (64%) returned at least one survey. Total completed surveys at the 1M, 7M, and 19M timepoints were 103 (39%), 93 (35%), and 71 (27%), respectively. 50 (36%) completed only one survey, while 41 (39%) completed surveys at all three timepoints. Of all respondents, median age was 67.7 years (IQR: 64.8-70.3), 88 (64%) were male, 128 (93%) white, and 126 (91%) were non-Hispanic. Among respondents of the 1M, 7M and 19M surveys, 82 (80%), 78 (84%), 65 (92%) received alloHCT, respectively. Mean OOP costs by category are presented in Table 1. While alloHCT patients experienced higher OOP costs compared to non-alloHCT patients across all categories (outpatient services, prescription medications, and accommodations for healthcare), none of the comparisons were statistically significant. However, total OOP costs were significantly higher among alloHCT patients at 1 and 7 months (mean over past 30 days, 1M: $889 vs. $217, p=0.046; 7M: $678 vs. $349, p=0.03; 19M: $328 vs. $103, p=0.15). Financial hardship indicators are presented in Table 2. While alloHCT patients experienced greater financial hardship compared to non-alloHCT patients across all categories (mobilized reserve funds, borrowed money to pay for care, developed debt due to care), none of the comparisons were statistically significant. However, alloHCT patients were significantly more likely to experience at least one of these factors of financial hardship at 7 months (1M: 38% vs. 19%, p=0.13; 7M: 42% vs. 13%; p=0.04; 19M: 37% vs. 0%, p=0.08). Discussion Patients with MDS who received alloHCT incurred substantially more OOP costs overall compared to patients who did not receive alloHCT, most notably for accommodations related to alloHCT. Similarly, alloHCT patients were more likely to develop financial hardship, including mobilizing reserve funds, borrowing money, and developing debt. These findings demonstrate that treatment-related OOP expenditures associated with alloHCT are substantial and may pose significant burdens for some populations receiving alloHCT. Financial navigation and assistance, which have been shown to help patients address cost burden in solid tumor settings, should be considered for MDS patients prior to alloHCT. Small final sample sizes limited our analyses significantly; further research into OOP costs and financial hardship among larger cohorts of alloHCT patients are necessary to confirm these findings. Acknowledgement Support for this study was provided by grants #U10HL069294 and #U24HL138660 to the Blood and Marrow Transplant Clinical Trials Network from the National Heart, Lung, and Blood Institute and the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.