RMTD-08 LEFT-DIGIT AGE BIAS REDUCES SURGICAL INTERVENTION AT DECADE TRANSITIONS IN PATIENTS WITH BRAIN METASTASES: IMPLICATIONS FOR CLINICAL DECISION-MAKING, RETROSPECTIVE ANALYSES, AND TRIAL DESIGN.

Abstract BACKGROUND Left-digit bias is a cognitive heuristic in which the leftmost digit of a patient’s age disproportionately influences clinician decision-making, particularly at decade transitions such as age 60, 70, or 80. OBJECTIVE We evaluated whether this bias affects the decision to perform surgery relative to radiation therapy in patients with secondary metastatic brain tumors. METHODS Using the TriNetX research network, which aggregates de-identified data from 104 healthcare institutions, we analyzed 159,223 patients aged 50–89 with brain metastases, including 7,995 who underwent surgery and 25,282 who received radiation. To quantify left-digit bias, we calculated the surgery-to-radiation ratio—defined as the number of surgeries divided by the number of radiation treatments—and applied three complementary METHODS (1) two-year interval comparisons around decade thresholds using proportions z-tests, (2) a weighted linear regression model fit to ages ending in digits 3–8 to establish expected trends, with residual deviation testing at ages ending in 9, 0, or 1, and (3) control comparisons both before and after each junction (e.g., 66–67 vs. 68–69 and 72–73 vs. 70–71) to assess specificity. RESULTS Significant declines in surgery-to-radiation ratios were observed at ages 60 (33.9% to 29.6%, p=0.042), 70 (31.8% to 28.7%, p=0.044), and 80 (34.0% to 30.8%, p=0.025). Regression-based analysis confirmed a significant negative deviation at age 60 (observed 28.2%, predicted 34.3%; Z = -2.19). Control comparisons showed no significant differences (e.g., 66–67 vs. 68–69, p=0.512; 72–73 vs. 70–71, p=0.289; 62–63 vs. 60–61, p=0.379). CONCLUSION These findings suggest that left-digit bias systematically reduces the likelihood of surgery for brain metastases at decade transitions. This has critical implications for patient outcomes, interpretation of retrospective age-based treatment studies, and clinical trial design, where rigid age cutoffs may inadvertently reflect provider behavior rather than true biological or functional thresholds.