<h3>Purpose/Objective(s)</h3> Growing evidence suggests that the thalamus plays a central role in neurocognition. The thalamus is comprised of several nuclei, each with different anatomic connectivity and functional properties. We sought to understand the impact of microstructural injury to the thalamus and thalamic nuclei on neurocognition following RT in primary brain tumor patients, using quantitative neuroimaging biomarkers. <h3>Materials/Methods</h3> On a prospective trial, 63 primary brain tumor patients receiving fractionated RT underwent high resolution volumetric and diffusion brain MRI along with prospective neurocognitive assessments (Wechsler Adult Intelligence Scale [WAIS IV], Hopkins Verbal Learning Test [HVLT], Brief Visuospatial Memory Test [BVMT], and Delis-Kaplan Executive Function System [D-KEFS]) at baseline (pre-RT) and 3, 6, and 12 months post-RT. Validated autosegmentation was performed, generating bilateral (R/L) thalami and individual thalamic nuclei. Nuclei were grouped by anatomical region (anterior, lateral, ventral, intralaminar, medial, posterior) into subfields. Tumor, surgical bed and edema were censored to avoid confounding. Quantitative neuroimaging biomarkers were analyzed: volume, fractional anisotropy (FA; decreases with injury) and mean diffusivity (MD; increases with injury). Linear mixed-effects models assessed change in biomarkers with time and dose, and as longitudinal predictors of cognitive function, controlling for time and subject-specific effects. <h3>Results</h3> We found significant atrophy over time after RT in the L/R-anterior (p=0.038, p=0.016), L/R-lateral (p=0.022, p=0.033), R-ventral (p=0.032), R-intralaminar (p<0.001), R-medial (p=0.036), and R-posterior (p<0.001) thalamic subfields. Increased dose was associated with decreased volume (p=0.006) in the lateral geniculate nucleus (LGN) of the L-posterior field and increased MD (p=0.006, p=0.014) in the LGN of the L/R posterior fields. Increased dose was associated with decreased FA (p=0.043) in the R-ventral field. Decreased volume in the L/R-anterior (p=0.011, p=0.033), L/R-ventral (p=0.0125, p=0.012), L/R-intralaminar (p=0.006, p=0.046), L/R-medial (p<0.001, p=0.027), and L-posterior fields (p=0.013) was associated with worse performance on WAIS-IV Digit Span Forward Test. Decreased FA in L/R-anterior fields predicted worse performance on DKEFS Letter Fluency Test (p=0.048, p=0.013). Increased MD in L/R thalamus (p=0.004, p=0.021) and L/R-posterior fields (p=0.022, p=0.011) was associated with worse performance on BVMT Total Recall Test. <h3>Conclusion</h3> Microstructural injury to the thalamus and thalamic nuclei is dose-dependent and progressive over time. Injury to these regions is associated with worse performance in executive function, visuospatial memory, and auditory attention. Dose avoidance efforts in these regions should be explored.