We sought to identify candidate biomarkers for early brain metastasis (BM) recurrence in patients who underwent craniotomy followed by adjuvant stereotactic radiosurgery. RNA sequencing was performed on eight resected brain metastasis tissue samples and revealed B-cell related genes to be highly expressed in patients who did not experience a distant brain failure and had prolonged overall survival. To translate the findings from RNA sequencing data, we performed immunohistochemistry to stain for B and T cell markers from formalin-fixed parffin-embedded tissue blocks on 13 patients. CD138 expressing plasma cells were identified and quantitatively assessed for each tumor sample. Patients’ tumor tissues that expressed high levels of CD138 plasma cells (N = 4) had a statistically significant improvement in OS compared to low levels of CD138 (N = 9) (p = 0.01). Although these findings are preliminary, the significance of CD138 expressing plasma cells within BM specimens should be investigated in a larger cohort. Immunologic markers based on resection cavity analysis could be predictive for determining patient outcomes following cavity-directed SRS.