Monocyte-Driven Systemic Biomarkers and Survival After Pulmonary Metastasectomy in Metachronous Lung-Limited Oligometastatic Disease: A Retrospective Single-Center Study

<b>Background/Objectives:</b> Metachronous lung-limited oligometastatic disease represents a biologically heterogeneous state in which patient selection for pulmonary metastasectomy remains challenging. While systemic inflammation-nutrition indices have shown prognostic value across malignancies, their relevance in this strictly defined surgical setting is not well established. <b>Methods:</b> We conducted a retrospective single-center cohort study including 109 patients with isolated metachronous pulmonary recurrence who underwent curative intent R0 metastasectomy between September 2015 and April 2024. Preoperative systemic biomarkers, including neutrophil-to-lymphocyte ratio (NLR), systemic immune-inflammation index (SII), systemic inflammation response index (SIRI), pan-immune-inflammation value (PIV), and monocyte-to-albumin ratio (MAR), were evaluated using receiver operating characteristic (ROC) analysis and multivariable Cox models to determine their association with overall survival (OS) and progression-free survival (PFS). Clinicopathological variables, such as lymph node involvement and metastatic burden, were incorporated into the adjusted models. <b>Results:</b> The median age of the cohort was 61 years (range, 29-82 years), and the sex distribution was balanced (48.6% female and 51.4% male), with 62.4% of patients being younger than 65 years. Among the systemic indices evaluated, monocyte-weighted biomarkers demonstrated the strongest prognostic performance. The MAR showed the highest discriminative ability for mortality (AUC, 0.749; <i>p</i> < 0.001), followed by the SIRI (AUC, 0.682; <i>p</i> = 0.007). In multivariable analyses, MAR independently predicted OS (<i>p</i> = 0.043) and PFS (<i>p</i> = 0.023), while SIRI independently predicted PFS (<i>p</i> = 0.043). Lymph node involvement remained the dominant adverse prognostic factor for both outcomes (<i>p</i> < 0.001); however, monocyte-weighted indices provided additional prognostic value beyond conventional anatomic criteria. <b>Conclusions:</b> Preoperative SIRI and MAR capture host immune-metabolic states that are relevant to postoperative trajectories and may refine risk stratification in candidates for pulmonary metastasectomy. These readily obtainable markers warrant prospective validation within biologically integrated selection frameworks.