Significant Correlation of Dynamic Pan-Immune Inflammation Value and Magnetic Resonance Imaging Response in Patients with Locally Advanced Rectal Cancer Treated with Total Neoadjuvant Therapy

Background: Accurate identification of complete or near-complete responders after total neoadjuvant therapy (TNT) is critical for selecting candidates for non-operative management in locally advanced rectal cancer (LARC). While static inflammatory biomarkers have been widely investigated, the predictive value of dynamic changes in systemic inflammation during TNT remains unclear. This study evaluated whether changes in the Pan-Immune Inflammation Value (PIV) correlate with magnetic resonance imaging tumor regression grade (mrTRG) and regrowth risk. Methods: Seventy-three patients with LARC treated with TNT between 2018 and 2024 were retrospectively analyzed. Patients were classified as complete/near-complete responders (C-NCRG; mrTRG1–2) or residual disease group (RDG; mrTRG3–5). The PIV was calculated as (platelet × monocyte × neutrophil)/lymphocyte at treatment initiation (first-PIV), three weeks after completion of chemotherapy (last-PIV), and at regrowth (Rg-PIV). Dynamic changes were defined as Δ-PIV (last-PIV minus first-PIV) and ΔRg-PIV (Rg-PIV minus first-PIV). Receiver operating characteristic analysis identified optimal cutoffs, and logistic regression assessed independent associations. Results: The baseline PIV did not differ between groups. Last-PIV and Δ-PIV were significantly lower in the C-NCRG compared with RDG (p = 0.006). A Δ-PIV cutoff of −36.9 discriminated responders (AUC = 0.705; sensitivity 53.7%; specificity 84.4%) and remained independently associated with MRI response in multivariate analysis. In the non-operative management cohort, ΔRg-PIV was significantly lower in patients without regrowth (p = 0.001), with a cutoff of −77.72 (AUC = 0.794; sensitivity 88.9%; specificity 66.7%). Overall survival was superior in the C-NCRG (p = 0.01). Conclusions: A dynamic reduction in the PIV during TNT is significantly associated with favorable MRI response and lower regrowth risk in LARC. PIV dynamics may serve as a noninvasive complementary biomarker to support response assessment and stratification in organ-preservation strategies.