Inflammation within tumor microenvironments has been correlated to numerous malignancies. This study aims to explore its significance in thyroid cancer (TC).

A retrospective analysis of the records of 157 TC and 40 benign nodules involved initial univariate analysis. The neutrophils to lymphocytes ratio (NLR), platelets to lymphocytes ratio (PLR), lymphocytes to monocytes ratio (LMR), and platelet count × neutrophil count / lymphocyte count (Systemic Immune-inflammation Index, SII) were related to some TC characteristics and to the number and location of involved lymph nodes. Chi-square or Fischer's exact tests for categorical variables and Student's t-tests for continuous ones were used. A 1:1 propensity score matching balanced TC and benign nodules groups based on age, sex, and tumor size. Post-matching, a multivariate logistic model integrated sex, age, N-L Ratio, and SII Index. Statistically significant immune index values underwent ROC curve analysis for determining cut-offs. Among the 157 TC, median test and density plots were performed.

The SII index emerged as a predictor of malignancy in both univariate and multivariate analyses (p-value<0.01). The ROC curve indicated a cut-off SII value of 465.71, (specificity=58% [95% CI: 0.43-0.73]; sensitivity=80% [95% CI: 0.68-0.93]). Median SII Index values for T-tumor size of 1 and >1 were 522.8 and 654.8, respectively (p-value=0.016). When Central Lymph Nodes (CLN) involvement was considered (CLN = 0 vs. CLN>0), median SII values were 530.7 and 1121.7, respectively (p-value=0.011).

The SII index appears to be a valuable tool in the suspicion of TC, showing significant correlations with malignancy, tumor size and CLN involvement.