Capsular invasion and lymphovascular invasion (LVI) are key microscopic features in differentiated thyroid cancer. Their relationship with TNM staging remains uncertain. This study assesses whether capsular invasion and LVI correlate with primary T stage, post-RAI residual tumor, locoregional nodal metastasis, and distant metastasis, including specific metastatic sites.

A retrospective analysis was conducted for DTC patients treated with radioactive iodine (RAI) therapy between 1 July and 15 November 2025. Patients without complete histopathology reports or post-therapy I-131 whole-body scans were excluded. Absence of documented invasion on the histopathology report was considered negative. Primary T stage (preoperative tumor size) was obtained from the histopathology report. Residual T after RAI was defined among patients who had received at least one prior RAI ablation, with post-therapy imaging used to evaluate residual functional thyroid tissue/tumor. All analyses of N and M included the full cohort of DTC RAI patients (from RAI-naïve to those with repeated RAI). Associations between capsular invasion, LVI, and outcomes were assessed using chi-square testing; subgroup analyses examined site-specific distant metastases (lung, bone, brain, liver).

Among 267 patients, capsular invasion showed no significant association with locoregional lymph node metastasis (p = 0.850) or distant metastasis (p = 0.395). LVI showed no significant association with locoregional lymph node metastasis (p = 0.097) but was strongly associated with distant metastasis (p = 0.001). Subgroup analysis demonstrated that LVI was significantly associated with lung (p = 0.013), bone (p = 0.002), brain (p = 0.042), and liver (p = 0.042) metastases. On primary T stage, both capsular invasion (p = 0.044) and LVI (p = 0.001) were significantly associated with tumor size. For residual T after RAI, capsular invasion was significantly associated with residual tumor size (p = 0.022), whereas LVI showed no significant association (p = 0.332).

In this cohort of RAI-treated DTC patients, LVI was a strong predictor of distant metastasis overall and of site-specific distant spread, while capsular invasion was not associated with metastasis. Both capsular invasion and LVI were associated with larger primary tumors, but only capsular invasion correlated with larger residual T after prior RAI. These findings suggest that routine reporting of LVI provides valuable prognostic information for distant metastatic risk, whereas capsular invasion may be more informative regarding local tumor burden and residual disease, offering additional insight for TNM-based risk stratification and patient management.