Impact of microscopic margin involvement on recurrence of papillary thyroid carcinoma

Introduction: A positive surgical margin (PSM) is identified in about 10 % of patients with thyroid cancer. Macroscopic PSM indicates overt residual cancer, which can cause remnant tumor regrowth or metastasis, leading to worse outcomes. However, most patients with PSM have only microscopic margin involvement, whose prognostic implications remain unclear. This study investigated the impact of microscopic PSM on recurrence in patients with papillary thyroid carcinoma (PTC). Materials and methods: Between 2009 and 2023, 1495 patients who underwent total thyroidectomy for PTC were enrolled. A 1:3 propensity score matching was performed to adjust for potential confounders. The primary outcome measure was 5-year recurrence-free survival. Results: A microscopic PSM was found in 68 (4.5 %) patients and was associated with larger tumor size (p = 0.003), extrathyroidal extension (p < 0.001), multifocality (p = 0.001), and lymph node metastasis (p = 0.016). After a mean follow-up of 7.7 years, 37 patients experienced disease recurrence: 6 (9.2 %) in the microscopic-PSM group and 31 (2.2 %) in the clear-margin group. The 5-year recurrence-free survival rate was significantly worse in the microscopic-PSM group (90.1 % vs. 98.2 %; p = 0.003). A microscopic PSM increased the risk of recurrence by approximately fivefold (hazard ratio 4.958, 95 % confidence interval 1.996–12.31) compared with a clear margin. Conclusion: Microscopic margin involvement was associated with an increased risk of recurrence in patients with PTC. Patients with microscopic PSM require judicious treatment and follow-up strategies.