Anatomical location matters: risk modelling and surgical outcomes in central lymph node metastasis of isthmus PTC

Background: Isthmus papillary thyroid carcinoma (iPTC) represents a distinct clinical entity with a higher propensity for central lymph node metastasis (CLNM) and greater local aggressiveness compared to lobe-confined PTC. Despite increasing recognition of its unique risk profile, controversy persists regarding optimal surgical management, particularly for small or indolent tumours. Methods: This retrospective cohort study included 319 patients with histologically confirmed iPTC who underwent comprehensive ultrasonographic assessment and thyroid surgery from February 2019 to June 2024 at a high-volume tertiary centre. Lobe-confined PTC cases were matched as controls using propensity score methodology. Clinicopathological features, risk factors for CLNM, surgical outcomes, and recurrence patterns were analysed using univariate and multivariate analyses and ROC/AUC modelling. A predictive nomogram for CLNM was developed and internally validated using 1,000 bootstrap resamples with optimism correction. Results: The iPTC cohort exhibited a significantly higher CLNM burden than matched lobe-confined PTC controls, independent of tumour size or multiplicity. Multivariate analysis identified male sex, increased isthmus tumour diameter, and calcification as independent preoperative predictors of CLNM. The derived nomogram demonstrated acceptable discrimination (optimism-corrected AUC 0.722, 95% CI 0.645–0.802) and internal calibration. Surgical extent (total thyroidectomy versus less extensive resection) was associated with increased perioperative morbidity but did not improve early recurrence rates in selected cases. There were no lymph node or distant recurrences on intermediate-term follow-up. Conclusions: Isthmus location independently increases central nodal metastatic risk in PTC. Risk modelling with simple clinical and ultrasonographic factors quantifies the probability of central lymph node metastasis and highlights the trade-off between more extensive thyroid resection and increased perioperative morbidity. We did not observe an early recurrence or survival advantage associated with more extensive surgery in this cohort, within the limits of intermediate follow-up and routine central dissection; instead, they support individualised, risk-adapted decision-making that requires validation in prospective outcome studies.