Supplementary file 1_Anatomical location matters: risk modelling and surgical outcomes in central lymph node metastasis of isthmus PTC.docx

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.</p>Conclusions<p>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.</p>