The significance of unrecognized histological high-risk features on response to therapy in papillary thyroid carcinoma measuring 1-4 cm: implications for completion thyroidectomy following lobectomy.

Background: Although lobectomy is an alternative to total thyroidectomy (TT) for 1-4 cm papillary thyroid carcinoma (PTC) without high-risk features (HRFs) such as aggressive histology, vascular invasion, lymphovascular invasion (LVI), microscopic extrathyroidal extension, positive margin, nodal metastasis >5 mm and multifocality, these HRFs are not recognized until after surgery. Therefore, the chance of completion TT being required following lobectomy might be high. We evaluated the frequency of unrecognized HRFs and how they affected the response to therapy following TT and radioiodine (RAI).

Methods: Altogether, 1513 patients were analysed. Only 1-4 cm PTCs without recognizable HRFs were included. For response-to-therapy evaluation, only patients who had TT and post-RAI-stimulated thyroglobulin were analysed. Patients without an excellent response were defined as having 'incomplete response'. A multivariate analysis for incomplete response was performed.

Results: Of the 600 patients eligible for lobectomy, 257 (42·8%) had ≥1 unrecognized histological HRF before surgery. The prevalence of unrecognized HRFs was similar between 1-2 cm and >2-4 cm PTCs (P = 0·393). Of the 330 patients eligible for response-to-therapy evaluation, 260 (78·8%) had an excellent response while 70 (21·2%) had an incomplete response. LVI was the only independent unrecognized HRF for incomplete response (P = 0·021).

Conclusions: The prevalence of unrecognized histological HRFs under the current recommendations is relatively high among 1-4 cm PTCs. Among the unrecognized histological HRFs, LVI was the only one which independently associated with an incomplete response (i.e. posing an increased risk of persistent/recurrent disease after curative surgery). These findings may have implications for patients who undergo lobectomy for 1-4 cm PTCs with no clinically recognizable HRFs under the current recommendations.