Accuracy of World Health Organisation‐grade parameters (necrosis and mitotic activity) and foci of vascular invasion in predicting prognosis of papillary thyroid carcinoma. A case–control validation study

Aims Tumour necrosis and/or increased mitoses define high‐grade papillary thyroid carcinoma (PTC). It is unclear whether angioinvasion is prognostic for PTC. Cut‐offs at five or more mitoses/2 mm 2 and four or more angioinvasive foci have been empirically defined based upon data from all forms of aggressive non‐anaplastic thyroid carcinomas. Performance of tumour necrosis, mitoses and vascular invasion in predicting distant metastases when specifically applied to PTC is undefined. Methods We analysed 50 consecutive PTC cases with distant metastases (DM‐PTC): 16 synchronous and 34 metachronous. A total of 108 non‐metastatic PTC (N‐DM‐PTC, 15.0‐year median follow‐up) were used as controls. Invasive encapsulated follicular variant PTC was excluded. Necrosis, mitoses and angioinvasion were quantified. Receiver operating characteristics (ROC) and area under the curve (AUC) analyses determined best sensitivity and specificity cut‐offs predictive of distant metastases. Results Metastases correlated with necrosis (any extent = 43.8% all DM‐PTC, 53.1% metachronous DM‐PTC versus 5% N‐DM‐PTC; P < 0.001), mitoses ( P < 0.001) and angioinvasion ( P < 0.001). Mitoses at five or more per 2 mm 2 was the best cut‐off correlating with distant metastases: sensitivity/specificity 42.9%/97.2% all DM‐PTC (AUC = 0.78), 18.8%/97.2% synchronous DM‐PTC (AUC = 0.63), 54.6%/97.2% metachronous DM‐PTC (AUC = 0.85). Angioinvasive foci at five or more was the best cut‐off correlating with distant metastases: sensitivity/specificity 36.2%/91.7% all DM‐PTC (AUC = 0.75), 25%/91.7% synchronous DM‐PTC (AUC = 0.79) and 41.9%/91.7% metachronous DM‐PTC (AUC = 0.73). Positive/negative predictive values (PPV/NPV) were: necrosis 22.6%/98.2%; five or more mitoses 32.3%/98.2%; five or more angioinvasive foci 11.8%/97.9%. After multivariable analysis, only necrosis and mitotic activity remained associated with DM‐PTC. Conclusion Our data strongly support PTC grading, statistically validating World Health Organisation (WHO) criteria to identify poor prognosis PTC. Angioinvasion is not an independent predictor of DM‐PTC.