Discussion

Intra-nodular vascularity in isolation did not appear to be a reliable predictor of thyroid malignancy. This supports the majority of studies in the world literature.^{Reference Yang and Fried7,Reference Remonti, Kramer, Leitao, Pinto and Gross8,Reference Khadra, Bakeer, Hauch, Hu and Kandil9} Notably, this group had a 50.4 per cent malignancy rate which at first glance appears more significant than the commonly quoted 25–30 per cent malignancy rate for thy3f nodules, many of which will undergo diagnostic excision.^{Reference Alexander, Rudd, Walker, Wong, Lunt and Hamakarim10,Reference Chng, Kurzawinski and Beale11} However, in our series, all nodules had one or more additional features suspicious enough to warrant surgery (e.g. solid nodule, equivocal echogenic foci, disrupted peripheral calcification, microcalcification, presence of pathological lymph nodes, hypoechogenicity, ‘taller-than-wide’). The only difference being that in one group, the nodules additionally demonstrated intra-nodular flow. This is evidenced by the fact that the control group also had a reasonably high malignancy rate of 42.2 per cent.

Because the incidence of thyroid nodules is high but the prevalence of thyroid malignancy is relatively low, there has been considerable interest in determining which sonographic features confer the greatest accuracy for reliably predicting malignancy. A recent meta-analysis by Khadra et al. in 2016 showed that intra-nodular flow on Doppler sonography did not accurately predict malignancy.^{Reference Khadra, Bakeer, Hauch, Hu and Kandil9} Moreover, other findings such as nodule size, calcifications, echogenicity, margins and shape also did not appear to accurately predict malignancy when looked at individually. Indeed, the authors noted that no individual sonographic feature reliably differentiated between benign and malignant thyroid nodules.

However, several studies suggest that interpreting intra-nodular vascularity in combination with other suspicious ultrasound findings improves the predictive value.^{Reference Rago, Vitti, Chiovato, Mazzeo, De Liperi and Miccoli12,Reference Sebag, Vaillant-Lombard, Berbis, Griset, Henry and Petit13} For example, Sebag et al.^{Reference Sebag, Vaillant-Lombard, Berbis, Griset, Henry and Petit13} noted that the combination of hypoechogenicity, microcalcifications and intra-nodular vascularity increased the specificity and positive predictive value for detecting malignancy. Unfortunately, attempts at multivariate analyses in our study proved unreliable because of collinearity, making interpretation of potential associations difficult and inconclusive.

Our intention was to analyse the histopathology of all nodules removed for suspicion of thyroid cancer, comparing them with their ultrasound reports. We divided the findings into two groups, namely those with and those without central vascularity to study the effect this finding had on malignancy rate. We found it made no difference, and we note that the North American ultrasound classification system, Thyroid Imaging, Reporting and Data System,^{Reference Tessler, Middleton, Grant, Hoang, Berland and Teefey14} does not include this variable. The authors quote a Brazilian study by Rosario et al. ^{Reference Rosario, Silva, Borges and Calsolari15} which found that benign and malignant nodules were not reliably differentiated using colour Doppler ultrasound.

From the data in our study, because we analysed all nodules removed for suspicion of thyroid cancer, central vascularity in isolation was very rarely the sole factor in upgrading the U classification from U2 to U3, accounting for 4 out of 65 U3 nodules (6.2 per cent), only 1 of which was malignant. This is likely to be a relatively uncommon occurrence given the multiple factors conferring a U3 classification (see Table 1).

Following on from this, we speculated whether central vascularity should be disregarded in the likely rare occasions where it is the only abnormal finding. Although we are unable to provide a definitive statement on this, it would appear from our study that when assessing suspicious nodules (U3–U5), intra-nodular or central vascularity does not appear to confer any additional risk for malignancy. This finding supports the fact that the Thyroid Imaging, Reporting and Data System does not include this observation for malignancy risk stratification, and this should perhaps be considered in future British Thyroid Association guidelines.

To conclude, our study has shown that intra-nodular vascularity is not a statistically significant predictor of thyroid nodule malignancy in isolation, but the literature suggests that the predictive value is enhanced when combined with other parameters. Further larger studies including analyses of different combinations of ultrasound findings will aid identification of the most reliable sonographic patterns for prediction of malignancy.