Tumour volume is a predictor of lymphovascular invasion in differentiated small thyroid cancer.

Objectives: For small thyroid cancers (≤2 cm), tumour volume may better predict aggressive disease, defined by lymphovascular invasion (LVI) than a traditional single measurement of diameter. We aimed to investigate the relationship between tumour diameter, volume and associated LVI.

Methods: Differentiated thyroid cancers (DTC) ≤ 2 cm surgically resected between 2007 and 2016 were analysed. Volume was calculated using the formula for an ellipsoid shape from pathological dimensions. A 'larger volume' cut-off was established by receiver operating characteristic (ROC) analysis using the presence of lateral cervical lymph node metastasis (N1b). Logistic regression was performed to compare the 'larger volume' cut-off to traditional measurements of diameter in the prediction.

Results: During the study period, 2405 DTCs were surgically treated and 523 met the inclusion criteria. The variance of tumour volume relative to diameter increased exponentially with increasing tumour size; the interquartile ranges for the volumes of 10, 15 and 20 mm diameter tumours were 126, 491 and 1225 mm^3, respectively. ROC analysis using volume to predict N1b disease established an optimal volume cut-off of 350 mm³ (area under curve = 0.59, P = 0.02) as 'larger volume'. 'Larger volume' DTC was an independent predictor for LVI in multivariate analysis (odds ratio (OR) = 1.7, P = 0.02), whereas tumour diameter > 1 cm was not (OR = 1.5, P = 0.13). Both the volume > 350 mm³ and dimension > 1 cm were associated with greater than five lymph node metastasis and extrathyroidal extension.

Conclusion: In this study for small DTCs ≤ 2 cm, the volume of >350 mm³ was a better predictor of LVI than greatest dimension > 1 cm.