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Background A thyroidectomy can be performed via a cervical incision in most patients with retrosternal goiter. Aims To investigate the correlation between the volume of the mediastinal portion of the thyroid gland and the need for an extra-cervical approach for retrosternal goiter. Study design Diagnostic accuracy study. Methods The measurement of craniocaudal length and the volume of the mediastinal component of the thyroid gland on computerised tomography images was performed in 47 patients with retrosternal goiter. Of these 47 patients, 8 (17%) required an extra-cervical approach and were classified as group 1, and 39 (83%) patients that required a cervical incision were classified as group 2. Receiver operating characteristic analysis was performed to determine the cut-off value for the craniocaudal length and the volume of the mediastinal thyroid mass, which significantly correlated with an extra-cervical approach for retrosternal goiter. Results Reoperative surgery was significantly more frequent in group 1 than in group 2 (50% vs 13%; p=0.03). The craniocaudal length of the mediastinal thyroid gland was significantly longer in group 1 than in group 2 (77±11 mm vs 31±21 mm, respectively; p=0.0001). The volume of the mediastinal component was significantly larger in group 1 compared to group 2 (264±106 cm3 vs 40±41 cm3, respectively; p=0.0001). The receiver operating characteristic curve of craniocaudal length and the volume of the mediastinal component identified ≥66 mm and ≥162 cm3 as the cut-off values with the maximum accuracy, respectively. The craniocaudal length of the thyroid mass below the thoracic inlet ≥66 mm or a volume of the mediastinal portion ≥162 cm3 were significantly associated with an extra-cervical approach (p=0.0001). For predicting an extra-cervical approach, the sensitivity, positive predictive value and negative predictive value of the cut-off value for craniocaudal length was 87.5%, 64% and 97%, respectively. For predicting an extra-cervical approach, the sensitivity, positive predictive value and negative predictive value of the cut-off values for the mediastinal volume were 100%, 89% and 100%, respectively. Conclusion A thyroid volume of ≥162 cm3 extending below the thoracic inlet was a significant determining factor for an extra-cervical approach, with a negative predictive value for the extra-cervical approach of 100% for retrosternal goiter with smaller volumes. Further studies with an increased number of patients are needed to determine the value of volumetric analysis of retrosternal goiter to predict the need for an extra-cervical approach in retrosternal goiter.