CT and MR imaging of thyroid carcinoma showing thymus-like differentiation (CASTLE): a report of ten cases.

Objective: To study the CT and MR imaging features of carcinoma showing thymus-like differentiation (CASTLE) and to raise awareness of this rare thyroid tumour. Methods: The imaging appearances of 10 CASTLE tumours confirmed by surgical pathology were retrospectively reviewed and correlated with clinical and histological findings. Results: Seven patients with newly diagnosed and three patients with recurrent tumours were identified (six males and four females). CASTLE tumours were commonly located in the lower neck between the inferior pole of the thyroid and the upper mediastinum. The average tumour size was 4.2cm (range: 2.5-6 cm). On plain CT scans, most tumours were ill-defined nodular masses of uniform density. After enhancement, most cases showed mild enhancement, while heterogeneous enhancement could be seen in more than half the cases. On the MR images, tumours presented with homogeneous isointensity on T-1 weighted images and they appeared to be slightly hyperintense on T-2 weighted images. On post-contrast images, marked enhancement was seen in two patients, and heterogeneous enhancement was seen in three cases. Aggressive local infringements mainly included the ipsilateral strap muscle, tracheoesophageal groove area and tracheal wall. The specimens stained positively for CD5 and CD117, indicating thymic differentiation. Conclusion: CASTLE has no characteristic imaging features when compared with other thyroid nodules, except for its unique location in the lower neck between the inferior pole of the thyroid and the upper mediastinum. CD5- and CD117-specific immunoreactivity is useful for diagnosis. Advances in knowledge: We reported 10 cases of CT and MR images illustrating the features of CASTLE, and we raised the level of awareness of this rare malignant thyroid tumour.