MRI of a recurrent adenoid cystic carcinoma of the trachea, treated with fast neutron therapy.

Adenoid cystic carcinoma (ACC) of the trachea is a rare tumour, which responds well to high linear energy transfer radiation, such as neutron therapy. Both CT and MRI are useful for its diagnosis, with MRI being superior at determining the extent of disease and perineural involvement. Identification of these disease characteristics with MRI helps to determine lesion resectability and decide on the most appropriate treatment strategies. MRI is also useful in the differentiation of post-radiation change from disease recurrence, with post-radiation change displaying low signal intensity compared with tumour recurrence displaying intermediate to high signal intensity. Furthermore, MR diffusion-weighted imaging can be useful in the distinction between the two. We present a case of tracheal ACC treated with fast neutron therapy and followed with MRI.