Primary thyroid lymphoma: MR appearance.

Magnetic resonance imaging was evaluated in six patients with primary thyroid lymphoma. Five patients had a solitary localized tumor and one had multiple tumors. These masses showed homogeneous iso- or high intensity compared with the uninvolved thyroid tissue on T1-weighted images and homogeneous high intensity on T2-weighted images. Diffuse lobulated goiter with homogeneous intensity was seen in the sixth case, where the differentiation between lymphoma and Hashimoto thyroiditis was impossible. Extrathyroidal tumor extension occurred in two cases and cervical lymphadenopathy in one case. In five of six cases, identification and staging of primary thyroid lymphoma were possible with MR imaging. Magnetic resonance imaging was comparable to CT in the detection of tumors (five cases), extrathyroidal extension (two cases), and cervical lymphadenopathy (one case). In three cases, the pseudocapsule (a low-intensity band forming the border between tumor and thyroid tissue) was detected only by MR imaging. In one case, the discrimination between tumor and uninvolved thyroid gland was clearer on MR imaging than on CT.