The role of Tc-99m-2-Methoxy-Isobutyl-Isonitrile Single Photon Emission Computed Tomography in visualizing anterior mediastinal tumor and differentiating histologic type of thymoma.

OBJECTIVE To evaluate the usefulness of (99m)Tc-2-Methoxy-Isobutyl-Isonitrile Single Photon Emission Computed Tomography (MIBI-SPECT) for assessing anterior mediastinal mass, and distinguishing the grade of malignancy of thymic epithelial tumors to offer guidance for decision making. METHODS From January 2006 to November 2009, 31 consecutive patients with an anterior mediastinal mass at CT were enrolled. All patients underwent (99m)Tc-MIBI-SPECT before invasive diagnostic procedures and/or surgical resection. The uptake of the mediastinal tumor (T) was compared with the normal tissue (N) and the T/N ratio was calculated to define the metabolic activity of the lesions. Patients were divided into six groups according histologic patterns, which were then correlated to the T/N value: benign tumor (BT), lymphoma (LYM), other malignant tumor (OMT), and low-risk thymoma (LRT) including types A, AB, and B1, high-risk thymoma (HRT) including types B2 and B3, and thymic carcinoma (TC). RESULTS There were five BT, eight LRT (four = A, two = AB, and two = B1), four HRT (three = B2 and one = B3), four thymic carcinoma, six LYM and four OMT. The T/N value of BT, LRT, HRT, CT, LYM, and OMT was 1.3 ± 0.3, 1.3 ± 0.2, 1.8 ± 0.3, 2.7 ± 0.5, 2.8 ± 0.1, and 2.9 ± 0.2, respectively. The T/N ratio of BT and of LRT was significantly lower than that of HRT, of CT, of LYM, and of OMT (p < 0.05), while there is no significant difference of MIBI uptake between BT and LRT. MIBI uptake in HRT was significantly lower than that in TC, LYM, and OMT (p < 0.05), whereas no significant difference was found between the different types of malignant lesions (TC, LYM, and OMT). Regarding thymoma, the degree of MIBI accumulation significantly increased as the World Health Organization (WHO) classification shifted from type A to type B and to TC. Yet, the T/N value of stages I and II was significantly lower than that of stages III and IV. The size of the lesion and the presence of myasthenia were not correlated with MIBI uptake. CONCLUSION (99m)Tc-MIBI-SPECT seems to be useful in the evaluation of malignancy in anterior mediastinal mass, and is significantly correlated with the WHO classification and the Masaoka stage. Thus, this technique may add further information to morphological studies for decision making.