Detection of Thymoma on 99mTc MIBI Scintigraphy: Revisiting the Past

We report a case of a 68-year-old man with history of inferior wall myocardial infarction 12 years back, who presented to the cardiologist with recent onset exertional dyspnea for past couple of months. He was then referred to our nuclear cardiology laboratory for Tc-99m MIBI (Methoxy Isobutyl Isonitrile) myocardial perfusion SPECT (MPS) to look for any inducible ischemia. He underwent treadmill exercise stress on Bruce protocol and was imaged. The study revealed scarred myocardium in the mid and proximal inferior wall, with mild stress induced ischemia in the inferolateral wall. Surprisingly, accumulation of radiotracer was also noted in the mediastinum just above the heart. To further evaluate this abnormal extra cardiac activity, the patient underwent CECT thorax, which showed a soft tissue density anterior mediastinal mass with necrosis and calcification. Based upon CECT features two differential diagnoses were considered: germ cell tumor (GCT) and thymoma. Later, the diagnosis was confirmed to be thymoma on biopsy.

Thymoma is a rare tumor affecting mainly adults in their fifth and sixth decade. Most of the thymomas arise from the anterior mediastinum.[1] Pathologically they can be either benign (type A, AB, B1), high risk (B2 and B3), or malignant (type C).[2] However, Tc-99m MIBI uptake has been reported in both malignant and benign thymomas.[3456] And it has been noted that MIBI uptake in thymoma directly correlates with its pathological grading that is more intense is the uptake higher is the grading.[6] The pathophysiology behind MIBI accumulation is increased number of mitochondrias in the hypercelluar matrix of the thymomas.[3] The incidence of these extra cardiac MIBI accumulations like thymoma have been reported in around 1% of patients undergoing MPS.[7] So it is of utmost importance to look for such abnormal tracer distributions during MPS reporting.