PET imaging of anterior mediastinal masses

Objectives To assess the feasibility of F18-FDG PET in evaluating anterior mediastinal masses and correlate with aggressiveness of tumor and histopathology.

Methods We retrospectively evaluated F18 FDG PET-CT scans of 33 patients with anterior mediastinal mass for pattern of uptake, invasion to surrounding structures, presence of metastasis, tumor necrosis, and quantitative parameters [Maximum SUV (SUVm) as well as the mean SUVmax)]. These finding were correlated with histopathology.

Results Our study included 12 thymic neoplasms, 15 lymphomas (7 diffuse large B cell, 5 Hodgkin’s, and 3 lymphoblastic varieties), 3 adenocarcinoma from lung primary, 1 rhabdomyosarcoma, 1 neuroendocrine tumor, and 1 germ cell tumor. 4 low risk thymomas (WHO types A, AB, and B1) had mean SUVm 2.4 (range 1.3-3.2), 3 high risk thymomas (type B2 and B3) had mean SUVm7.1(range 3.9-9.3), 5 Thymic Cancers (type C) had mean SUVm 20.7 (range 9.6-42). 3 thymic cancer and 1 high risk thymomas had necrosis on PET, and no necrosis was noted on low risk thymomas. 7 large cell lymphomas (mean SUVm 20.2, range 8.3-32), 5 Hodgkin’s lymphomas (mean SUVm 11.7, range 3.1 -16), 3 lymphoblastic lymphoma/ leukemia (mean SUVm 6.5, range 5.7-8), 1 rhabdomyosarcoma (SUVm 12.7), 1 germ cell tumor (SUVm 5), 1 neuroendocrine tumor (SUVm 14.6), 3 mets from lung primary (SUVm 7.7, Range 8.3-15).

Conclusions Low risk thymomas (WHO class A, AB and B1) and benign tumors had low FDG uptake (mean SUVmax <3.2). Aggressive neoplasms, i.e. invasive thymomas (B2 and B3), thymic cancers (type C), lymphomas, neuroendocrine, rhabdomyosarcoma, mets from lung primary; all had intense uptake (SUVm>5). PET was also helpful in diagnosing distant metastases in 9/30 patients with aggressive tumors. We conclude 18 FDG PET/CT is of a significant value in differentiating between aggressive vs non-aggressive neoplasm, but was of limited value in differentiating between types of malignant neoplasms. Thus, PET imaging is complementary to other imaging modalities as well as to the histopathology.