Thymoma is a n uncommon cancer of the thymus. Limited information is available about sensitivity and specificity of PET for thymojma. Unfortunately, baseline thymic uptake is common. One study examined thymic uptake at 18F-FDG PET in 94 patients ranging from 18 to 29 years of age and found that 32 of these patients exhibited normal physiologic thymic uptake. The criteria for “normal” in these patients included a normal thymus identified at CT, absence of clinical symptoms of thymus-related disease, and absence of mediastinal tumor at follow-up ranging from 6 to 69 months.
Sasaki et al found a mean SUV of 7.2 ± 2.9 for patients with thymic carcinoma (n = 9). This value was significantly greater than the values found for invasive thymoma (3.8 ± 1.3) and noninvasive thymoma (3.0 ± 1.0). By using an SUV of 5.0 as a cutoff, the authors achieved reasonable sensitivity (84.6%), specificity (92.3%), and accuracy (88.5%) in differentiating thymic carcinoma from thymoma. They found no statistically significant difference in SUV between invasive and noninvasive thymomas, a finding that is consistent with the histologic similarity of the two tumors. This finding is also consistent with earlier work by Liu et al, which failed to demonstrate a significant difference in 18F-FDG uptake between different stages of thymoma. Brink et al also presented one case of thymic carcinoma with an SUV of 9.6. On the basis of these limited data, it appears that 18F-FDG PET will prove to be effective in differentiating thymic carcinoma from other entities within the thymus but will likely prove equivocal in differentiating invasive and noninvasive thymoma from each other and from thymic hyperplasia. More study is clearly required.
Brett Ferdinand, Pramod Gupta, and Elissa L. Kramer Spectrum of Thymic Uptake at 18F-FDG PET RadioGraphics 2004 24: 1611-1616.
M. M. Abouzied, E. S. Crawford, and H. A. Nabi
18F-FDG Imaging: Pitfalls and Artifacts
J. Nucl. Med. Technol., September 1, 2005; 33(3): 145 – 155.