Usefulness of positron‐emission tomography for predicting the World Health Organization grade of thymic epithelial tumors

BACKGROUND: It is often difficult to distinguish between thymoma and thymic carcinoma by preoperative radiological tests. While there have been some reports that the maximum standardized uptake value (SUV(max)) in positron emission tomography‐computed tomography (PET‐CT) is useful to this end, no large‐scale analysis has been performed. We therefore analyzed the usefulness of the SUV(max) and tumor size (TS) for differentiating thymic epithelial tumors. METHODS: From 2011 to 2019, 129 patients with thymic epithelial tumor who underwent PET‐CT before surgical treatment were enrolled. The relevance of the SUV(max) to the World Health Organization (WHO) histological type was assessed. To reduce the impact of the TS, the ratio of the SUV(max) to the TS was also investigated. RESULTS: A total of 99 thymoma cases and 30 thymic carcinoma cases were enrolled into the study. The SUV(max) and SUV(max)/TS of thymic carcinoma were significantly higher than those of thymoma (SUV(max): 7.7 ± 3.4 vs. 3.3 ± 1.3, p < 0.01; SUV(max)/TS: 1.5 ± 0.7 vs. 0.6 ± 0.4, p < 0.01). Focusing on the patients with a moderate SUV(max) of ≤5 (84 thymoma and 4 thymic carcinoma), the SUV(max)/TS values of thymic carcinoma were still significantly higher than those of thymoma (1.6 ± 0.8 vs. 0.6 ± 0.4, p < 0.01). CONCLUSIONS: PET‐CT might provide significant information for differentiating images of thymoma and thymic carcinoma. We experienced several cases of thymic carcinoma with a moderate SUV(max) of ≤5, and SUV(max)/TS was considered a useful parameter for differentiating such cases.