Abstract 6273: GGCT expression is a useful marker for the diagnosis of follicular thyroid carcinoma

Abstract Background: Follicular thyroid carcinoma (FTC) and follicular adenoma (FA) have similar histological findings. FTC is distinct from FA based on the presence of capsular or vascular invasion in the tissues, but it is difficult to confirm the diagnosis in some cases. In this study, we aimed to establish useful molecular maker for differential diagnosis between FTC and FA. Methods: This study included FTC (n = 32), FA (n = 64), and follicular tumors of uncertain malignant potential (FT-UMP, n = 5). Invasive front tissue and tumor center tissue from 3 cases of FTC were collected using a laser microdissection system; RNA was extracted, followed by microarray analysis to examine molecules that were commonly highly expressed in the invasive front. Subsequently, immunohistochemical staining of gamma-glutamyl cyclotransferase (GGCT) was performed. For assessment of GGCT expression, the intensity of the nuclear staining was stratified using a four-tiered scale (0 to 3) and percentage of positive cells (0-100%) in the hotspot. Multiplying intensity score and percent of positive cells, an expression score ranging from 0 to 300 was calculated. The Ki-67 labeling index was examined in 20 cases of FTC, 25 cases of FA, and 4 cases of FT-UMP from the same cohort. We counted at least 2,000 tumor cells in the hot spot for the assessment of Ki-67 labeling index. Results: Microarray analysis revealed GGCT exhibited high expression levels at the invasive front of FTC. In immunohistochemical assessment, GGCT expression scores were significantly higher in FTC than in FA (118.5 ± 51.4 vs. 57.3 ± 34.7, p < 0.0001). The median Ki-67 labeling index were 3.9% (range 0.2 - 16.1) for FTC and 2.4% (range 0.3 - 7.0) for FA. FTC showed a higher Ki-67 index than FA (Wilcoxon rank sum test, p = 0.0444). With GGCT expression score, applying a cutoff value of 101.1, the distinction between FTC and FA resulted in a sensitivity of 68.8% and a specificity of 87.5% (AUC: 0.832). With Ki-67 labeling index, applying a cutoff value of 4.0%, the distinction between FTC and FA resulted in a sensitivity of 50.0% and a specificity of 80.0% (AUC: 0.677). The GGCT expression score was positively related with the Ki-67 labeling index in the FTC cases. (Spearman's ρ = 0.5293, p = 0.0164). There were no significant associations between GGCT expression scores and clinicopathological parameters, including age, gender, tumor diameter, pT stage, pN stage, pattern of capsular invasion, number of foci of vascular invasion, and the presence of distant metastasis. Conclusions: GGCT is a potential marker for differentiating FTC from FA. The GGCT expression of FTC may contribute to estimating the invasive activity and proliferative activity. Citation Format: Toshiyuki Mitsuhashi, Sachiko Ogasawara, Masamichi Nakayama, Reiichiro Kondo, Jun Akiba, Kenta Murotani, Takeharu Ono, Fumihiko Sato, Hirohito Umeno, Hirohisa Yano. GGCT expression is a useful marker for the diagnosis of follicular thyroid carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6273.