Single‐Cell RNA Sequencing Reveals the Heterogeneity in Differentiation Trajectory and Tumor Microenvironment Leading to More Aggressive Phenotypes of Papillary Thyroid Cancer in Children and Young Adult Patients

Abstract The tumor ecosystem heterogeneity of papillary thyroid carcinoma (PTC) is poorly characterized in children and young adult patients (CAYA‐PTC). In this study, single‐cell RNA sequencing is used to profile transcriptomes from the paratumor and tumor tissues of 11 patients. Compared to adult, CD4T_Tfh and CD8T_Tex cells are significantly more prevalent in CAYA‐PTC patients. Three phenotypes are identified within the thyrocytes through differentiation trajectory analysis, including normal, BRAF‐like, and Fusion‐like. Notably, the data reveal that CAYA‐PTC patients lack the “mild‐state (BRAF‐like)” malignant thyrocyte population. This variation in differentiation states indicates that PTC cells in CAYA patients rapidly develop into invasive and metastatic forms, whereas in adult patients, this progression occurs gradually over a longer period. Additionally, extracellular matrix cancer‐associated fibroblasts (emCAFs_LAMP5) interact with endothelial cells and thyrocytes, promoting tumor angiogenesis and metastasis more prominently in CAYA patients. Fibroblast activation protein (FAP) expression is high in emCAF_LAMP5 and positively correlated with LAMP5 in CAYA‐PTC tissues. Consequently, 68Ga‐FAPI‐PET emerges as a promising diagnostic method for CAYA patients who are not effectively diagnosed by traditional 18F‐FDG‐PET. Collectively, the findings provide insight into the CAYA‐PTC ecosystem that suggests distinct diagnostic, prognostic, and therapeutic implications compared to adults.