Intrathyroid thymic carcinoma (ITTC), also referred to as carcinoma showing thymus-like elements (CASTLE), is a rare malignant neoplasm that accounts for 0.083 % of all primary malignant thyroid neoplasms in Japan and 0.15 % in China [1]. Miyauchi et al. first reported this condition as intrathyroidal epithelial thymoma in 1985 [2]. Four categories were reported by Chan and Rosai in 1991: ectopic hamartomatous thymomas, ectopic cervical thymomas, spindle epithelial tumors with thymic-like differentiation (SETTLE), and CASTLE [3]. CASTLE is a rare tumor that commonly occurs in the thyroid and soft tissues of the neck. They can also develop in the sublingual, parotid, and submandibular glands [[4], [5], [6]]. The World Health Organization (WHO) currently defines the CASTLE category of thyroid cancer as ITTC. Most reported ITTCs occur in Asia, especially in China and Japan, and their etiology is still unclear [7].

The histological and immunophenotypic characteristics of ITTC are identical to those of eutopic thymic squamous cell carcinoma (TSCC) [8]. The tumor cells demonstrate a solid, nested, or sheet-like pattern and express immunohistochemical (IHC) markers, such as CD5 and c-KIT (CD117) [[9], [10], [11], [12]]. ITTC does not express thyroid follicular epithelial cell markers, such as thyroid transcription factor (TTF1) or thyroglobulin (TG), indicating that its carcinogenesis is independent of thyroid follicular cells [13,14].

Owing to its rarity, there is still no sufficient understanding or consensus regarding the clinical treatment of ITTC. In the present study, the clinicopathological, therapeutic, and prognostic data of 22 patients with ITTC were retrospectively analyzed to improve our understanding of ITTC and provide a reference for the diagnosis and treatment of the disease. Importantly, we showed that immune checkpoint inhibitors targeting PD-L1 may be a therapeutic option for ITTC.