Carcinoma showing thymus‐like differentiation of the parotid gland: The brief report of cytomorphology and review of the literature

Abstract

Here, we report an extremely rare case of carcinoma with thymus‐like differentiation (CASTLE) of the parotid gland. CASTLE is a rare malignant epithelial tumor with thymic epithelial differentiation that arises in the thyroid gland or perithyroidal soft tissue. CASTLE of salivary gland origin is rare, with only nine published case reports to date (reported as “CASTLE” or “thymic carcinoma”). It is critical to diagnose salivary gland tumors using fine needle aspiration cytology. However, this tumor is rare, and there have been few studies on its cytomorphological features. Therefore, it is important to understand the cytological diagnostic characteristics of CASTLE. Herein, we review the cytological features and diagnostic characteristics of salivary gland CASTLE. We also report the genotype results obtained using targeted exome sequencing, which we analyzed with DNA extracted from formalin‐fixed paraffin‐embedded tissue.

Carcinoma showing thymus‐like elements (CASTLE) with amyloid deposition in the parotid gland

“…However, CASTLE has lower levels of keratinization or intercellular bridging. 14 It has been suggested that CD5 is a useful marker for identifying parotid CASTLE, though its use is limited because the characteristics of parotid CASTLE have not yet been fully elucidated. 11,12 Thyroidal CASTLE is thought to originate from the ectopic thymus, vestiges of thymopharyngeal duct, or branchial pouch remnants (including remnants of the postbranchial body formed from the fourth pharyngeal pouch or solid cell nest of the thyroid gland).…”

“…Similar to SCC, CASTLE is positive for p63 and CK5/6. However, CASTLE has lower levels of keratinization or intercellular bridging 14 . It has been suggested that CD5 is a useful marker for identifying parotid CASTLE, though its use is limited because the characteristics of parotid CASTLE have not yet been fully elucidated 11,12 …”